thirty-seventh annual thirty-seventh annual...
TRANSCRIPT
THIRTY-SEVENTH ANNUAL
a
INSTITUTE ON LAKE SUPERIOR GEOLOGY
PROCEEDINGS: PART 1 - ABSTRACTS
MAY 1-4, 1991EAU CLAIRE, WISCONSIN
oenp4iáoh/e —
THIRTY-SEVENTH ANNUAL
INSTITUTE ON LAKE SUPERIOR GEOLOGY
PROCEEDINGS: PART 1 - ABSTRACTS
MAY 1-4, 1991 EAU CLAIRE, WISCONSIN
U
Organization Committee, 37th Annual Meeting ILSG, 1991
General Chairman: Paul E. Myers
PHONE: 715-836-3713
Editor: Luff! S. Chan
Geology Department LiUniversity of Wisconsin
Eau Claire, WI 547024004I
PROCEEDINGS VOLUME 37
PART I: ABSTRACTS (Includes Meeting Program) U
PART 2: FIELD TRIPS
Reference to material in this volume should follow the example below.
Morey, Glen B., and Southwick, David L,, 1991, Manganese mineralization in early IProterozoic iron-formation of the Emily District, Cayuna Range, east-centralMinnesota, (abst.j; Institute on Lake Superior Geology Proceedings, 37th AnnualMeeting, Eau Claire, WI, 1991; v. 37, part 1, p. 75-76.
UPublished and Distributed by:
Institute on Lake Superior GeologyLi
M.G. Mudrey, Jr., Secretary/TreasurerWisconsin Geological and Natural History Survey
3817 Mineral Point RoadMadison, WI 53705
LiPURCHASE OF PROCEEDINGS VOLUMES
Copies of the 37th ILSG Proceedings (Part I, Abstracts and Part H, Field Trips) may be purchased duringthe meeting for $5.00 (US) each.
Payable to the institute on Lake Superior Geology
Issues of Proceedings and Abstracts, Part 1, and Field Guidebook, Part 2. from this and previous meetingsmay be ordered from:
Michael G. Mudrey, Jr.Address, See above PHONE: 608-263-1705
The cost of each part is $6.00 U.S. Orders will be filled while supplies last. All volumes back to 1955 areavailable for photocopying at the prevailing rate from the Michigan Technological University Library throughMr. M.S. Spence, Archivist. Phone 906-487-2505. L
1
Organization committee, 37th Annual Meeting ILSG, 1991
General Ch- Paul E Mvers
PHONE; 715-836-3713
Geology Department University of Wisconsin
Eau Claire. WI S4702.4004
PROCEEDINGS VOLUMEJiZ
PART I: ABSTRACTS (Includes Meeting Program)
PART 2: FIELD TRIPS
Refereace to material in this volume should follow the example below.
Many, Glen B., and Southwick, David L., 1991, Mangame mineralization in early Pm€erow i r o i i - f i ' o n of the Emily District, Cquna Range, east-central Minnesota, [tabst.]; Institute en Luke Superior Geology Proceedings, 37th Annual
I Meeting, EM Ctairv, WI, 1991; v. 37, part 1, p. 75-76. I
Published and Distributed by: Institute on Lake Superior Geology
M.G. Mudrey, Jr., Secretary/Treasurer Wisconsin Geological and Natural History Survey
3817 Mineral Point Road Madison, WI 53705
PURC&iSE OF PROCEEDIh'GS VOLUMES
Copies of the 37th ILSG Proceedings (Part I, Abstracts and Part II, Field Trips) may be purchased during the meeting for $5.00 (US) each.
I Payable to the Institute on Lake Superior Geology I
Issues of Proceedings and Abstracts, Part 1, and Field Guidebook, Part 2. from this and previous meetings may be ordered from: I
Michael G. Mndrey, Jr. Address, See above PHONE: 608-263-1705
The cost of each part is $6.00 US. Orders will be tilled while supplies last, All volumes back to 1955 are available for photocopying a t the prevailing rate from the Michigan Technological University Library through Mr. MS. Spence, Archivist. Phone 906487-250s. I
a
37th Annual
Institute on Lake SuperiorGeology
Proceed iii gs
Ray Wachs Civic center
Eati Claire1 VisconSin
May 1-4, 1991
Orqaizized kv
Mvcrs. I.Jniversitv of \Visconsin-1 ZIU Claire
i ..s. i ;fliLNitV of Wise' )nsin—Lau Claire
Volume 37
Part 1 Abstracts
TABLE OF CONTENTS
Institutes on Lake Superior Geology to 1991
L Constitution of the Institute on Lake Superior Geology ii
LBy-Laws of the Institute on Lake Superior Geology iii
Goldich Medal Guidelines iv
LStudent Travel Award v
Board of Directors vi
L Local Committee vi
r Student Paper Award Committee vi
L Goldich Medal Committee vi
[ Goldich Medal Recipient vii
Banquet Speaker viii
L Acknowledgements viii
Report of the Chairs of the 36th Annual Institute ix
L Calendar of Events xi
[ Programs Xiii
Poster Papers XV
L Abstracts 1
L
L
L
L
L
TABLE OF CONTENTS
Institutes on Lake Superior Geology to 1991
Constitution of the Institute on Lake Superior Geology
By-Laws of the Institute on Lake Superior Geology
Goldich Medal Guidelines
Student Travel Award
Board of Directors
Local Committee
Student Paper Award Committee
Goldich Medal Committee l
Goldich Medal Recipient
Banquet Speaker
Acknowledgements
Report of the Chairs of the 36th Annual Institute
Calendar of Events
Programs
Poster Papers I I
Abstracts
ii
iii
iv
\
vi
vi
vi
V.
vii
viii
viii
ix
xi
xiii
XV
INSTITUTES ON LAKE SUPERIOR GEOLOGY
INSTITUTE NUMBER DATE PLACE
1 1955 Minneapolis, MN2 1956 Houghton, MI
3 1957 East Lansing, MI
4 1958 Duluth, MN
5 1959 Minneapolis, MN
6 1960 Madison, WI
7 1961 Port Arthur, Ont. (Thunder Bay)
8 1962 Houghton, MI
9 1963 Duluth, MN
10 1964 Ishpeining, MI
11 1965 St. Paul, MN
12 1966 Sault Ste. Marie, MI
13 1967 East Lansing, MI14 1968 Superior, WI
15 1969 Oshkosh, WI
16 1970 Thunder Bay, Ont.
17 1971 Duluth, MN
18 1972 Houghton, MI
19 1973 Madison, WI
20 1974 Sault Ste. Marie, Ont.
21 1975 Marquette, MI
22 1976 St. Paul, MN
23 1977 Thunder Bay, Ont.
24 1978 Milwaukee, WI
25 1979 Duluth, MN
26 1980 Eau Claire, WI
27 1981 East Lansing, MI
28 1982 International Falls, MN
29 1983 Houghton, MI
30 1984 Wausau, WI
31 1985 Kenora, Ont.
32 1986 Wisconsin Rapids, WI
33 1987 Wawa, Ont.
34 1988 Marquette, MI
35 1989 Duluth, MN
36 1990 Thunder Bay, Ont.
37 1991 Eau Claire, WI
1
INSTITUTES ON LAKE SUPERIOR GEOLOGY
INSTITUTE NUMBER
1 2 3
Minneapolis, MN Houghton, MI East Lansing, MI Duluth, MN Minneapolis, MN Madison, WI Port Arthur, Ont. (Thunder Bay) Houghton, MI Duluth, MN Ishpeming, MI St. Paul, MN Sault Ste. Marie, MI East Lansing, MI Superior, WI Oshkosh, WI Thunder Bay, Ont. Duluth, MN Houghton, MI Madison, WI Sault Ste. Marie, Ont. .,:
Marquette , MI : c ,
St. Paul, MN Thunder Bay, Ont. . , ~ , . Milwaukee, WI Duluth, MN Eau Claire, WI
.: .,,i East Lansing, MI .,-
International Falls, MN Houghton, MI Wausau, WI Kenora, Ont. Wisconsin Rapids', WI Wawa, Ont. Marquette, MI Duluth, MN Thunder Bay, Ont. Eau Claire, WI
CONSTITUTION OF INSTITUTE ON LAKE SUPERIOR GEOLOGY
Article I NameThe name of the organization shall be the 'Institute on Lake Superior Geology.
Article II ObjectivesThe objectives of this organization are:
A. To provide a means whereby geologists in the Great Lakes region may exchange ideas andscientific data.
B. To promote better understanding of the geology of the Lake Superior region.C. To plan and conduct geological field trips.
Article III StatusNo part of the income of the organization shall inure to the benefit of any member or individual.tn the event of dissolution the assets of the organization shall be distributed to
_____________
(some tax free organization).
(To avoid Federal and State income taxes, the organization should be not only scientiflcor educational, but also non-profit".)
Minn. Stat. Anno. 290.01, subd. 4Minn. Stat. Anno. 290.05(9)1954 Internal Revenue Code s. 50I(c)(3)
Article IV MembershipThe membership of the organization shall consist of the board of directors- Any geologistinterested shall be permitted to attend and participate in and vote at the annual meetings.
Article V MeetingsThe organization shall meet once a year, preferably during the month of April. The place andexact date of each meeting will be desginated by the board of directors.
Article VI DirectorsThe board of directors shall consist of the Chairman, Secretary-Treasurer, and the last threepast Chairman; but if the board should at any time consist of fewer than five persons, by rcaonof unwillingness or inability of any of the above persons to serve as directors, the vacancies onthe board may be filled by the annual meeting so as to bring the membership of the board upto five members.
Article VII OfficersThe officers of this organization shall be a Chairman and Secretary-Treasurer.
A. The Chairman shall be elected each year by the board of directors, who shall give dueconsideration to the wishes of any group that may be promoting the next annual meeting.His term of office as Chairman wilt terminate at the close of the annual meeting over whichhe presides or when his successor shall have been appointed. He will then serve for aperiod of three years as a member of the board of directors.
B. The Secretary-Treasurer shall be elected at the annual meeting. His term of office shallbe two years or until his successor shall have been appointed.
Article VIII AmendmentsThis constitution may be amended by a majority vote of those persons who are personallypresent at, participating in, and voting at any annual meeting of the organization.
II
Article I
Articlc I1
Article I11
Article IV
Article V
Articlc VI
Article VII
Article VIII
CONSTITUTION OF INSTITUTE ON LAKE SUPERIOR GEOLOGY
kb!!s The name of the organization shall be the "Institute on Lake Superior Geology".
The objectives of this organization are:
A. To provide a means whereby geologists in the Great Lakes region may exchange ideas and scientific data.
B. To promote better understanding of the geology of the Lake Superior region. C. To plan and conduct geological field trips.
aaus No part of the income of the organization shall inure t o the benefit of any member or individual. In the event of dissolution the assets of the organization shall be distributed to (some tax free organization).
(To avoid Federal and State income taxes, the organization should be not only "scientific" or "educational", but also "non-profit".)
Minn. Stat. Anno. 290.01. subd. 4 Minn. Stat. Anno. 290.05(9) 1954 Intcrnal Revenue C d c s. 501(c)(3)
Membership The membership of the organization shall consist of the board of directors. Any geologist interested shall be permitted to attend and participate in and vote at the annual meetin&%
The organization shall meet once a year, preferably during the month of April. The place and exact date of each meeting will be desginatcd by the board of directors.
The board of directors shall consist of the Chairman, Sccreta~y-Treasurer, and the last three past Chairman; but if the board should at any time consist of fewer than five persons, by rcaon of unwillingness or inability of any of the above persons to serve as directors, the vacancies on the board may be filled by the annual meeting so as t o bring the membership of the board up t o five members.
ixGQxs The officers of this organization shall be a Chairman and Secretary-Treasurer.
A. The Chairman shall be elected each year by the board of directors, who shall give due consideration to the wishes of any group that may be promoting the next annual meeting. His term of office as Chairman will terminate a t the close of the annual meeting over which he presides or when his successor shall have been appointed. H e will then serve for a period of three years as a member of the board of directors.
B. The Secretary-Treasurer shall be elected a t the annual meeting. His term of office shall be two years or until his successor shall have been appointed.
Afwdnwm This constitution may be amended by a majority vote of those persons who are personally present at, participating in, and voting at any annual meeting of the organization.
BY-LAWS
Duties of the Officers and Directors
A. It shall be the duty of the Annual Chairman to;
1. Preside at the annual meeting.2. Appoint all committees needed for the organization of the annual meeting.3. Assume complete responsibility for the organization and financing of the
annual meeting over which he presides.
B. It shall be the duty of the Secretary-Treasurer to:
1. Keep accurate attendance records of alt annual meetings.2. Keep accurate records of all meetings of, and correspondence between,
the board of directors.3. Hold all funds that may accrue as profits from annual meetings or field
trips and to make these funds available for the organization and operationof future meetings as required.
C. It shall be the duty of the board of directors to plan locations of annual meetingsand to advise on the organization and financing of all meetings.
II. Duties and Expenses
1. , There shall be no regular membership dues.2. Registration fees for the annual meetings shall be determined by the Chairman
in consultation with the board of directors, it is strongly recommended thatthese be kept at a minimum to encourage attendance of graduate students.
ill. Rules of Order
The rules contained in Robert's Rules of Order shall govern this organization in allcases to which they are applicable.
IV. Amendments
These by-laws may be amended by a majority vote of those persons who arepersonally present at, participating in, and voting at any annual meeting of theorganization; provided that such modifications shall not conflict with the constitutionas presently adopted or subsequently amended,
Ill
.Duties of the Officers and Directors
.4 It shall be the duty of the Annual Chairman to:
1. Preside at the annual meeting. 2 Appoint all committees needed for the organization of the annual meeting. 3. Assume complete responsibility for the organization and financing of the
annual meeting over which he presides.
1. 2.
3.
C. I t :
B. It shall be the duty of the Secretary-Treasurer to:
Keep accurate attendance records of all annual meetings. Keep accurate records of all meetings of, and correspondence between, the board of directors. Hold all funds that may accrue as profits from annual meetings or field trips and to make these funds available for the organization and operation of future meetings as required.
shall be the duty of the board of directors to plan locations of annual meetings and to advise on the organization and financing of all meetings.
and F.mensas
1. . There shall be no regular membership dues. 2. Registration fees for the annual meetings shall be determined by the Chairman
in consultation with the board of directors. It is strongly recommended that these be kept at a minimum to encourage attendance of graduate students.
The rules contained in Robert's Rules of Order shall govern this organization in all cases to which they are applicable.
. . . - : : ~ . . . , ~ . .
These by-laws may be amended by a majority vote of those persons who are personally present at, participating in, and voting at any annual meeting of the organization; provided that such modifications shall not conflict with the constitution as presently adopted or subsequently amended.
iii
Award GuidelinesSAM GOLDICH MEDAL
PreambleThe Institute on Lake Superior Geology was born on or around 1955, as documented bythe fact that the 27th annual meeting will be held in 1981. The Institutes are exemplaryin their continuing objectives of dealing with those aspects of geology that are relatedgeographically to Lake Superior; of encouraging the discussion of subjects and sponsoringfield trips which will bring together geologists from academia, government surveys, andindustry; and of maintaining an exceedingly informal but highly effective mode of operation.
During the course of its existence the membership of the Institute (that is, those geologistswho indicate an interest in the objectives of the I.LS.G. by attending) has become awareof the fact that certain of their colleagues have made particularly noteworthy andmeritorious contributions to the improvement of understanding of "Lake Superior" geologyand its mineral deposits.
The exemplary award was made by I.L.S.G. to Sam Goldich in 1979 for his manycontributions to the geology of the region extending over about 50 years.
Award Guidelines1) The medal shall be awarded annually by the LL.S.G. Board of Directors to a geologist
whose name is associated with a substantial sustained interest in, or a major contributionto, the geology of the Lake Superior region.
2) The Board of Directors, LL.S.G. shall appoint the Nominating Committee. The initialappointment will be of three members, one to serve for three years, one for two, andone for one year, the member with the briefest incumbency to be chairman. After thefirst year the Board of Directors shall appoint at each spring meeting one new memberwho will serve for three years. In the thrid year this member shall be the chairman.The Committee membership should reflect the main fields of interest and geographicdistribution of I.LS.G. membership.
3) By November 1, the Goldich Medal Nominating Committee shall make itsrecommendation to the Chairman of the Board of Directors who will then inform theBoard of the nominee.
4) The Board of Directors normally will accept the nominee of the Committee, will informthe medalist immediately, and will have one medal engraved appropriately forpresentation at the next meeting of the Institute.
5) It is recommended that the Institute set aside annually from whatever sources, suchfunds as will be required to support the continuing costs of this award.
April 4, 1981
J. Kalliokoski, ChairmanBill CannonFred KehlenbeckGlenn MoreyGreg Mursky
iv
Award Guidelines SAM GOLDICH MEDAL
Preamble The Institute on Lake Superior Geology was born on or around 1955, as documented by the fact that the 27th annual meeting will be held in 1981. The Institutes are exemplary in their continuing objectives of dealing with those aspects of geology that are related geographically to Lake Superior; of encouraging the discussion of subjects and sponsoring field trips which will bring together geologists from academia, government surveys, and industry; and of maintaining an exceedingly informal but highly effective mode of operation.
During the course of its existence the membership of the Institute (that is, those geologists who indicate an interest in the objectives of the I.L.S.G. by attending) has become aware of the fact that certain of their colleagues have made particularly noteworthy and meritorious contributions to the improvement of understanding of "Lake Superior" geology and its mineral deposits.
The exemplary award was made by I.L.S.G. to Sam Goldich in 1979 for his many contributions to the geology of the region extending over about 50 years.
Award G u ~ d a . . 1) The medal shall be awarded annually by the I.L.S.G. Board of Directors to a geologist
whose name is associated with a substantial sustained interest in, or a major contribution to, the geology of the Lake Superior region.
2) The Board of Directors, I.L.S.G. shall appoint the Nominating Committee. The initial appointment will be of three members, one to serve for three years, one for two, and one for one year, the member with the briefest incumbency to be chairman. After the first year the Board of Directors shall appoint at each spring meeting one new member who will serve for three years. In the thrid year this member shall be the chairman. The Committee membership should reflect the main fields of interest and geographic distribution of I.L.S.G. membership.
3) By November 1, the Goldich Medal Nominating Committee shall make its recommendation to the card of Directors who will then inform the
, .z.:,: , . ..~ , ~ . ~ Board of the nominee. , .
4) The Board of Directors normally will accept the nominee of the Committee, will inform the medalist immediately, and will have one medal engraved appropriately for presentation a t the next meeting of the Institute.
5) It is recommended that the Institute set aside annually from whatever sources, such funds as will be required to support the continuing costs of this award.
April 4, 1981
J. Kalliokoski, Chairman Bill Cannon Fred Kehlenbeck Glenn Morey Greg Mursky
STUDENT TRAVEL AWARD
l'hc 1986 Board 4)1 Directors established the 1.LS.G. Student Travel Award to sLIl)I)orLsttideiit participation at the annual Institutes. The awards will be made from a special fundset up for this purpose. This award is intended to help defray some of the direct travelcosts to the Institute and includes a waiver of registration fees, but excludes expenses formeals, lodging, and field trip registration. The number of awards and value are determinedby the annual Chairman in consultation with the Secretary-Treasurer and will be announcedat the annual banquet.
The following general criteria will be considered by the annual Chairman, who isresponsible for the selection:
1) The applicants must have active resident (undergraduate or graduate) studentstatus at the time of the Institute, certified by the department head.
2) Students who are the senior author on either an oral or poster paper will begiven favored consideration.
3) It is desirable for two or more students to jointly request travel assistance.
4) In general, priority will be given to those in the Institute region who arefarthest away.
5) Each travel award request shall be made in writing, to the annual Chairman,with an explanation of need, possible author status or other significant details.
Successful applicants will receive their awards at the time of registration for the Meeting.
V
STUDENT TRAVEL AWARD
I'hc I080 Board of Directors established the I.L.S.G. Student Travel Award to siipport s~iidcnt participation at the annual Institutes. The awards will be made from a special fund set up for this purpose. This award is intended to help defray some of the direct travel costs to the Institute and includes a waiver of registration fees, but excludes expenses for meals, lodging, and field trip registration. The number of awards and value are determined by the annual Chairman in consultation with the Secretary-Treasurer and will be announced at the annual banquet.
The following general criteria will be considered by the annual Chairman, who is responsible for the selection:
1) The applicants must have active resident (undergraduate or graduate) student status at the time of the Institute, certified by the department head.
2) Students who are the senior author on either an oral or poster paper will be given favored consideration.
3) It is desirable for two or more students to jointly request travel assistance.
4) In general, priority will be given to those in the Institute region who arc farthest away.
5) Each travel award request shall be made in writing, to the annual Chairman, with an explanation of need, possible author status or other significant details.
Successful applicants will receive their awards at the time of registration for the Meeting.
BOARD OF DIRECTORS
1991 P. E. Myers (with L. S. Chan)Department of Geology, University of Wisconsin, Eau Claire. Wisconsin54701
1990 H. H. KehlenbeckDepartment of Geology, Lakehead University, Thunder Bay, Ontario P7B 5El
1989 ft. W. Ojakangas (with J. C. Green and T. B. Holst)Department of Geology, University of Minnesota, Duluth, Minnesota 55812
1988 J. S. Klasner (with J. D. Hughes and K. J. Schulz)Department of Geology, Western Illinois University, Macomb, lilnois61455
Secretary-TreasurerM. G. Mudrey, Jr.Wisconsin Geological and Natural History Survey, 3817 Mineral PointRoad, Madison, WI 53705
LOCAL COMMITTEE
P. E. Myers General Chairman
L. S. Chan Program and Abstract Editorand Field Trip Guidebook Editor
ft. P. Willis Registration Coordinator
STUDENT PAPER AWARD CoMMITTEE 1991
J. S. Klasner, Chairman Western Illinois University, Macomb, Illinois
GOLDICH MEDAL COMMITTEE 1990-91
M. C. Mudrey, Jr. Wisconsin Geologic and Natural History Survey,Madison, WI
F. W. Cambray Dept. of Geological Sciences, Michigan StateUniversity, East Lansing, MI j
H. H. Halls Dept. of Geology, Erindall College, Mississauga,Ont. LSL 1CO
j
'1
viU
j
BOARD OF DIRECTORS
1991 P. E. Myers (with L. S. Chan) Department of Geology, University of Wisconsin, Eau Claire, Wisconsin 54701
1990 M. M. Kehlenbeck Department of Geology, Lakehead University, Thunder Bay, Ontario P7B 5E1
1989 R. W. Ojakangas (with J. C . Green and T. B. Hoist) Department of Geology, University of Minnesota, Duluth, Minnesota 55812
1988 J. S. Klasner (with J. D. Hughes and K. J. Schulz) Department of Geology, Western Illinois University, Macomb, Illnois 61455
Secretary-Treasurer M. G. Mudrey, Jr. Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, WI 53705
LOCAL COMMITTEE
P. E. Myers General Chairman
L. S. Chan Program and Abstract Editor and Field Trip Guidebook Editor
Registration Coordinator
STUDENT PAPER AWARD COMMITTEE 1991
' S. Klasner, Chairman Western Illinois University, Macomb, Illinois
GOLDICH MEDAL COMMITTEE 1990-91
M. G. Mudrey, Jr. Wisconsin Geologic and Natural History Survey, Madison, WI
F. W. Cambray Dept. of Geological Sciences, Michigan State University, East Lansing, MI
H. H. Halls Dept. of Geology, Erindall College, Mississauga, Ont. L5L 1CO
Biographical Sketch ofWILLIAM J. HINZE
1991 Goldich Medal Recipient
William J. Hinze began his earth-science career at the University of Wisconsin, where in 1951he received a bachelor of science in geology. From 1954 to 1955 he served in the U.S. AnnyCorps of Engineers at their Research and Development Laboratory, after which he returned to theUniversity of Wisconsin, receiving his doctorate in geology and physics in 1957. From 1956 to1958 he worked as a staff geophysicist for Jones & Laughlin Steel Company. In 1958 he joinedthe faculty of the department of geology at Michigan State University. where he held appointmentsof assistant to full professor. In 1972 he took his present post as professor of geophysics in theDepartment of Earth and Atmospheric Sciences at Purdue University. In 1985 he served as avisiting professor at the University of Lausanne in Switzerland.
In the course of his career Hinze has made many significant and lasting contributions topotential-field geophysics. In addition to numerous abstracts and technical reports, he avengesseveral formal papers per year, either as chief author or as co-author—a truly remarkable record ifhis fonnidable professional duties are considered. For instance, from 1975 to 1982 he waschairman of the U.S. National Magnetic Anomaly Map Committee, and from 1985 to 1989 he co-chaired the North American Magnetic Anomaly Map Committee. He edited The Utility of RegionalGravity and Magnetic Anomaly Maps, which was published by the Society of ExplorationGeophysicists in 1985. From 1986 to 1988 he co-chaired the U.S. National Science Foundation'sjoint U.S./Indian workshop "Regional Geophysical Lineaments: Their Tectonic and EconomicSignificance" and co-edited the workshop volume, which was published by the Geological Societyof India in 1989. Since 1985 he has dealt with problems concerning the distribution and archivingof geophysical data for the worldwide scientific community and, related to this task, has served invarious posts with the U.S. Geodynamics Committee, the International Lithospheric Commission,and the National Academy of Science/National Research Council. From 1988 to the present he hasserved as a consultant to and member of the Advisory Committee on Nuclear Waste of the NuclearRegulatory Commission. He has served from 1986 both as a member and vice-chainnan of theboard of directors of Deep Observation and Sampling of the Earth's Continental Crust, Inc.(DOSECC), and as a co-chairman of the Potential Fields Committee of the Great LakesInternational Program on Crustal Evolution (GLIMPCE). He has been a member of the U.S.Geodynamics Committee of the National Academy of Science/National Research Council since1989.
During his long and successful career Hinze has been at the forefront of geophysical researchin the Lake Superior region and the surrounding Midcontinent. His doctoral research, a gravityinvestigation of the Baraboo Syncline region, which was published in the Journal of Geology in1959, was one of the most innovative gravity studies of its time. His 1960 Economic Geologypaper on the gravity method in iron-ore exploration, which was largely based on his experienceswith Jones & Laughlin Steel Company, became a classic work among explorationists; a slightlyrevised version reappeared in 1966 in the Society of Exploration Geophysicists' volumes on mininggeophysics. During the 1960s Hinze was the principal investigator of several major aeromagneticand gravity studies of the western Great Lakes, the Upper Peninsula of Michigan, and the MichiganBasin. The aeromagnetic studies over Lake Superior complemented the seismic investigations of
vii
Biographical Sketch of WILLIAM J. HINZE
1991 Goldich Medal Recipient
William J. Iiinze began his earth-science career at the University of Wisconsin, where in 1951 he received a bachelor of science in gwlogy. From 1954 to 1955 he served in the US. Army Corps of Engineers at their Research and Development Laboratory, after which he returned to the University of Wisconsin, receiving his doctorate in gwlogy and physics in 1957. From 1956 to 1958 he worked as a staff geophysicist for Jones & Laughlin Steel Company. In 1958 he joined the faculty of the department of gwlogy at Michigan State University, where he held appointments of assistant to full professor. In 1972 he took his present post as pmfessor of gwphysics in the Department of Earth and Atmospheric Sciences at Purdue University. In 1985 he served as a visiting professor at the University of Lausanne in Switzerland.
In the course of his career Hinze has made many significant and lasting contributions to potential-field geophysics. In addition to numerous abstracts and technical reports, he averages several formal papers per year. either as chief author or as w-author- truly remarkable rewrd if his formidable professional duties are considered. For instance, from 1975 to 1982 he was chairman of the US. National Magnetic Anomaly Map Committee, and from 1985 to 1989 he w- chaired the North AmericanMagnetic Anomaly Map Committee. He edited The Utility ofRegional Gravity and Magnetic Anomaly Maps, which was published by the Society of Exploration Geophysicists in 1985. From 1986 to 1988 he w-chaired the US. National Science Foundation's joint U.S./hdian workshop "Regional Geophysical Lineaments: Their Tectonic and Economic Significance" and co-edited the workshop volume, which was published by the Geological Society of M i a in 1989. Since 1985 he has dealt with problems concerning the distribution and archiving of gwphysical data for the worldwide scientific community and, related to this task, has served in various posts with the U.S. Geodynamics Committee, the International Lithospheric Commission, and the National Academy of Sciendational Research Council. From 1988 to the present he has served as a consultant to and member of the Advisory Committee on Nuclear Waste of the Nuclear Regulatory Commission. He has served from 1986 both as a member and vice-chairman of the board of directors of Deep Observation and Sampling of the Earth's Continental Crust, Inc. (DOSECC), and as a co-chairman of the Potential Fields Committee of the Great Lakes International Program on Crustal Evolution (GLIMPCE). He has been a member of the US. Geodynamics Committee of the National Academy of ScienceJNational Research Council since 1989.
During his long and successful career Hinze has been at the forefront of gwphysical research in the Lake Superior region and the surrounding Midcontinent. His doctoral research, a gravity investigation of the Bamboo Syncline region, which was published in the Journal of Geology in 1959, was one of the most innovative gravity studies of its time. His 1960 Economic Geology paper on the gravity method in iron-ore exploration, which was largely based on his experiences with Jones & Laughlin Steel Company, became a classic work among explorationists; a slightly revised version reappeared in 1966 in the Society of Exploration Geophysicists' volumes on mining gwphysics. During the 1960s Hinze was the principal investigator of several major aeromagnetic and gravity studies of the western Great Lakes, the Upper Peninsula of Michigan, and the Michigan Basin. The aeromagnetic studies over Lake Superior complemented the seismic investigations of
vii
the Lake Superior Experiment, and in 1966 the results were published with the seismic studies inthe frequently cited The Earth Beneath the Continents (American Geophysical Union GeophysicalMonograph 10). In 1982 R.J. Wold and he co-edited Geology and Tectonics of the Lake SuperiorBasin (Geological Society of America Memoir 156). which remains today the definitive referenceon Lake Superior geology and geophysics. In recognition of his contributions to Midcontinentgeology, he was awarded an honorary membership in the Michigan Basin Geological Society in1986.
Today Hinze maintains his high profile in geophysical studies of the Lake Superior region.Since the mid-1970s he has been a leading investigator of Mideontinent seismicity and its probablerelationship to basement structure. Through his involvement with DOSECC and other agencies, hehas been a leading pioponnent of deep scientific drilling in the Lake Superior region. He and hisstudents are presently conducting combined studies of seismic-reflection, gravity, and magneticdata to investigate crusthi structure beneath Lake Superior. a task that epitomizes the stronggeneralist approach that characterizes his career.
Perhaps Bill Hinze's greatest legacy is his students. Through organizational skills that areunsurpassed and an enthusiasm for his work that is contagious, he has inspired many to learn andachieve far beyond what they thought they were capable. The careers of many successfulgeophysicists worldwide proudly bear his imprint.
His is truly a carter that has made a difference.VAL W. CHA1'DLS
Minnesota Geological Survey
Ban quet Speaker
Paul IC. Sims, U.S. Geological Survey, Denver, Colorado, will be the speaker at the Annual ILSGBanquet in Ray Wachs Civic Center (Rm A) at 8:30 pm on Thursday, May 2, 1991. His topic will be'Archean and Early Proterozoic Geology of the Lake Superior Region -- An Overview.' He will beintroduced by Glen Morcy of the Minnesota Geological Survey.
Acknowledgements
We greatly appreciate the enthusiastic and vital assistance of the many UW-Eau Claire geologystudent drivers, registration helpers, projectionists, and gofers. We also thank those who volunteeredto serve as session chairs, speakers, and authors; their dedication to the philosophy of the ILSG isconspicuous.
viii
the Lake Superior Experiment, and in 1966 the results were published with the seismic studies in the frequently cited The Earth Beneath the Continents (American Geophysical Union Geophysical Monograph 10). In 1982 R.J. Wold and he co-edited Geology and Tectonics of the Lake Superior Basin (Geological Society of America Memoir 156). which remains today the definitive reference on Lake Superior geology and geophysics. In recognition of his contributions to Midcontinent geology, he was awarded an honorary membership in the Michigan Basin Geological Society in 1986.
Today Hinze maintains his high profile in geophysical studies of the Lake Superior region. Since the mid-1970s he has been a leading investigator of Midcontinent seismicity and its probable relationship to basement structure. Through his involvement with DOSECC and other agencies, be has been a leading pmponnent of deep scientific drilling in the Lake Superior region. He and his students are presently conducting combined studies of seismic-reflection, gravity, and magnetic data to investigate crustal structure beneath Lake Superior, a task that epitomizes the strong generalist approach that characterizes his career.
Perhaps Bill Hinze's greatest legacy is his students. Through organizational skills that are unsurpassed and an enthusiasm for his work that is contagious, he has inspired many to learn and achieve far beyond what they thought they were capable. The careers of many successful geophysicists worldwide proudly bear his imprint.
His is truly a career that has made a difference.
VAL W. CHANDLER Minnesota Geological Survey
~ > '
Banquet Speaker
Paul K. Sims, US. Geological Survey, Denver. Colorado, will be the speaker at the Annual ILSG Banquet in Ray Wachs Civic Center (Rm A) at 8:30 pm on Thursday, May 2, 1991. His topic will be "Archean and Early Proterozoic Geology of the Lake Superior Region - An Overview." He will be introduced by Glen Morey of the Minnesota Geological Survey.
Acknowledgements
We greatly appreciate the enthusiastic and vital assistance of the many UW-Eau Claire geology student drivers, registration helpers, projectionists, and gofers. We also thank those who volunteercd to serve as session chairs, speakers, and authors; their dedication to the philosophy of the ILSG is conspicuous.
viii
36th ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGYThunder Bay, Ontario 1990
The 36th annual meeting of the Institute on Lake Superior Geology was heldfrom May 9 through May 12 in Thunder Bay, Ontario. The meeting was hosted byLakehead University, Thunder Bay. Manfred M. Kehlenbeck served as GeneralChairman, Graham Borradaile edited the Field Trip Guidebook, and Philip W. Fralickassisted with the program organization.
The technical sessions and banquet were held at the Airlane Motor Hotel onThursday, May 10 and Friday, May 11. A total of 191 geologists registered for themeeting. Thirty-five oral and twenty poster papers were presented on a wide variety ofsubjects pertinent to the geology of the Lake Superior Region.
Three of the four field trips were held as both pre- and post- meeting trips onMay 9 and May 12 respectively. The fourth field trip was held on May 12. RichardSutcliffe of the Ontario Geological Survey led 2 trips to the L.ac des mes area toexposures of mafic intrusions, PGE mineralization and granitoid rocks. GrahamBorradaile of Lakehead University led 2 field trips to examine the geology of theShebandowan and Quetico subprovinces. Two trips led by Maurice Lavigue and JohnScott of the Ministry of Northern Development and Mines offered insigbt on base metalmineralization in the Shebandowan greenstone belt. The final field trip was led by SteveKissin of Lakehead University and focused on granitoid-related mineral deposits in thewestern Superior region. A total of 187 geologists registered for the seven fieldexcursions. To transport the entire group to relatively isolated areas required a total of10 vans and 3 large buses. In addition to the trip leaders, the following members of theGeology Department acted as van drivers: P. Fralick, M. Kehlenbeck, S. Kissin, D.Nicol, S. Spivak, B. Seemayer and it. Viitala.
A total of 140 people attended the annual banquet on May 10. ManfredKehienbeck presented the 1990 Goldich Medal to Ken Card of the Geological Survey ofCanada for his many significant contributions to the geological understanding of therocks in the Great Lakes Region. J.J. Brummer, M.G. Mudrey and F.W. Cambrayserved on the Goldich Medal Committee. Dedication of the 1990 I.LS.G. publication toH.L. James in recognition of his life-time achievements as a geologist in the LakeSuperior area was presented by Mike Mudrey. After numerous unsatisfactory replies toinquiring participants during the day-long program, we were delighted to have HerrDoktor Professor Wolfgang Gottlieb von Schlunimerklutz (a.k.a. Richard Ojakangas),Direktor of the Wissenschaftliche Institut fuer Welt Probleme entertain all assembledwith his presentation of recent research on the origin of Panzerenkotklotzen. Hismeticulous research and marvellous illustrations made a lasting impression.
Recognition of the importance of student participation in the meetings of theInstitute led to the continued financial support of 16 student contributors in 1990. Fundsfor this assistance which includes waiver of registration fees were drawn equally from thetwo treasuries of the Institute. A total of $2,060.00 was spent in 1990. Four awards were
ix
36th ANNUAL INSTITUTE ON LAKE SUPERIOR GEOLOGY under Bay, Ontario 1990
g of the Institute on Lake Sup Geol from May 9 through May 12 in Thunder Bay, Ontario. The meeting was hosted by Lakehead University, Thunder Bay. Manfred M. Kehlenbeck served as General Chairman, Graham Borradaile edited the Field Trip Guidebook, and Philip W. Fralick assisted with the program organization.
. ~ . . .;. .. . ... , . , ' ,:- 7--. <,.* <<A . , :. . ,. The technical sessions and banquet were held at 'the Airlane Motor Hotel on
Thursday, May 10 and Friday, May 11. A total of 191 geologists registered for the meeting. Thirty-five oral and twenty poster papers were presented on a wide variety of subjects pertinent to the geology of the Lake Superior Region.
Three of the four field trips were held as both pre- and post- meeting trips on May 9 and May 12 respectively. The fourth field trip was held on May 12. Richard Sutcliffe of the Ontario Geological Survey led 2 trips to the Lac des Dies area to exposures of mafic intrusions, PGE mineralization and granitoid rocks. Graham Borradaile of Lakehead University led 2 field trips to examine the geology of the Shebandowan and Quetico subprovinces. Two trips led by Maurice Lavigue and John Scott of the Ministry of Northern Development and Mines offered insight on base metal mineralization in the Shebandowan greenstone belt. The final field trip was led by Steve Kissin of Lakehead University and focused on granitoid-related mineral deposits in the western Superior region. A total of 187 geologists registered for the seven field excursions. To transport the entire group to relatively isolated areas required a total of 10 vans and 3 large buses. In addition to the trip leaders, the following members of the Geology Department acted as van drivers: P. Frali Nicol, S. Spivak, B. Seemayer and R. Viitala. : i...
A total of 140 people attended the annual b Kehlenbeck presented the 1990 Goldich Medal to Ken Card of the Geological Survey of Canada for his many significant contributions to the geological understanding of the rocks in the Great Lakes Region. J.J. Brummer, M.G. Mudrey and F.W. Cambray served on the Goldich Medal Committee. Dedication of the 1990 I.L.S.G. publication to H.L. James in recognition of his life-time achievements as a geologist in the Lake Superior area was presented by Mike Mudrey. After numerous unsatisfactory replies to inquiring participants during the day-long program, we were delighted to have Herr Doktor Professor Wolfgang Gottlieb von Schlummerklutz (a.k.a. Richard Ojakangas), Direktor of the Wissenschaftliche Institut filer Welt Probleme entertain all assembled with his presentation of recent research on the origin of Panzerenkotklotzen. His meticulous research and marvellous illustrations made a lasting impression.
Recognition of the importance of student participation in the meetings of the Institute led to the continued financial support of 16 student contributors in 1990. Funds for this assistance which includes waiver of registration fees were drawn equally from the two treasuries of the Institute. A total of $2,060.00 was spent in 1990. Four awards were
presented for the best oral and poster papers by students for a total of $600.00. Therecipients of the 1990 Best Student Paper Awards were: Stephen D. Geerts, NaturalResources Research Institute, Duluth, MN.; Joseph D. Jablinski, University ofMinnesota, Duluth; Steven R. Koehler, University of Wisconsin, River Falls; and DavidB. Saja, Michigan Technological University, Houghton J. Kiasner, W. Kangas and D.Southwick served on the Best Student Paper Committee.
The I.LS.G. Board of Directors met on May 4. The meeting was attended by J.Kalliokoski, M. Kehienbeck, J. Klasner, Lung Chan, M. Lavigne, P. Myers, M. Mudrey,R. Ojakangas, R. Sage, and M. Smyk.
The following items were discussed:
(1) The site for the 1991 meeting will be at Eau Claire, WI - Paul Myers as GeneralChairman.
(2) Voted to accept Henry Halls on the Goldich Medal Committee to replace J.J.Brummer whose term has expired.
(3) To continue the practice of travel support for students presenting papers. Fundsto be drawn equally from the ILSG treasuries in the United States and Canada.
(4) The treasurer's report by J. Kalliokoski was received. The balance in the Generalaccount as of March 27, 1990 was $8,463.75. The Goldich Medal Fund contained$911.76. Fourteen Medals are in care of G.B. Morey. Since the 1990 meetingwas held in Thunder Bay the account was actively changing. Consequently, nomeaningful statement could be made. M. Kehlenbeck, however, assured theBoard that all expenses would be met and that no deficits would be incurred.
(5) Offers to host future meetings of the Institute were received from the Ministry ofNorthern Development and Mines (Marathon, Ontario), Michigan TechnologicalUniversity, and the Minnesota Geological Survey.
(6) Alter many years of service, Joe Kalliokoski tendered his resignation as Secretary-Treasurer of the Institute. Joe has not only been carrying out the duties of thisoffice, but has been very active in many aspects of the functioning of the Instituteand has a sustained record of contributions. His sage advise has been a greathelp and comfort to many general chairmen in the past. Voted to ask MikeMudrey to assume the office of Secretary-Treasurer.
I wish to thank all those who dedicated their time and expertise to make the 1990meeting a success.
Respectfully submitted,
M.M. Kehlenbeck
x
presented for the best oral and poster papers by students for a total of $600.00. The recipients of the 1990 Best Student Paper Awards were: Stephen D. Geerts, Natural Resources Research Institute, Duluth, MN.; Joseph D. Jablinski, University of Minnesota, Duluth; Steven R. Koehler, University of Wisconsin, River Falls, and David B. Saja, Michigan Technological University, Houghton. J. Klasner, W. Kangas and D. Southwick served on the Best Student Paper Committee.
The I.L.S.G. Board of Directors met on ~a~ 4. The meeting was attended by J. Kalliokoski, M. Kehlenbeck, J. Klasner, R. Ojakangas, R. Sage, and M. Smyk.
The following items were discussed:
The site for the 1991 meeting will be at Eau Claire, WI - Paul Myers as General Chairman. Voted to accept Henry Halls on the Goldich Medal Committee to replace J.J. Bmmmer whose term has expired. To continue the practice of travel support for students presenting papers. Funds to be drawn equally from the ILSG treasuries in the United States and Canada. The treasurer's report by J. Kalliokoski was received. The balance in the General account as of March 27, 1990 was $8,463.75. The Goldich Medal Fund contained $911.76. Fourteen Medals are in care of G.B. Morey. Since the 1990 meeting was held in Thunder Bay the account was actively changing. Consequently, no meaningful statement could be made. M. Kehlenbeck, however, assured the Board that all expenses would be met and that no deficits would be incurred. Offers to host future meetings of the Institute were received from the Ministry of Northern Development and Mines (Marathon, Ontario), Michigan Technological University, and the Minnesota Geological Survey. After many years of service, Joe Kalliokoski tendered his resignation as Secretary- Treasurer of the Institute. Joe has not only been carrying out the duties of this office, but has been very active in many aspects of the functioning of the Institute and has a sustained record of contributions. His sage advise has been a great help and comfort to many general chairmen in the past. Voted to ask Mike Mudrey to assume the office of Secretary-Treasurer.
I wish to thank all those who dedicated their time and expertise to make the 1990 meeting a success.
Respectfully submitted,
M.M. Kehlenbeck
CALENDAR OF EVENTS
TUESDAY, APRIL 30
3:00-5:00 p.m. Early Registration for Field Trips #1 and #2 Participants - CivicCenter Inn (CCI) Lobby
FIELD TRIP #1 P.K. Sims, and J.S. Kiasner (Leaders) "The Mountain shear zone - a
post-Penokean discrete ductile deformation zone"5:00p.m. Two vans depart for Antigo (Park Inn)8:30 p.m., Park Inn, Pre-Field Trip Conference
WEDNESDAY, MAY 1
FIELD TRIP #1 8:00 a.m. Vans depart from Park Inn, Antigo, for the mountain area. Vansreturn to CCI by about 9:00 p.m.
FIELD TRIP #2 L.S. Chan, P.E. Myers, and R.L. Hay, (Leaders): "Features andsignificance of the Precambrian-Cambrian contact in westernWisconsin"8:00 a.m. Pre-trip Seminar CCI, Farwell Rm.8:20 a.m. Vans depart from CCI Lobby
2:00-5:00 p.m. Pre-Conference Planning Session - Staff, CCI Gibson Room
4:00-8:00 p.m. Registration and check-in for pre-registrants, Civic Center Inn Lobby
8:00-11:00 p.m. Informal Get.together and Cash BarMusic by: Bill Jordan Jazz Quintet
THURSDAY, MAY 2
7:15 a.m Registration - Lobby, Civic Center Inn (CCI)
TECHNICAL SESSION
All Technical Sessions are in Room C, Ray Wachs Civic Center (RWCC); Poster Sessionsin RWCC Rooms B and E. Schedules of oral papers are shown on pages xiii and xiv.Contributors should have posters up by 9:00 a.m.
8:00 a.m. Introduction and Welcome: Paul Myers, Conference Chairman, and LarrySchnack, Chancellor, University of Wisconsin - Eau Claire
8:20-10:00 a.m. Morning Session #1. Chairs: R.W. Ojakangas and A.B. Dkkas
10:00 a.m. COFFEE BREAK
10:20-11:50 a,m. Morning Session #2. Chairs: G.L. LaBerge and K. Schultz
xi
300-500 p.m.
FIELD TRIP #1
FIELD TRIP #1
FIELD TRIP #2
CALENDAR OF EVENTS
TUESDAY, APRIL 30
Early Registration for Field Trips #1 and #2 Participants - Civic Center Inn (CCI) Lobby
P.K. Sims, and J.S. Klasner (Leaders) "The Mountain shear zone - a post-Penokean discrete ductile deformation zone" 500 p.m. Two veins depart for Antis (Park Inn) 8:30 p.m., Park Inn, Pre-Field Trip CoIfference
WEDNESDAY, MAY 1
8:00 a.m. Vans depart from Park Inn, Antis, for the mountain area. Vans return to CCI by about 9:00 p.m.
L.S. Chan, P.E. Myers, and R.L. Hay, (Leaders): "Features and significance of the Precambrian-Cambrian contact in western Wisconsin" 8:00 a.m. Pre-mp Seminar CCI, Farwell Rm. 8:20 a.m. Vans depart from CCI Lobby
Pre-Conference Planning Session - Staff, CCI Gibson Room
Registration and check-in for pre-registrants, Civic Center Inn Lobby
Informal Get-together and Cash Bar Music by: Bill Jordan Jazz Quintet
THURSDAY, MAY 2
Registration - Lobby, Civic Center Inn (CCI)
TECHNICAL SESSION
All Technical Sessions are in Room C, Ray Wachs Civic Center (RWCC); Poster Sessions in RWCC Rooms D and E. Schedules of oral papers are shown on pages xiii and xiv. Contributors should have posters up by 9:00 a.m.
8:00 a.m. Introduction and Welcome: Paul Myers, Conference Chairman, and Lorry Schnewk, Chancellor, University of Wisconsin - Eau Claire
8:20-10:OO a.m. Morning Session #I. Chairs: R.W. Ojakangas and A.B. Dickas
10:OO a.m. COFFEE BREAK
10:20-11:50 a.m. Morning Session #2. Chairs: G.L. LaBerge and If. Schultz
NOON-2:00 p.m. BOARD LUNCHEON AN]) ANNUAL MEETING: Farwell Room, CCI
12:30-1:30 p.m. POSTER SESSION #1 - Authors present (Poster papers, p. xv)
1:40-3:00 p.m. Afternoon Session #1. Chairs: V.W. Chandler and G.B. Morey
3:00 p.m. COFFEE BREAK -- Remove Poster Papers
3:20-4:50 p.m. Afternoon Session #2. Chairs: MM. Kehienbeck and C. Craddock
6:00-7:30 p.m. CASH BAR: RWCC ROOM A
7:30-10:00 p.m. ANNUAL ILSG BANQUET -- RWCC Rooms A and B
Presentation of the Goldich Medal to WJ. Hinze, Purdue Universityby V.W. Chandler
Speaker: Paul K. Sims, "Archean and Early Proterozoic Geology of theLake Superior Region -- An Overview"; introduced by G.B. Morey.
FRIDAY, MAY 3, 1991
8:00 a.m.-3:30 p.m. Registration and Sale of Extra Proceedings Volumes
8:20 a.m. Morning Session #1. Chairs: M. G. Mudrey, Jr. and 7'. A. DeMatties
10:00 a.m. COFFEE BREAK
10:20-11:30 a.m. Morning Session #2. Chairs: G.W. Adams and BA. Brown
12:30-1:30 p.m. POSTER SESSION #2 -- Authors present (poster papers, p. xvi)
1:40-2:40 p.m. Afternoon Session #1. Chairs: W.S. Cordua and M. Jirsa
2:40 p.m. Student Award Presentation, John Klasner
3:00 p.m. COFFEE BREAK -- Remove Poster Papers
3:20-4:30 p.m. Afternoon Session #2. Chairs: J. Kaliokoski and J. C. Green
7:00-8:30 p.m. Pre-Trip #3 Seminar, CCI, Farwell Room, M.G. Mudrey, Jr. and B.A.Brown
SATURDAY, MAY 4, 1991
FIELD TRIP #3 M.G. Mudrey, Jr., and B.A. Brown (Leaders), "Proterozoicvolcanogenic massive sulfide deposits of northwestern Wisconsin'8:00 a.m. Bus departs from front entrance of CCI
xli
BOARD LUNCHEON AND ANNUAL MEETING: Fanvell Room, CCI
POSTER SESSION #1 - Authors present (Poster papers, p. xv)
Afternoon Session #1. Chairs: V.W. Chandler and G.B. Morq
COFFEE BREAK - Remove Poster Papers
Afternoon Session #2. Chairs: MM. &hienbeck and C. Craddock
CASH BAR: RWCC ROOM A
ANNUAL ILSG BANQUET -- RWCC Rooms A and B
Presentation of the Goldich Medal to WJ. Hinze, Purdue University by V.W. Chandler
Speaker: Paul K. Sims, "Archean and Early Proterozoic Geology of the Lake Superior Region -- An Overview"; introduced by G.B. Morey.
FRIDAY, MAY 3,1991
8:00 a.m.-330 p.m. Registration and Sale of Extra Proceedings Volumes
FIELD TRIP #3
Morning Session #l. Chairs: M. G. Mudrey, Jr. and T. A. DeMatties
COFFEE BREAK
Morning Session #2. Chairs: G.W. Adam and BA. Brown
POSTER SESSION #2 -- Authors present (poster papers, p. xvi)
Afternoon Session #l. Chairs: W.S. Cordua and M. Jirsa
Student Award Presentation, John Klasner
COFFEE BREAK - Remove Poster Papers
Afternoon Session #2. Chairs: J. SalioImski a d . C. Green
Pre-Trip #3 Seminar, CCI, Farwell Room, M.G. Mudrey, Jr. and B.A. Brown
SATURDAY, MAY 4,1991
M.G. Mudrey, Jr., and BA. Brown (Leaders), "Proterozoic volcanogenic massive sulfide deposits of northwestern Wisconsin" 8:00 a.m. Bus departs from front entrance of CCI
xii
L
L
L
L
L
L
L
L
PROGRAM - DAY 1, THURSDAY, MAY 2, 1991
Ii Inc Atil hors j 11 tie
8:20 AM Allen, David J. and WilliamJ. Hinze
The Wisconsin Gravity Minimum: Source & Implications
8:40 AM AJlen, David 1., William LHinze, and William F.Cannon
The Relationship of Topography and Gravity over the Lake SuperiorSwell: Evidence for a Keweenaw Hot Spot
9:00 AIvI Chandler, Val W. Aeromagnetic Surveying Program in Minnesota: Past and FuturePerspectives
9:20 AM Osterberg, Steven A. and IanR. Morrison
Physical Volcanology of the Footwall Rocks at the Winston LakeMassive Sulfide Deposit
9:40 AM Dickas, AJbert B. and MG.Mudrey, Jr.
Geology & Petrography of the Amnicon Pluton, Douglas County.Wisconsin
10:00 AM COFFEE BREAK10:20 AM Read, William F. The Neda Iron Formation -- A Product of Vulcanism?10:40 AM Anderson, Raymond It. The Amoco MG. Eischeid Oil Test: 4200 Meters of Clastic Rocks of
Midcontinent Rift System
11:00 AM Morey, G.B. and D.LSouthwick
Manganese Mineralization in Early Proterozoic Iron-Formation of theEmily District, Cuyuna Range, East-Central Minnesota
11:20 AM Bachman, R.L. The Homestake Mine: An Early Proterozoic Iron-Formation HostedGold Deposit
11:40 AM CONCLUDING REMARKS BY SESSION CHAIRS11:50 AM END OF MORNING SESSION
12 30 PM__J POSTER SESSION
1:40 PM Laberge, Gene L. Early Proterozoic Rocks in the Monico, Wisconsin Area: Implicationson the Wisconsin Magmatic Terranc
2:00 PM Hemming, S., GN. Hanson,S.M. McLennan, and W.D.Sharp
Isoclinal Slump Folds in the Lower Pokegama Ouartzite: Evidencefor Seismicity and Slope Instability During Deposition of theAnimikie Group
2:20 PM Craddock, John P. andAndrew Moshoian
Continuous Strike-Slip Fault-En Echelon Fracture Arrays inDeformed Archean Rocks: Implications for Fault PropagationMechanics
2:40 PM Jerde, Eric A Magmatic Evolution in the Midcontinent Rift: Evidence fromHypabyssal Rocks of the North Shore of Lake Superior
3:00 PM COFFEE BREAK3:20 PM Cambray, F. William and
Kazuya FujitaCollision Induced Ripoffs, Ancient and Modern: The MidcontinentRift System and the Red Sea-Gulf of Aden Compared
3:40 PM Cambray, F. William andGlenn Scott
A Late Keweenawan Thrust ? Marquette County, Michigan
4:00 PM 'Manson, M.L. and H.C.Halls
The Geology and Geophysics of Major Post-Keweenawan Faults inthe Eastern Lake Superior Region
4:20 PM Canibray, F. William andJoseph J. Maneuso
Detachment Faulting and the Origin of the Asymmetric DepositionalPattern of the Marquette Trough
4:40 PM CONCLUDING REMARKS BY SESSION CHAIRS4:50 AM END OF AFTERNOON SESSION
XIII
PROGRAM - DAY 1, THURSDAY, MAY 2,1991
8:40 AM "Allen, David J., William 1. The Relationship of Topography and Gravity over the Lake Superior Hinze. and William F. Swell: Evidence for a Keweenaw Hot SDOI I Cannon 1
9:00 AM 1 Chandler, Val W. 1 Acromagnetic Surveying Program in Minnesota: Past and Future
Dickas, Albert B. and M.G. Geology & Petrography of the Amnicon Pluton, Douglas County, Mudrev. J r I isc cons in . .
10:00 AM 1 1 COFFEE BREAK
10:20 AM 1 Read, William F. 1 The Neda Iron Formation - A Product of Vulcanism?
10:40 AM 1 Anderson, Raymond R 1 The Amoco M.G. Eischcid Oil Test: 4200 Meters of Clashc Rocks of 1 1 Midcontinent Rift System
11:00 AM I Morev. G.B. and D.L. 1 Mansanese Mineralization in Earlv Proterozoic Iron-Formation of the
on the Wisconsin Magmatic Terrane
200 PM Hemming, S., G.N. Hanson, Isoclinal Slump Folds in the Lower Pokegama Quartzite: Evidence S.M. McLennan, and W.D. for Seismicity and Slope Instability During Deposition of the Sharp Animikie Group
220 PM 'Craddock, John P. and Continuous Strike-Slip Fault-En Echelon Fracture Arrays in Andrew Moshoian Deformed Archean Rocks: Implications for Fault Propagation
Mechanics
240 PM Jerde , Eric A Magmatic Evolution in the Midcontinent Rift: Evidence from Hypabyssal Rocks of the North Shore of Lake Superior
300 PM COFFEE BREAK
3 2 0 PM Cambray, F. William and Collision Induced Ripoffs, Ancient and Modern: The Midcontinent Kazuya Fujita Rift System and the Red Sea-Gulf of Aden Compared
3 4 0 PM Camhray, F. William and A Late Keweenawan Thrust ? Marquette County, Michigan Glenn Scott
400 PM "Manson, M.L and H.C. The Geology and Geophysics of Major Post-Keweenawan Faults in Halls the Eastern Lake Superior Region
4:20 PM Cambray, F. William and Detachment Faulting and the Origin of the Asymmetric Depositional Joseph 1. Mancuso Pattern of the Marquetie Trough
4:40 PM CONCLUDING REMARKS BY SESSION CHAIRS
450 AM END OF AFTERNOON SESSION
xiii
PROGRAM - DAY 2, FRIDAY, MAY 3, 1991
'runt- Authors—
— Title
Student Contribution
xiv-J
J
A.M.: SYMPOSIUM ON MINERAL RESOURCES POTENTIAL OF NORTHERN WiSCONSIN
8:20 Mv! DeMatties, TA. and MG.Mudrey, Jr.
Geological Setting of the Early Proterozoic Base- and PreciousMetal-Rich Metavolcanic Belt of Wisconsin
8:40 AM DeMatties, TA. and W.F.Rowell
Bend, A Lower Proterozoic, Copper- and Gold-EnrichedVolcanogenic Massive-Sulfide Deposit in Taylor County, Wisconsin
9:00 AM Hanson, Jay C. Ground Geophysical Surveys Leading to the Bend Copper-GoldDiscovery
9:20 AM Kennedy, Lawrence P., TeresaA. Harding, John A Schaff,and Anne M. Zielinski
The Lynne Massive Sulfide Deposit, Oneida County, Wisconsin
9:40 AIM Thresher, Jr., J. Geology and Mineralogy of the Flambeau Deposit, Oneida Counts'.
Wisconsin
10:00 AM COFFEE BREAK
10:20 AM Mudrey, Jr., M.G. and BA.Brown
Platinum Group Element Potential of Keweenawan Intrusive Rockin Wisconsin
10:40 AM Brown, Bruce, A. Nonmetallic Mineral Resources and Minor Metals Potential ofNorthern Wisconsin
11:00AM Evans, Thomas I. Regulating Metallic Mineral Development in Wisconsin
11:20 AM CONCLUDING REMARKS BY SESSION CHAIRS
11:30 AM END OF MORNING SESSION
1230 PM POSTER SESSION
1:40 PM Klasner, John S. and P.K. Sims Thick-Skinned Backthrusting in the Felch-Calunict Troughs Region,Northern Michigan -- A comparison with the Southern Alps
2:00 PM Middleton, Michael D. A Preliminary Study of Rejuvenated Movement Along aPrecambrian Fault, St. Croix County, Wisconsin
2:20 PM Miller, J.D., Jr., J.B. Paces,and RE. Zartman
Geochronology of the Duluth Complex: A Progress Report
2:40 PM STUDENT AWARD PRESENTATION
3:00 PM COFFEE BREAK
3:20 PM Seifert, Karl E., Zell F.Peterman, and Scott E.Thieben
The Nature and Source of Midcontinent Rift Igneous Rocks
3:40 PM Windom, Kenneth E, W.R.Van Schmus, Karl E. Seifert,E.T. Wallin, and R.R.Anderson
Archean and Proterozoic Tectono-Magmatic Activity Along theSouthern Margin of the Superior Province in Northwestern Iowa,USA
4:00 PM Chan, Lung S. Paleomagnetism of Central Wisconsin Dike Swarm: Constraints onThermomechanical Model of Midcontinent Rift
4:20 PM CONCLUDING REMARKS BY SESSION CHAIRS
4:30 PM END OF AFTERNOON SESSION
J
J
jJ
Li
Li
I R - DAY 2, FRIDAY, MAY 3,1991
A.M.: SYMPOSIUM ON MINERAL RESOURCES POTENTIAL O F NORTHERN WISCONSIN
DeMatties, T.A and M.G. 1 Geological Setting of the Early Proterozoic Base- and Precious 8:20 AM 1 Mudrw. Jr. Metal-Rich Meiavolcanic Belt of Wisconsin
840 AM I DeMatties, T.A. and W.F. I Bend, A Lower Proterozoic, Copper- and Gold-Enriched Rowell 1 Volcanogenic Massive-Sulfide ~ e b i t in Taylor County, Wisconsin
900 AM 1 Hanson. Jav C. 1 Ground Geophysical Surveys Leading to the Bend Coppcr.Gold , . . . - . . Discovery
920 AM Kennedy, Lawrence P., Teresa The Lynne Massive Sulfide Deposit, Oneida County, Wisconsin A Hardina. John A. Schaff, 1 and ~ n n e k . Zielinski
940 AM 1 Thresher, Jr.. J. 1 Geology and Mineralogy of the Flambeau Deposit, Oneida County.
I . . 10:00 AM COFFEE BREAK
la20 AM 1 Mudrey, Jr., M.G. and B.A. 1 Platinum Group Element Potential of Keweenawan Intrusive Rock 1 Brown 1 in Wisconsin
la40 AM Brown, Bruce, A Nonmetallic Mineral Resources and Minor Metals Potential of Northern Wisconsin
11:00 AM Evans, Thomas J. Regulating Metallic Mineral Development in Wisconsin
11:20 AM CONCLUDING REMARKS BY SESSION CHAIRS
11:TOAM I END OF MORNING SESSION . . . . . . - . -~ - - - -~ - ~
1230 PM POSTER SESSION
1:40 PM Klasner. John S. and P.K. Sims I Thick-skinned back thrust in^ in the Fclch-Calumcl Trouch* Ki;Wn, Northern Michigan - A comparison with the Southern Alps
2:00 PM Middleton, Michael D. A Preliminary Study of Rejuvenated Movement Along a Precambrian Fault. St. Croix Counlv. Wisconsin
2:20 PM 1 Miller, J.D., Jr., J.B. Paces, I Geochronolo&y of the Duluth Complex: A Progress Report and R.E. Zartman
240 PM STUDENT AWARD PRESENTATION
300 PM 1 COFFEE BREAK
320 pM I Seifert, Karl E., Zell E. I The Nature and Source of Midcontinent Rift Igneous Rocks Peterman, and ScoU E. Thieben
340 PM Windom, Kenneth E., W.R. Archean and Proterozoic Tectono-Magmatic Activity Along the Van Schmus, Karl E. Seifert, Southern Margin of the Superior Province in Northwestern Iowa, E.T. Wallin. and R.R. USA Anderson
4:00 PM Chan, Lung S. Palmmagnetism of Central Wisconsin Dike Swarm: Constraints on Thermomechanical Model of Midcontinent Rift
'Student Contribution
xiv
POSTER SESSION
DAY 1, THURSDAY, MAY 2, 1991,12:30 PM - 1:30 PM, ROOMS D & E, RW CWIC CENTER
Brehm, Daniel J., C. Patrick Ervin,M.G. Mudrey, Jr., B.A. Brown, &M. L. Czechanski
Extension of Gravity Filed Coverage in East-CentralWisconsin: 1990 COGEOMAP Program
Brown, B.A. and R.S. Maass Preliminary Bedrock Geology Map of Eau Claire County,Wisconsin
*Dahl, Linda J. and Susan E. Brink Preliminary Drill Core Study of Two Holes Drilled on theCuyuna Iron Range and Emily Manganiferous Iron FormationDistrict of Minnesota
*Mariano, J. and W.J. Hinze Geophysical Investigations of the Midcontinent Rift inEastern Lake Superior Using Variable MagnetizationModeling
Nicholson, SW., Ki. Schulz, W.F.Cannon, and L.G. Woodruff
The Porcupine Mountains Area, Michigan - a KeweenawanCentral Volcano?
Reichhoff, J.A., S.A. Hauck, and DL.Southwick
Lithogeochemistry and Geological Mapping in the VermilionGreenstone Bely, Minnesota, as an Aid to MineralExploration
Severson, Mark 3. and Steven A.Hauck
Correlation of Igneous Units at the Minnamax Deposit. NEMinnesota
Teskey, D. Three-Dimensional Modelling of the Magnetic Anomaly -Central Lake Superior
Turnock, AC., D.C. Kamineni, andR. McGregor
Chemistry and Metamorphism of an Archean Pillow Basalt
Welsh, James L. and Jayne Reichhoff Geochemistry of Archean Rocks form the Virginia Horn area:Preliminary Interpretations
Witthuhn, Kathleen Stress Analysis of the Midcontinent Rift in Michigan andMinnesota
Zanko, L., A. Gokee, B. Dewey, S.Hauck, and J. Pastor
Geostatistical and GIS Evaluation of Biochemical andEcological Data From Three Mineralized Sites (Au & Cu-Ni-PGE), Northeastern Minnesota: Implications for MineralExploration in a Boreal Forest
Student Contribution
xv
POSTER SESSION
DAY 1, THURSDAY, MAY 2,1991, 12:30 PM - 1:30 PM, ROOMS D & E, RW CIVIC CENTER
M.G. Mudrey, Jr., B.A. Brown, & Wisconsin: 1990 COGEOMAP Program M.L. Czechanski
*Dahl, Linda J. and Susan E. Brink Preliminary Drill Core Study of Two Holes Drilled on the Cuyuna Iron Range and Emily Manganiferous Iron Formation District of Minnesota
I 'Mariano, J. and W.J. Hinze Geophysical Investigations of the Midcontinent Rift in Eastern Lake Superior Using Variable Magnetization
1 Modeling 11 Nicholson, S.W., K.J. Schulz, W.F. The Porcupine Mountains Area, Michigan - a Keweenawan Cannon, and L.G. Woodruff Central Volcano?
Reichhoff, J.A., S.A. Hauck, and D.L. Lilhogwchemistry and Geological Mapping in the Vermilion Southwick Greenstone Bely, Minnesota, as an Aid to Mineral
Exploration I Severson, Mark J. and Steven A. Correlation of Igneous Units at the Minnamax Deposit, NE Hauck Minnesota
Teskey, D. Three-Dimensional Modelling of the Magnetic Anomaly - Central Lake Superior
Turnock, A.C., D.C. Kamineni, and Chemistry and Metamorphism of an Archean Pillow Basalt R. McGregor
Welsh, James L. and Jayne Reichhoff Geochemistry of Archean Rocks form the Virginia Horn area: 1 Preliminary Interpretations
I1 'Witthuhn, Kathleen
Zanko, L., A. Gokee, B. Dewey, S. Hauck, and J. Pastor
Stress Analysis of the Midcontinent Rift in Michigan and Minnesota
Geostatistical and GIS Evaluation of Biochemical and Ecological Data From Three Mineralized Sites (Au & Cu-Ni- PGEt. Northeastern Minnesota: Irn~lications for Mineral ~ x ~ l & a t i o n in a Boreal Forest
*Student Contribution
POSTER SESSION
DAY 2, FRIDAY, MAY 3, 199112:30 PM-1:30 PM, ROOMS D & E, RW CIVIC CENTER
Campbell, Frederick K. The Feasibility of Recovering Metals From a HazardousWaste Site; A Brief Case Study
Geerts, Stephen D. Geology of Platinum Group Element-Enriched Horizonswithin the Dunka Road Copper-Nickel Prospect, St. LouisCounty, Minnesota
Green, John C., Ed. Venzke,and Tom Lawler
Drift Pebble Lithology of the Tomahawk Road Area, LakeCounty, Minnesota, Used to Help Infer Local Bedrock
Heine, John J., Tom A. Toth,Steven A. Hauck, andGeorge W. Shurr
Geology of the Meridian Aggregates Quany and theSurrounding Area, St. Cloud, Minnesota: A Study of theBedrock Influences on the Pre-Late Cretaceous WeatheringProfile
Hinz, Peter Industrial Minerals of Northwestern Ontario
Hinz, Peter Exploration and Mining Activity in Northwestern Ontario
TMJohnson, James S., Denise A.Stavish, Mary K. Tozer, andGeorge W. Schurr
Surface Expression of Major Bedrock Structural Features
Morrison, Ian R. Pick Lake Zinc-Copper-Silver Deposit
Thompson, M.D., L.D.McGinnis, MG. Mudrey, Jr.,and CA'. Ervin
Midcontinent Rift Structure Interpreted From theGNtJArgonne Seismic Data Set
Toth, Thomas A., John 3.Heine, & Steven A. 1-lauck
Regional Stratigraphic Model of Late Cretaceous Sedimentsand their Relationship with the Underlying Pre-LateCretaceous Weathering Profile along the Minnesota RiverValley, Minnesota
Woodzick, Thomas L., GaryP. Murdock, and Douglas E.Pride
A Study of Thematic Mapper Lineaments in NorthwestNevada
J
U
1
J
*Student Contribution
U
xvi
J
U
J
POSTER SESSION
DAY 2, FRIDAY, MAY 3 , 1991 1 2 3 0 PM-1:30 PM, ROOMS D & E, RW C M C CENTER
Campbell, Frederick K.
Geeris, Stephen D.
Green, John C., Ed. Venzke, and Tom Lawler
Heine, John J., Tom A. Toth, Steven A. Hauck, and George W. Shurr
Hinz, Peter
Hinz, Pete1
'Johnson, James S., Denise A. Slavish, Mary K. Tozer, and George W. Schurr
Morrison, Ian R.
Thompson, M.D., L.D. McGinnis, M.G. Mudrey, Jr., and C.P. Ervin
Toth. Thomas A.. John I. Heine, & Steven A. Hauck
Woodzick, Thomas L., Gary P. Murdock, and Douglas E. Pride
"'Student Contribution
The Feasibility of Recovering Metals From a Hazardous Waste Site; A Brief Case Study
Geology of Platinum Group Element-Enriched Horizons within the Dunka Road Copper-Nickel Prospect, St. Louis County, Minnesota
Drift Pebble Lithology of the Tomahawk Road Area, Lake County, Minnesota, Used to Help Infer Local Bedrock
Gwlogy of the Meridian Aggregates Quarry and the Surrounding Area, St. Cloud, Minnesota: A Study of the Bedrock Influences on the Pre-Late Creiaceous Weathering Profile
Industrial Minerals of Northwestern Ontario
Exploration and Mining Activity in Northwestern Ontario
Surface Expression of Major Bedrock Structural Features
Pick Lake Zinc-Copper-Silver Deposit
Midcontinent Rift Structure Interpreted From the GNIIArgonne Seismic Data Set
Regional Stratigraphic Model of Late Cretaceous Sediments and their Relationship with the Underlying Pre-Late Cretaceous Weathering Profile along the Minnesota River Valley, Minnesota
A Study of Thematic Mapper Lineaments in Northwest Nevada
L
L
C
[L
L
[ABSTRACTS
xvii
ABSTRACTS
The Wisconsin Gravity Minimum: Source and Implications
David J. Allen and William J. Hinze, Department of Earth and Atmospheric Sciences,Purdue University, West Lafayette, Indiana
The Wisconsin Gravity Minimum (Fig. 1) attains its most negative values ofapproximately -95 milliGals over the outcrop of the 1.5 Ga Wolf River Batholith, but extendsdiscordantly across the Archean Marshfield Terrane, the 1.9-1.8 Ga Penokean Pembine-Wausau Terrane, and the 1.76 Ga anorogenic rhyolite-epizonal granite terrane of southernWisconsin.
The edges of the minimum are characterized by sharp gradients indicating that thesource of the anomaly lies at shallow depths. The north-northeast trend of the anomaly isdiscordant with the east-west Penokean structural grain in Wisconsin, suggesting that thesource of the anomaly may be younger than 1.8 Ga. Thus, we propose that the entireWisconsin Gravity Minimum is produced by the Wolf River Batholith. Although the batholithoutcrops only in northeast Wisconsin, the anomaly suggests that it lies buried beneath a thincover of older Precambrian roof rocks and early Paleozoic sedimentary rocks throughout asubstantial portion of Wisconsin.
Forward modeling of gravity data along several profiles across the Wisconsin GravityMinimum supports the buried Wolf River Batholith hypothesis. South and west of the itsoutcrop, the Wolf River Batholith may be buried beneath approximately 4 km of olderPrecambrian rock, and the batholith extends to depths between 10 and 15 km (Allen, 1990).Figure 2 is an interpreted geologic cross-section based on a gravity model along anorthwest-southeast profile (Fig. 1). In northern Wisconsin, the negative gravity anomalyis produced by the density contrast between the granitic rocks of the Wolf River Batholithand the denser Penokean and Archean rocks. In southern Wisconsin, however, the 1.76 Gaanorogenic rhyolites-epizonal granites and Baraboo quartzites have densities similar to thatof the Wolf River Batholith; thus, the source of the gravity anomaly in southern Wisconsin isthe density contrast between the Wolf River Batholith and the denser, pre-l.76 Ga rockswhich underlie the anorogenic granites, rhyolites, and quartzites. The lower crustal structureshown in Figure 2 was obtained by projecting a crustal model based on a combined seismicreflection-gravity interpretation along GLIMPCE line H in Lake Michigan (Cannon et al.,1991) west along geophysical-anomaly strike into Wisconsin. As shown in Figure 2, thelower crustal structure does not significantly contribute to the Wisconsin Gravity Minimum.
The emplacement of the Wolf River Batholith may have been controlled by pre-existing structures. For example, the Eau Pleine Shear Zone, a 1.8 Ga suture between thePembine-Wausau Terrane and the Marshfield Terrane, has been identified beneath LakeMichigan (Cannon et al., 1991) and projects through the most negative portion of theWisconsin Gravity Minimum where the Wolf River Batholith is thickest (Fig. 2). The shearzone may have acted as a conduit along which magma was preferentially transported at 1.5Ga. Also, the Wolf River Batholith outcrops north of the shear zone, but is buried to thesouth. The northern portion of the batholith may have been uplifted relative to the southernportion at a later time during shear zone reactivation. Alternatively, the batholith originallymay have been emplaced to shallower levels in the Pembine-Wausau Terrane than in theMarshfield Terrane.
The southeast margin of the Wisconsin Gravity Minimum closely approximates a
The Wisconsin Gravity Minimum: Source and Im~lications
''
David J. Allen and William J. Hinze, D Purdue University, West Lafayette, Indiana
The Wisconsin Gravity Minimum (Fig. 1) attains its most negative values of approximately -95 milliGals over the outcrop of the 1.5 Ga Wolf River Batholith, but extends discordantly across the Archean Marshfield Terrane. the 1.9-1.8 Ga Penokean Pembine- Wausau Terrane, and the 1.76 Ga anorogenic rhyolite-epizonal granite terrane of southern Wisconsin.
The edges of the minimum are characterized by sharp gradients indicating that the source of the anomaly lies at shallow depths. The north-northeast trend of the anomaly is discordant with the east-west Penokean structural grain in Wisconsin, suggesting that the source of the anomaly may be younger than 1.8 Ga. Thus, we propose that the entire Wisconsin Gravity Minimum is produced by the Wolf River Batholith. Although the batholith outcrops only in northeast Wisconsin, the anomaly suggests that it lies buried beneath a thin cover of older Precambrian roof rocks and early Paleozoic sedimentary rocks throughout a substantial portion of Wisconsin.
Forward modeling of gravity data along several profiles across the Wisconsin Gravity Minimum supports the buried Wolf River Batholith hypothesis. South and west of the its outcrop, the Wolf River Batholith may be buried beneath approximately 4 km of older Precambrian rock, and the batholith extends to depths between 10 and 15 km (Allen, 1990). Figure 2 is an interpreted geologic cross-section based on a gravity model along a northwest-southeast profile (Fig. 1). In northern Wisconsin, the negative gravity anomaly is produced by the density contrast between the granitic rocks of the Wolf River Batholith and the denser Penokean and Archean rocks. In southern Wisconsin, however, the 1.76 Ga iinorogenic rhyolites-epizonal granites and Baraboo quartzites have densities similar to that of the Wolf River Batholith; thus, the source of the gravity anomaly in southern Wisconsin is the density contrast between the Wolf River Batholith and the denser, pre-1.76 Ga rocks which underlie the anorogenic granites, rhyolites, and quartzites. The lower crustal structure shown in Figure 2 was obtained by projecting a crustal model based on a combined seismic reflection-gravity interpretation along GLIMPCE line H in Lake Michigan (Cannon et al., 1991) west along geophysical-anomaly strike into Wisconsin. As shown in Figure 2, the lower crustal structure does not significantly contribute to the Wisconsin Gravity Minimum.
The emplacement of the Wolf River Batholith may have been controlled by pre- existing structures. For example, the Eau Pleine Shear Zone, a 1.8 Ga suture between the Pembine-Wausau Terrane and the Marshfield Terrane, has been identified beneath Lake Michigan (Cannon et al., 1991) and projects through the most negative portion of the Wisconsin Gravity Minimum where the Wolf River Batholith is thickest (Fig. 2). The shear zone may have acted as a conduit along which magma was preferentially transported at 1.5 Ga. Also, the Wolf River Batholith outcrops north of the shear zone, but is buried to the south. The northern portion of the batholith may have been uplifted relative to the southern portion at a later time during shear zone reactivation. Alternatively, the batholith originally may have been emplaced to shallower levels in the Pembine-Wausau Terrane than in the Marshfield Terrane.
The southeast margin of the Wisconsin Gravity Minimum closely approximates a I
60
50
40
30
20
10
0
—10
—20
—30
—40
—50
—60
—70
—80
—90
—100
prominent magnetic boundary which separates a positive anomaly to the northwest from anegative anomaly to the southeast (Fig. 2). This magnetic boundary, herein called theWinnebago magnetic anomaly, continues northeast beyond the Wisconsin Gravity Minimum,suggesting that it is not the magnetic signature of the edge of the batholith, but insteadreflects a pre-1.5 Ga structure which controlled the location of the southeast margin of theWolf River Batholith.
The general correlation of the Wisconsin Gravity Minimum with the Wisconsin Archsuggests a possible genetic relationship. Since the Wolf River Batholith is less dense thanthe surrounding rocks, it may have been intermittently uplifted over the past 1.5 Ga inresponse to buoyancy forces.
-.4
-j
jJ
J
JF-- I
Scale Kilometers jFigure 1: Bouguer gravity anomaly map of the Wisconsin area based on four kilometergridded data. The line indicates the profile shown in Figure 2.
J
U
0 50 100 150 200 250
2
prominent magnetic boundary which separates a positive anomaly to the northwest from a negative anomaly to the southeast (Fig. 2). This magnetic boundary, herein called the Winnebago magnetic anomaly, continues northeast beyond the Wisconsin Gravity Minimum, suggesting that it is not the magnetic signature of the edge of the batholith, but instead reflects a pre-1.5 Ga structure which controlled the location of the southeast margin of the Wolf River Batholith.
The general correlation of the Wisconsin Gravity Minimum with the Wisconsin Arch suggests a possible genetic relationship. Since the Wolf River Batholith is less dense than the surrounding rocks, it may have been intermittently uplifted over the past 1.5 Ga in response to buoyancy forces.
0 50 100 150 200 250 I - -
1 : Kilometers
Figure 1: Bouguer gravity anomaly map of the Wisconsin area based on four kilometer gridded data. The line indicates the profile shown in Figure 2.
—as
0NW SE2 0
-0 0LI) Mi -. - a —_J . In_C . . .o •. .-. .
cn'0 i—. --. o>- ;-.. -.- .-- OR
— Lrrzcr Or LOWZA CAUSTMaGNETIC £NOSAL.Y
I I I '
0 50 100 ISO 200 250 300 350 400DISTANCE (KM)
WAUSAU WAUPACA APRENTICE
—
£1CRANITC SILW7U*LE
J I -,
+ I . • •_• -. 'vj7 gfJ 3açnQrsn\+ _ANOROCtN1C FELSIC ROCKS..r
_______ __________________
- -10\ ' L__'__JN LAG ABA J I I ' — 20
/ / — , — I' PEMBINE-WAtJSAU TERRA.•1
N N N N NI ICLEINE / / MARS1ELDTERRANE , / / / / /FAULT
1SHEARZONE N " N N N N N 'N \ S. \ S. ..SUPERIOR,,I,...,I .1 •I / , / / — — / — / / / — / / ,CRATON ..I..("S. S. N N N N N N S. N N N \ N \
• , / / ' ' ' / , ' — , — — -40' N N
____________________________
• ' UPPER M&E . -• • •M... . • • • . - •
50
Figure 2: Bouguer gravity and total intensity magnetic anomaly profiles and related geologicmodel showing major crustal units and terranes. The dashed line indicates the gravitationaleffect of density variations beneath 20km. See Figure 1 for the location of the profile.
REFERENCES CITEDAllen, D. J., 1990, The Wisconsin gravity minimum: source and implications. unpub. M. S.
Thesis, Purdue Univ., W. Lafayette, IN, 183 p.Cannon, W. F., Lee, M. W., Hinze, W. J., Schulz, K. J., and Green, A. C., 1991, Deep crustal
structure of the Precambrian basement beneath northern Lake Michigan, midcontinentNorth America: Geology 19, 207-2 10.
. . . . . . . . . . . . -0 . . . . . . ..-.. . .
o m -. . . . . ..... . . . . . . . . . . . . . . . ......... - : . ...... . . . . . . . a ; . : : . . ...... . . . . . .. . . 0 . :: : . . . . . . . 3 : . . . . . . .: . ...:.. . . . ;. : '. . . . . . . -0 - . :. . . . . . . ..:.
-. ...... - - - - - - - - - - - - - - -
- --- t r r c c ~ o r L O W ~ ~ m u s t ........ M A G N E T I C A N O M A L Y
0 SO 100 150 200 250 300 350
DISTANCE (KM)
Fiaure 2: Bouguer gravity and total intensity magnetic anomaly profiles and related geologic model showing major crustal units and terranes. The dashed line indicates the gravitational effect of density variations beneath 20 km. See Figure 1 for the location of the profile.
REFERENCES CITED
Allen, D. J., 1990, The Wisconsin gravity minimum: source and implications, unpub. M. S. Thesis, Purdue Univ., W. Lafayette, IN, 183 p.
Cannon, W. F., Lee, M. W., Hinze. W. J., Schulz, K. J., and Green, A. G., 1991. Deep crustal structure of the Precambrian basement beneath northern Lake Michigan, midcontinent North America: Geology 19,207-210.
4
The Relationship of Topography and Gravity over the Lake Superior Swell:Evidence for a Keweenaw Hot Spot?
David J. Allen, William J. Hinze, Department of Earth and Atmospheric Sciences,Purdue University, West Lafayette, IN, and William F. Cannon, USGS, Reston, VA
The Lake Superior Swell (Dutch, 1981) is a broad topographic dome -surrounding the Lake Superior Basin, the location of the most intense 1.1 GaKeweenawan rifting activity. Centered on Lake Superior is a radial drainage patternof approximately 1000 km diameter. Within this area, streams flow radiallyoutward from the relatively small inward-directed drainage system of the LakeSuperior Basin (Figure 1). In addition, recent analysis of the regional gravity andtopographic data indicates that a broad negative gravity anomaly is also centered onLake Superior and is related to isostatic compensation of the Lake Superior Swell.
The regional negative anomaly is defined by determining average gravity as afunction of radial distance from a central point in Lake Superior (Figure 1). Asshown in Figure 2, the average gravity decreases, in an approximately linearmanner, from a radial distance of 650 km to 300 km. Within 300 km, the gravityanomalies produced by the Midcontinent Rift obscure the regional anomaly, whilebeyond 650 km, the average gravity is influenced by remote features unrelated to theLake Superior Swell. Analysis of topographic data indicates that the averagetopography varies in an inverse manner to the gravity (Figure 2). Furthermore, therelationship between average gravity and average topography (Figure 3) suggeststhat the Lake Superior Swell is in approximate isostatic equilibrium (for a fullycompensated topographic load of density 2.67 g/cm3, slope = -112 mCals/km).
Recently, it has been proposed that a mantle plume (the Keweenaw hot spot)is related to the origin of the 1.1 Ga rifting and igneous event which led to the largevolume of basalts and the short time span of their eruption (Hutchinson et al., 1990;Nicholson and Shirey, 1990). We propose that the drainage pattern, the topographicdome, and the negative gravity anomaly may still reflect the short-lived Keweenawhot spot.
The persistence of the topographic dome must be isostatically related to afundamental change imprinted on the lithosphere by the mantle plume. Forexample, the Lake Superior Swell may be supported isostatically by a crust which hasbeen thickened by the addition of Keweenawan basaltic rocks. Beneath the LakeSuperior Basin, for example, the crust thickens by as much as 15 km. The negativegravity anomaly, however, extends well beyond the Lake Superior Basin, suggestingthat the crust may have been thickened (to a lesser degree) over a much broaderarea, well beyond the outcrop of the rift rocks. The observed gravity anomaly maybe accounted for by a gradual thickening of the crust (of approximate magnitude 1km) from 650 km to 300 km radial distance (Allen, 1990).
The source of the gravity anomaly, alternatively, may be a density contrast inthe mantle related to the extraction of the Keweenawan basalts. The depleted uppermantle rocks are expected to be less dense (by at most 0.10 g/cm3) than undepletedrocks due to the removal of the dense phase garnet and/or an increase in theMgO/FeO ratio (Boyd and McCallister, 1976; Oxburgh and Parmentier, 1977). The
David J. Allen, William J. Hinze, Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN, and William F. Cannon, USGS, Reston, VA
I The Lake Superior Swell (Dutch, 1981) is a broad topographic dome
surrounding the Lake Superior Basin, the location of the most intense 1.1 Ga Keweenawan rifting activity. Centered on Lake Superior is a radial drainage pattern of approximately 1000 km diameter. Within this area, streams flow radially outward from the relatively small inward-directed drainage system of the Lake Superior Basin (Figure 1). In addition, recent analysis of the regional gravity and topographic data indicates that a broad negative gravity anomaly is also centered on Lake Superior and is related to isostatic compensation of the Lake Superior Swell.
The regional negative anomaly is defined by determining average gravity as a function of radial distance from a central point in Lake Superior (Figure 1). As shown in Figure 2, the average gravity decreases, in an approximately linear manner, from a radial distance of 650 km to 300 km. Within 300 km, the gravity anomalies produced by the Midcontinent Rift obscure the regional anomaly, while beyond 650 km, the average gravity is influenced by remote features unrelated to the Lake Superior Swell. Analysis of topographic data indicates that the average topography varies in an inverse manner to the gravity (Figure 2). Furthermore, the relationship between average gravity and average topography (Figure 3) suggests that the Lake Superior Swell is in approximate isostatic equilibrium (for a fully compensated topographic load of density 2.67 g/cm3, slope = -112 mGals/km).
Recently, it has been proposed that a mantle plume (the Keweenaw hot spot) is related to the origin of the 1.1 Ga rifting and igneous event which led to the large volume of basalts and the short time span of their eruption (Hutchinson et al., 1990; Nicholson and Shirey, 1990). We propose that the drainage pattern, the topographic dome, and the negative gravity anomaly may still reflect the short-lived Keweenaw hot spot.
The persistence of the topographic dome must be isostatically related to a fundamental change imprinted on the lithosphere by the mantle plume. For example, the Lake Superior Swell may be supported isostatically by a aus t which has been thickened by the addition of Keweenawan basaltic rocks. Beneath the Lake Superior Basin, for example, the crust thickens by as much as 15 km. The negative gravity anomaly, however, extends well beyond the Lake Superior Basin, suggesting that the crust may have been thickened (to a lesser degree) over a much broader area, well beyond the outcrop of the rift rocks. The observed gravity anomaly may be accounted for by a gradual thickening of the aust (of approximate magnitude 1 km) from 650 km to 300 km radial distance (Allen, 1990).
The source of the gravity anomaly, alternatively, may be a density contrast'in the mantle related to the extraction of the Keweenawan basalts. The depleted upper mantle rocks are expected to be less dense (by at most 0.10 g/cm3) than undepleted rocks due to the removal of the dense phase garnet and/or an increase in the MgO/FeO ratio (Boyd and McCallister, 1976; Oxburgh and Parmentier, 1977). The
negative gravity anomaly may be accounted for by an upper mantle which becomesincreasingly depleted toward the center of the Lake Superior Swell. Again, thissuggests that the upper mantle was disturbed over a region much broader than theoutcrop of the rift rocks.
Figure 1: Map of part of central North America showing major drainage systems.Arrows denote direction of stream flow. Shaded areas are covered by Phanerozoicplatform sedimentary rocks; unshaded areas are exposed Precambrian rocks of theCanadian Shield. Heavy dashed line shows approximate axis of the MidcontinentRift (MCR). Short dashed line is outline of present Lake Superior drainage basin.Circle is 1000 km diameter. Cross in Lake Superior indicates position of centralpoint used in the radial averaging analysis.
100 75
40 4075
5
negative gravity anomaly may be accounted for by an upper mantle which becomes increasingly depleted toward the center of the Lake Superior Swell. Again, this suggests that the upper mantle was disturbed over a region much broader than the outcrop of the rift rocks.
Figure 1: Map of part of central North America showing major drainage systems. Arrows denote direction of stream flow. Shaded areas are covered by Phanerozoic platform sedimentary rocks; unshaded areas are exposed Precambrian rocks of the Canadian Shield. Heavy dashed line shows approximate axis of the Midcontinent Rift (MCR). Short dashed line is outline of present Lake Superior drainage basin. Circle is 1000 km diameter. Cross in Lake Superior indicates position of central point used in the radial averaging analysis.
REFERENCES CITEDAllen, D. J., 1990, The Wisconsin gravity minimum: source and implications, Junpub. M. S. Thesis, Purdue Univ., W. Lafayette, IN, 183 p.Boyd, F. R., and McCallister, R. H., 1976, Densities of fertile and sterile garnet
peridotites: Geophys. Res. Lett. 3, 509-512.Dutch, S. L, 1981, Isostasy, epeirogeny, and the highland rim of Lake Superior: Z.
Geomorph. N. F., Suppl.-Bd. 40, 27-41.Hutchinson, D. R., White, R. S., Cannon, W. F., and Schulz, K. J., 1990, Keweenaw
hot spot: geophysical evidence for a 1.1 Ga mantle plume beneath theMidcontinent Rift System: J. Geophys. Res. 95(7), 10869-10884.
Nicholson, S. W., and Shirey, S. B., 1990, Midcontinent Rift volcanism in the LakeSuperior region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin:J. Geophys. Res. 95(7), 10851-10868.
Oxburgh, E. R., and Parmentier, E. M., 1977, Compositional and density stratificationin oceanic lithosphere - causes and consequences: J. Geol. Soc. London 133,343-355.
AVERAGE GRAVITY AND AVERAGE TOPOGRAPHYVERSUS RADIAL DISTANCE
CENTRAL POINT AT 47.97 p4, 88.39W00
0—ILI-J
0p1 ->-I
>'CCrC
0In
I I• I I
)C
x I
I I
I I
xxI IC1
IC I Ix
I I
. I •I S
x S SS •I •
•GRI ITt IS'<CLEVATION I I
__________________________
-1-
0•o C/)1 Cr
UiLii
0>-oC.
'CCr000000- I-
AVERAGE GRAVITY VERSUS AVERAGE TOPOGRAPHYFOR RADIAL DISTANCES FROM 300 KM TO 650 KM
SLOPE —114 MGALS/KM. YIWT —9 MOALS. R= —0,963
'CC
>-
>'C',Cr 4o'
0
-4
jjJ
J
jj
.
0100 200 300 400 500 600 700 800
RADIAL DISTANCE (NM)
Figure 2: Average gravity and averagetopography vs. radial distance from thecentral point indicated in Figure 1.
00 250 360 3TOPOGRAPHY (M)
Figure 3: Average gravity vs. averagetopography for radial distance between300 km and 650 km.
jLi
6
AVERAGE GRAVITY AND AVERAGE TOPOGRAPHY VERSUS RADIAL DISTANCE
CENTRAL POINT AT 47.97 N. 8 8 . 3 9 W
Figure 2: Average gravity and average topography vs. radial distance from the central point indicated in Figure 1.
AVfRAGf GRAVITY VERSUS AVERAGE TOPOGRAPHY FOR RADIAL DISTANCES FROM 3 0 0 KM TO 6 5 0 KU
SLOPE= -114 UCALS/KU. Y 1 N k - 9 MGALS. R = - 0 . 9 6 3
' 0 2 5 0 3 0 0 TOPOGRAPHY (M)
Figure 3: Average gravity vs. average topography for radial distance between 300 km and 650 km.
REFERENCES CITED Allen, D. J., 1990, The Wisconsin gravity minimum: source and implications,
unpub. M. S . Thesis, Purdue Univ., W. Lafayette, IN, 183 p. Boyd, F. R., and McCallister, R. H., 1976, Densities of fertile and sterile garnet
peridotites: Geophys. Res. Lett. 3,509-512. Dutch, S. I., 1981, Isostasy, epeirogeny, and the highland rim of Lake Superior: 2.
Geomorph. N. F., Supp1.-Bd. 40,2741. Hutchinson, D. R., White, R. S., Cannon, W. F., and Schulz, K. J., 1990, Keweenaw
hot spot: geophysical evidence for a 1.1 Ga mantle plume beneath the Midcontinent Rift System: J. Geophys. Res. 95(7), 10869-10884.
Nicholson, S. W., and Shirey, S. B., 1990, Midcontinent Rift volcanism in the Lake Superior region: Sr, Nd, and Pb isotopic evidence for a mantle plume origin: J. Geophys. Res. 95(7), 10851-10868.
Oxburgh, E. R., and Parmentier, E. M., 1977, Compositional and density stratification in oceanic lithosphere - causes and consequences: J. Geol. Soc. London 133, 343-355.
THE AMOCO M.G. EISCHEID OIL TEST:4200 METERS OF CLASTIC ROCKS OF MIDCONTINENT RIFT SYSTEM
Raymond R. AndersonIowa Department of Natural Resources
Geological Survey Bureauiowa City, Iowa 52242
The M.G. Eischeid #1 deep petroleum test well was drilled by Amoco ProductionCompany between fipril and October, 1987, near the town of Flalbur (Carroll County) inwest-central Iowa. The well reached a total depth of 5355 m (17,851 feet) including 235feet of glacial drift, 130 feet of Cretaceous Dakota Formation, 2392 feet of Paleozoicstrata including Pennsylvanian through Cambrian units dominated by carbonates, 813feet of an unidentified sandstone unit ("Unit H"), and 14,085 feet of "Red Clastics"assigned to the Keweenawan Supergroup, before reaching t.d. 151 feet into a mafic dikein the crystalline basement. The well was primarily drilled by rock bit, with duplicatesamples sets presently reposited at the geological surveys of Iowa and Nebraska. Inaddition to the cuttings, five short cores were taken (see Fig. 1 for intervals cored)totaling about 66 feet. Splits are reposited at the Iowa Geological Survey Bureau.
A series of preliminary investigations of Eischeid rock samples and geophysicallogs were conducted shortly after the release of the materials. These investigationsincluded stratigraphic and petrographic studies, analysis of source rock potential,porosity studies, clay mineralogy analysis, fluid inclusion studies, integrated isotopic andfluid inclusion analysis of vein-filling minerals, and the determination of an age for thebasal dike. The results of these studies were published by the Iowa Geological SurveyBureau (Anderson, 1990a).
Of particular interest in the Eischeid well were the 14,000 feet of Keweenawan"Red Clastic" strata, one of the thickest clastic sections in the Midcontinent Rift system(MRS) strata ever examined. The six formations initially described and assigned to theKeweenawan Supergroup by Witzke (1990) constitute two groups temporarily named the"Upper" and "Lower Red Clastic Sequences" (Figure 1).
Unit H"Unit H" displays a coarser average grain size and is less well-cemented than the
rocks of underlying "Upper Red Clastic Sequence", and is more arkosic than the quartzarenites of the overlying Mt. Simon Fm. The unit was discussed by McKay (1990) whodescribed it as being dominated by two primary lithologies, a fine to very coarse arkosicsandstone and a micaceous, shaley to sandy siltstone. McKay reported trace amounts ofrnicrobrecciated lithic fragments in "Unit H", similar to fragments observed in all units ofthe "Red Clastic Sequences", and interpreted the fragments as the products of structuralmovements along fault zones that pervasively cut the MRS sequence. Grains withsimilar lithologic fabrics are not observed in the Mt. Simon interval of the well and havenot been reported in similar age rocks in the region. Consequently these rocks arethought to be of Proterozoic age and the may be associated with the "Upper Red ClasticSequence".
"Upper Red Clastic Sequence"The "Upper Red Clastic Sequence" is composed of three informal units totalling
6895 feet, and occupies the same stratigraphic position as the Bayfield Group ofWisconsin with which it is correlated. Both groups are dominated by fluvial deposition,but display different compositional characteristics. The Bayfield is more mature thanthe underlying Oronto Group, whereas Ludvigson and others (1990) reported that the"Upper Red Clastic Sequence" is less mature than its underlying "Lower Red ClasticSequence". An upward increase in the volcanic rock fragment component of "Upper Red
7
THE AMOCO M.G. EISCHEID OIL TEST: 1200 METERS OF CLASTIC ROCKS OF MIDCONTINENT RIFT SYSTEM
Raymond R. Anderson , :, . - Iowa Denartment of Natural Resources i.
. . . ',. ~:..'- ,.::. , > Geological Survey Bureau . . .. ... . , ., . Iowa City, Iowa 52242 , . . . ~. . - .
rhe M.G. Eischeid #1 deep petroleum test well was drilled bv Amoco Production Company between A ril and October, 1987, near the town of Halbu? (Carroll County) in ¥ west-central Iowa. e well reached a total depth of 5355 m (17,851 feet) including 235 feet of dacial drift. 130 feet of Cretaceous Dakota Formation. 2392 feet of Paleozoic
intervals cored)
basal dike. Bureau (Anderson, 1990a).
Of particular interest in the Eischeid well were the 14,000 feet of Keweenawan "Red Clastic" strata, one of the thickest clastic sections in the Midcontinent Rift system (MRS) strata ever examined. The six formations initially described and assigned to the Keweenawan Supergrou b Witzke (1990) constitute two groups temporarily named the "Upper" and "Lower Re 8 f . C astic Sequences" (Figure 1).
Unit H "Unit H" displa s a coarser average grain size and is less well-cemented than the
rocks of underlying "Upper Red Clastic Se uence", and is more arkosic than the uartz arenites of the overlying Mt. Simon Fm. The unit was discussed by McKay (1990) who described it as being dominated by two primary lithologies, a fine to very coarse arkosic sandstone and a micaceous, shaley to sand&sil~tone. McKay reported trace amounts of microbrecciated lithic fragments in "Unit , similar to fragments observed in all units of the "Red Clastic Se uences", and interpreted the fragments as the products of structural movements don fault zones that pervasively cut the MRS sequence. Grains with similar lithologic fabrics are not observed in the Mt. Simon interval of the well and have not been reported in similar age rocks in the region. Consequently these rocks are thought to be of Proterozoic age and the may be associated with the "Upper Red Clastic Sequence".
"Upper Red Clastic Sequence* The "Up er Red Clastic Sequence" .is composed of three informalunits totalling
6895 feet, an ? occupies the same strati aphic position as the Bayfield Group of Wisconsin with which it is correlated. Bot f groups are dominated by fluvial deposition, but dis la different compositional characteristics. The Bayfield is more mature than the un s er yin Oronto Group, whereas Ludvigson and others (1990 reported that the 4.0 "Upper Red Elastic Sequence" is less mature than its underlying wer Red Clastic Sequence". An upward increase in the volcanic rock fragment component of "Upper Red
7
-J
JClastic Sequence" sandstones may record the unroofing of basalts on the nearby centralhorst (Anderson, 1990b).
J"Lower Red Clastic Sequence"The "Lower Red Clastic Sequence" is also composed of three informal units and
totals 7190 feet of strata. It is the basal clastic sequence in the Eischeid well and bearsmany similarities to the Oronto Group of Wisconsin with which it is correlated. Theprimary difference between the two groups are the basal units: the Copper HarborConglomerate (Oronto Gp) being dominated by very coarse volcanic clasts, "Unit B'("Lower Red Clastic Sequence") ) being dominated by quartz sand grains. These -differences can be explained by the relative positions of the Oronto Gp and the Eischeidwell in the MRS. The Oronto Group is described from exposures on the central horst.Early in rift history the horst area was a graben, and the conglomerate clasts were Japparently eroded from volcanic rocks that capped the footwalls of the graben-boundingfaults. The Eischeid well was drilled off the central horst, and penetrated the fluvialsystem that drained the rift-bounding Proterozoic terrane. These fluvial systems hadtheir headwaters in the granites and gneisses that dominate the basement of the region,and were the source of the quartz sands that dominate "Unit B".
The middle unit in the "Lower Red Clastic Sequence", Unit C, is almost 1500 feetthick and displays close lithologic and sedimentological similarity to the Nonesuch Shaleof the Oronto Group. Both are characterized by gray to black siltstone and shales.Ludvigson and others (1990) identified a number of sedimentary structures that theyinterpreted as evidence of deposition in a lake or similar standing body of water. Calcite Jcements and vein fills are more abundant in "Unit C', than in other "Red Cla.stic" strata inthe Eischeid well, and cored intervals display high angle tectonic dips rangin from 65°to vertical and slightly overturned. Ludvigson and Spry (1990) described calcite veinletsand slickensided fault surfaces with rough facets indicating reverse faulting during thelater, compression phase of rift development. The presence of such a thick sequence oflake sediments outside of the former central graben suggests that the lakes that formedin the MRS were large, extending well beyond the limits of the graben-bounding faults.Seismic interpretations suggest they may have extended as much as 15 miles beyond thegraben (Anderson, 1990c).
-4
Figure 1: Correlation of Eischeid Red Clasticswith the Keweenawan Supergroup
meanframewoñ Eischeid Keweenawan Supergroup
— composhion
? QF17L4 Unit H
a.(1)
a.%
Q76F17L7 Unit G Chequarnegon SS ci.06m0
QJnL9 Unit F QeviI Island SS
Q5SL13 UnitE OrientaSs
Q75F1L UnitD FredaFm a.C)
____
0Cl)
QF24L3 Unit C Nonesuch Fm —Io
_________
C0
08
-J Unit B Copper Harbor Fm 0QVFI2L1
Clastic Se uence" sandstones may record the unroofing of basalts on the nearby central 5 horst (An erson, 1990b).
"Lower Red Clastic Sequence"' . ..-.vfi., The "Lower Red Clastic Sequeii&en is dsb cbfflbosed of three informal units and
totals 7190 feet of strata. It is the basal clastic sequence in the Eischeid well and bears many similarities to the Oronto Group of Wisconsin with which it is correlated. The primary difference between the two groups are the basal units: the Copper Harbor Conglomerate (Oronto Gp) being dominated by very coarse volcanic clasts, "Unit Bn ("Lower Red Clastic Sequence") ) being dominated by uartz sand grains. These differences can be explained by the relative positions of the a ronto Gp and the Eischeid well in the MRS. The Oronto Group is described from exposures on the central horst. Early in rift histo the horst area was a graben, and the conglomerate class were apparent! eroded rom volcanic rocks that capped the footwalls of the graben-bounding ^ 7 faults. T e Eischeid well was drilled off the central horst, and penetrated the fluvial s stem that drained the nft-bounding Proterozoic terrane. These fluvial systems had e i r headwaters in the ranites and gneisses that dominate the basement of the region, and were the source of t% e quartz sands that dominate "Unit B".
The middle unit in the "Lower Red Clastic Sequence", Unit C, is almost 1500 feet thick and displays close lithologic and sedimentological similarity to the Nonesuch Shale of the Oronto Group. Both are characterized by gray to black siltstone and shales. Ludvigson and others (1990) identified a number of sedimentary structures that they interpreted as evidence of deposition in a lake or similar standing body of water. Calcite cements and vein fills are more abundant in "Unit C". than in other "Red Clastic" strata in the Eischeid well, and cored intervals display high angle tectonic dips ranging from 65O to vertical and sli htly overturned. Ludvigson and Spry (1990) described calcite veinlets and slickensided ! ault surfaces with rough facets indicating reverse faulting during the later, compression phase of rift development. The presence of such a thick sequence of lake sediments outside of the former central graben suggests that the lakes that formed in the MRS were large, extending well beyond the limits of the graben-bounding faults. Seismic interpretations suggest they may have ext es beyond the graben (Anderson, 1990~).
Figure 1 : Correlation of Eischeid Red Clasti with the Keweenawan Supergroup
mean framework Eischeid - composition ,-,
? 1 Q,F,,L, Unit H J .g 1 Q ~ F , , L , 1 unit G 1
Q&LB Unit F a
QAGL,~ Unit E
Eischeid Petroleum PotentialAlthough no shows of liquid petroleum were encountered during the drilling of the
Eischeid well, trace amounts of methane and ethane were detected, and rareintergi-anular black residues (suggestive of oil movement) were observed in the "LowerRed Clastic Sequence". The dark shales and siltstones in the lower sequence were shownby Palacas and others (1990) to once have been organic-rich potential petroleum sourcerocks, but are presently overmature. Methane clathrates were identified in fluidinclusions in calcite veins by Ludvigson and Spry (1990) who suggested that petroleumwas mobile during the compression phase of MRS history. On the basis of comparisonsbetween fluid inclusions ans isotope data from vein-filling calcites in the rift-flankingbasins (Eischeid oil test) and faults bounding central horsts (Ludvigson and Anderson,1986), Ludvigson and Spry (1990) suggested that such fluid would be transported towardsthe flanks of the rift. This petroleum may still be trapped on the flanks of theMidcontinent Rift System.
The M.G. Eischeid #1 well offers a great potential to increase our understandinof the development of the Midcontinent Rift System and the clastic strata associated witit. Also, the rocks encountered in the well demonstrated that large volumes ofpetroleum was probably generated from organic-rich units and may still be trappedwithin the structure.
References
Anderson, R.R., 1990a, The Amoco M.G. Eischeid #1 deep petroleum test, Carroll County, Iowa. IowaDepartment of Natural Resources. Geological Survey Bureau, Special Report Series No.2, 185 p.
Ludvigson G.A. and Anderson, R.R., 1986, The Douglas Fault at Amnicon Falls State Park, Wisconsin:Brittle cataclastic textures in Keweenawan Rocks. Proceedings of the 32nd Annual Meeting, instituteof Lake Superior Geology, Wisconsin Geological and Natural History Survey, p SO-Si.
* all of the following references are in Anderson, 1990a * * * *
Anderson, R.R., 1990b, Review of current studies of Proterozoic rocks in the Amoco M.G. Eischeid #1petroleum test well, Carroll County, Iowa. p. 175-184.
Anderson, R.R., 1990c, Interpretation of geophysical data over the Midcontinent Rift System in the area ofthe M.G. Eischeid *1 petroleum test, Carroll County, Iowa. p. 27-38.
Ludvigson, (3.L., McKay, R.M., and Anderson, R.R, 1990, Petrology of Keweenawan sedimentary rocks inthe M.G. Eiscbeid #1 drillhole. p. 77-112.
Ludvigson, G.L., and Spry, P.G., 1990, Tectonic and paleobydrologic significance of carbonate veinlets in theKeweenawan sedimentary rocks of the Amoco MX3. Eischeid #1 drillhole. p. 153-168.
McKay R.M., 1990, Regional aspects of the Mt. Simon Formation and the placement of the Mt. Simon-pre-Mt. Simon sedimentary contact in the Amoco M.G. Eischeid #1 drillhole. p. 59-66.
Palacas, J.G., Sebmoker, J.W, Daws, T.A., Pawlewicz, M.J., and Anderson, Rit, 1990, Petroleumsource-rock assessment of Middle Proterozoic (Keweenawan) sedimentary rocks, Eiscbeid #1 well,Carroll County, Iowa. P. 119-134.
Witzke, BJ., 1990, General stratigraphy of the Phanerozoic and Keweenawan sequences in the area of theM.G. Eischeid #1 drillhole, Carroll County, Iowa. P. 39-58.
9
Eischeid Petroleum Potential Although no shows of liquid petroleum were encountered during the drilling of the
Eischeid well, trace amounts of methane and ethane were detected, and rare inter anular black residues su estive of oil movement) were observed in the "Lower a Red astic Sequence". The !i ar S8 shales and siltstones in the lower se uence were shown by Palacas and others (1990) to once have been organic-rich p t e n t i a troleum source rocks, but are presently overmature. Methane clathrates were icfcntified in fluid inclusions in calcite veins by Ludvigson and S ry 1990) who suggested that petroleum was mobile durin the compression phase of S istory. On the basis of corn arisons ^ & h between fluid inc usions ans isotope data from vein-filling calcites in the rift-flanking basins (Eischeid oil test) and faults boundin central horsts (Ludvigson and Anderson, 1986 , Ludvigson and Sp (1990) su ested that such fluid would be transported towards A as the anks of the rift. This petro eum may still be trapped on the flanks of the
. . Midcontinent Rift System.
The M.G. Eischeid #1 well offers a eat potential to increase our understandin of the development of the Midcontinent Ri f t System and the clastic strata associated wit F, it. Also, the rocks encountered in the well demonstrated that large volumes of petroleum was probably general ed within the structure.
Anderson, R.R., 1990a. The Amoco M.G. Eischeid #1 deep petroleum test, Carroll County, Iowa. Iowa Department of Natural Resources, Geological Survey Bureau, Special Report Series No. 2,185 p.
Ludvigson G.A. and Anderson. R.R., 1986, The Douglas Fault at Amnicon Falls State Park, Wisconsin: Brittle cataclastic textures in Keweenawan Rocks. Proceedings of the 32nd Annual Meeting, Institute of Lake Superior Geology, Wisconsin Geological and Natural History Survey, p. 50-51.
* all of the following references are in Anderson, 1990a * * * *
eview of current studies of Proterozoic rocks in the Amoco M.G. petroleum test well. Can-oil County, Iowa. p. 175-184.
Anderson, R.R., 1990~. Interpretation of geophysical data over the Midcontinent Rift System in the area of the M.G. E i i c i d #\ petroleum test, Carroll County, Iowa. p. 27-38.
Ludvigson, G.L., McKay, R.M., and Andersoo, R.R., 1990, Petrology of Keweenawan sedimentary rocks in the M.G. Eischeid #1 drillhole. p. 77-112.
Ludvigson, G.L., and Spry, P.G., 1990, Tectonic and paleahydrologic significance of carbonate veinlets in the Kewcenawan sedimentary rocks of the Amoco M.G. Eischeid #l drillhole. p. 153168.
McKay R.M., 1990, Regional aspects of the Mt. Simon Formation and the placement of the MI. Simon - pre-Mt. Simon sedimentary contact in the Amoco M.G. Eischeid #I drillhole. p. 59-66.
, Palacas. J.G., Schmoker, J.W., Daws. T.A., Pawlewicz, MJ., and Anderson, R.R., 1990, Petroleum source-rock assessment of Middle Proterozoic (Kcweenawan) sedimentary rocks, Eiscbeid #1 well, Carroll County, Iowa. P. 119-134.
WWte, BJ., 1990, General stratigraphy of the Phanerowic and Kcweenawan sequences in the area of the M.G. Eischeid #1 drillhole, Carroll County, Iowa. P. 39-58.
9
I
THE HOMESTAKE MINE
AN EARLY PROTEROZOIC IRON-FORMATION HOSTED GOLD DEPOSIT
R.L. Bachman JDistrict GeologistHomestake Mining Company
Lead, South Dakota jThe Homestake mine in the northern Black Hills of South Dakota, USA is the largest iron-
formation-hosted gold deposit known, and has prOduced 1,126 tonnes (36.2 million oz.) of gold from J130 million tonnes of ore milled. In 1990 the mine produced 12 tonnes of gold. The deposit wasdiscovered in 1876, and the mine has operated continuously to the present day. Gold ore is currentlymined for depths to 2,438 meters. Gold is the principal commodity produced along with a minor silverby-product. The gold/silver ratio averages 5:1; base metal content is negligible.
The deposit lies within the Early Proterozoic core of the Black Hills uplift, which represents the Jsouthern most exposure of Trans-Hudson orogen rocks. Tectonism and thermal activity associated withthe Trans-Hudson event occurred as a result of the collision of Wyoming and Superior Cratonscontemporaneous with the Penokean orogen. Proterozoic gold metallization is interpreted to be a resultof an evolved, long lived tecto-thermal process that relied on plate scale shear zones as fluid conduitsand zones of high geothermal gradient as fluid mobilizers. j
Gold ore bodies at Homestake are hosted almost exclusively by the Early Proterozoic HomestakeFormation, an iron-formation (15 to 35% total Fe) consisting of siderite phyllite and/or grunerite schist.The underlying Poorman Formation is composed of an upper sequence of sericite to.biotite-dominantcarbonate-quartz and graphitic phyllites and a lower voluminous sequence of hornblende-plagioclaseschist of tholeiitic affinity. The overlying Ellison Formation consists of sericite-quartz phyllite @eliticand tuffaceous strata) interbedded with quartzite.
Strata that contain the Homestake deposit were complexly deformed by a series isoclinal and sheathfold events that are synchronous with extensive ductile and ductile-brittle shearing. Mine area rocks
have been subjected to upper greenschist-lower amphibolite facies metamorphism; metamorphic intensity
increases to the northeast. Intrusion of a 1.72 Ga S-type granite northeast of the mine post-datedregional metamorphism, and appears contemporaneous with late stage semi-brittle deformation(Bachman et al., 1990).
Individual ore bodies are contained within plunging synclinal fold structures of FlomestakeFormation known as ledges. Ten of these structures have produced gold from relatively undeformed,elongate tabular zones of quartz, siderite, chlorite, pyrrhotite, arsenopyrite, minor pyrite, and native
gold. Ore mineralization is developed within and adjacent to dilated segments of late-stage ductile-brittle shears (Caddey et aL, 1990). These shears and associated ore bodies cross-cut earlier folds andoverprint metamorphic fabric.
10
THE HOMESTAKE MINE
AN EARLY PROTEROZOIC IRON-FORMATION HOSTED GOLD DEPOSIT
R.L. Bachman District Geologist
Homestake Mining Company Lead, South Dakota
The Homestake mine in the northern Black Hills of South Dakota, USA is the largest iron- formation-hosted gold deposit known, and has produced 1,126 tonnes (36.2 million oz.) of gold from 130 million tomes of ore milled. In 1990 the mine produced 12 tonnes of gold. The deposit was discovered in 1876, and the mine has operated continuously to the present day. Gold ore is currently mined for depths to 2,438 meters. Gold is the principal commodity produced along with a minor silver by-product. The goldlsilver ratio averages 5:l; base metal content is negligible.
The deposit lies within the Early Proterozoic core of the Black Hills uplift, which represents the southern most exposure of Trans-Hudson orogen rocks. Tectonism and thermal activity associated with the Trans-Hudson event occurred as a result of the collision of Wyoming and Superior Cratons contemporaneous with the Penokean orogen. Proterozoic gold metallization is interpreted to be a result of an evolved, long lived tecto-thermal process that relied on plate scale shear zones as fluid conduits and zones of high geothermal gradient as fluid mobilizers.
Gold ore bodies at Homestake are hosted almost exclusively by the Early Proterozoic Homestake Formation, an iron-formation (15 to 35% total Fe) consisting of siderite phyllite and/or grunerite schist. The underlying Poorman Formation is composed of an upper sequence of sericite to-biotite-dominant carbonate-quartz and graphitic phyllites and a lower voluminous sequence of hornblende-plagioclase schist of tholeiitic affinity. The overlying Ellison Formation consists of sericite-quartz phyllite (pelitic and tuffaceous strata) interbedded with quartzite.
Strata that contain the Homestake deposit were complexly deformed by a series isoclinal and sheath fold events that are synchronous with extensive ductile and ductile-brittle shearing. Mine area rocks have been subjected to upper greenschist-lower arnphibolite facies metamorphism; metamorphic intensity increases to the northeast. Intrusion of a 1.72 Ga S-type granite northeast of the mine post-dated regional metamorphism, and appears contemporaneous with late stage semi-brittle deformation (Bachman et al., 1990).
Individual ore bodies are contained within plunging synclinal fold structures of Homestake Formation known as ledges. Ten of these structures have produced gold from relatively undeformed, elongate tabular zones of quartz, siderite, chlorite, pyrrhotite, arsenopyrite, minor pyrite, and native gold. Ore mineralization is developed within and adjacent to dilated segments of late-stage ductile- brittle shears (Caddey et al., 1990). These shears and associated ore bodies cross-cut earlier folds and overprint metamorphic fabric.
At least two stages of mineral alteration are observed, one of which issynmetamorphic and appearsto predate gold mineralization, and a second synchronous with gold mineralization. The pre-goldmineral alteration stage is represented by extensive potassium and carbonate metasomatism. Ore-stagehydrothermal alteration appears retrogressive producing extensive chlorite and siderite replacement ofthe Homestake Formation in and adjacent to the ore bodies.
Timing of gold mineralization at Homestalce, even though not dated directly, is regarded as earlyProterozoic. Dating of zircon from tuffaceous sediments within the Ellison Formation gave an age of1.97 Ga (Redden et al., 1990) and provide an approximate age for Homestaice Formation deposition.The approximate age of regional metamorphism and major regional ductile deformation is 1.84 Ga(Zartman and Stem, 1967). The Crook Mountain Granite and associated deformation at 1.72 Ga appearsto post-date gold mineralization. It is interpreted that gold was introduced after peak metamorphism(1.84 Ga) and prior to granite emplacement (1.72 Ga) by epigenetic processes.
REFERENCES
Bachman, R.L., Campbell, T.l., and Sneyd, D.S., 1991, Crook Mountain granite and its relation toEarly Proterozoic gold mineralization at the Homestalce mine, northern Black Hills, SD: Geol Soc.America, Abstracts with Programs, v. 22, no. 6, p.2.
Caddey, S.W., Bachman, R.L., Campbell, T.J., Reid, R.R., Otto, R.P., 1990, The Homestalce goldmine, an Early Proterozoic iron-formation hosted gold deposit, Lawrence County, South Dakota:U.S. Geol Survey Bull. 1857-3 (in press).
Redden, J.A., Peterman, Z.E., Zartman, R.E., and DeWitt, E., 1990, U-Th-Pb geochronology andpreliminary interpretation of Precambrian tectonic events in the Black Hills, South Dakota: (3eol.Assoc. Canada Spec. Paper on the Trans-Hudson Orogen (in press).
Zartman, R.E., and Stem, T.W., 1967, Isotopic age and geologic relationships of the Little ElicGranite, northern Black Hills, South Dakota; U.S. Geol. Survey Prof. Paper 575-D, p. D157-Dl 63.
Ill
I At least two stages of mineral alteration are observed, one of which is synmetarnorphic and appears to predate gold mineralization, and a second synchronous with gold mineralization. The pre-gold mineral alteration stage is represented by extensive potassium and carbonate metasomatism. Ore-stage
I hydrothermal alteration appears retrogressive producing extensive chlorite and siderite replacement of the Homestake Formation in and adjacent to the ore bodies.
I Timing of gold mineralization at Homestake, even though not dated directly, is regarded as early Proterozoic. Dating of zircon from tuffaceous sediments within the Ellison Formation gave an age of
I 1.97 Ga (Redden et al., 1990) and provide an approximate age for Homestake Formation deposition. The approximate age of regional metamorphism and major regional ductile deformation is 1.84 Ga (Zartman and Stem, 1967). The Crook Mountain Granite and associated deformation at 1.72 Ga appears
I to post-date gold mineralization. It is interpreted that gold was introduced after peak metamorphism (1.84 Ga) and prior to granite emplacement (1.72 Ga) by epigenetic processes.
I 1
REFERENCES
I Bachman, R.L., Campbell, T.J., and Sneyd, D.S., 1991, Crook Mountain granite and its relation to Early Proterozoic gold mineralization at the Homestake mine, northern Black Hills, SD: Gwl Soc.
I America, Abstracts with Programs, v. 22, no. 6, p.2.
Caddey, S.W., Bachman, R.L., Campbell, T.J., Reid, R.R., Otto, R.P., 1990, The Homestake gold mine, an Early Proterozoic iron-formation hosted gold deposit, Lawrence County, South Dakota:
I U.S. Geol Survey Bull. 1857-1 (in press). Redden, J.A., Petennan, Z.E., Zartman, R.E., and DeWitt, E., 1990, U-Th-Pb geochronology and
preliminary interpretation of Precambrian tectonic events in the Black Hills, South Dakota: Gwl.
I Assoc. Canada Spec. Paper on the Trans-Hudson Orogen (in press). Zartman, R.E., and Stem, T.W., 1967, Isotopic age and gwlogic relationships of the Little Elk
Granite, northern Black Hills, South Dakota; U.S. Geol. Survey Prof. Paper 575-D, p. D157-
EXTENSION OF GRAVITY FILED COVERAGE IN EAST-CENTRALWISCONSIN: 1990 COGEOMAP PROGRAM
Daniel J. Brehm, C. Patrick Ervin, Dept. of Geology, Northern JIllinois University, DeKalb, IL 60115M.G. Mudrey, Jr., B.A. Brown, M.L. Czechanski, Wisconsin
Geological and Natural History Survey, 3817 Mineral PointRd. • Madison, WI 53705
During the summer of 1990, approximately 4800 additionalmeasurements of the earth's gravity field were made in an areabounded by 44 & 45 degrees latitude and by 88 & 90 degreeslongitude. Most stations are located at surveyed elevationpoints posted on USGS topographic sheets and are spaced one mileapart, except where access was limited. Northern IllinoisUniversity's Lacoste-Romberg gravity meter G409 was used. Dataare tied to the State of Wisconsin Primary Gravity Base StationNetwork (Ervin, 1983), which is in turn tied to the InternationalGravity Standardization Networlc-1971 (DMAAC, 1974). Normallyaccepted field and processing procedures were used. These datahave been combined with earlier data to produce a much moredetailed Bouguer gravity anomaly map.
The northwest section of the area is underlain by primarilyProterozoic and Late Archean rocks, comprised of qüartzite,granite, rhyolite, granodiorite, gneiss, metavolcanics, andmetasediments. This variability is reflected in the complexcharacter of the Bouguer gravity anomalies.
The north-central part of the region contains the southernpart of the Middle Proterozoic Wolf River Batholith, which iscomposed primarily of granitic and syenitic rocks, withinclusions of quartz monzonite, anorthosite, and gabbro. Thegreater homogeneity of the Wolf River Batholith produces asmoother Bouguer gravity field than exists in the northwestsection.
The southern and eastern areas of the map largely overlyPaleozoic sedimentary rocks, possibly underlain at depth by anextension of the Wolf River Batholith (Allen, 1990), resulting ina Bouguer gravity field dominated by longer wavelength anomaliesthan in the rest of the survey region.
REFERENCES
Allen, D.J. , 1990, The Wisconsin Gravity Minimum: Source andImplications, unpub. Ms thesis, Purdue Univ., West Lafayette,IN., 183 p.
Ervin, C.P., 1983, Wisconsin Gravity Base Station Network, Wis.Geol. & Nat. Hist. Sun. Misc. Pap. 83-1, 43 p.
-jDMAAC, 1974, World Relative Gravity Reference Network - North
America, DMAAC Ref. Pub. No. 25, 1974 Supplement.
12
J
Daniel J. Brehm, C. Patrick Ervin, Dept:of Geology, Northern Illinois University, DeKalb, IL 60115
M.G. Mudrey, Jr., B.A. Brown, M.L. Czechanski, Wisconsin Geological and Natural History Survey, 3817 Mineral Point Rd., Madison, WI 53705
During the summer of 1990, approximately 4800 additional measurements of the earth's gravity field were made in an area bounded by 44 & 45 degrees latitude and by 88 & 90 degrees longitude. Most stations are located at surveyed elevation points posted on USGS topographic sheets and are spaced one mile apart, except where access was limited. Northern Illinois .University's LaCoste-Romberg gravity meter G409 was used. Data are tied to the State of Wisconsin Primary Gravity Base Station Network (Ervin, 1983), which is in turn tied to the International Gravity Standardization Network-1971 (DMAAC, 1974). Normally accepted field and processing procedures were used. These data have been combined with earlier data to produce a much more
tailed Bouguer gravity anomaly map.
The northwest section of the area is underlain by primarily Proterozoic and Late Archean rocks, comprised of quartzite, granite, rhyolite, granodiorite, gneiss, metavolcanics, and metasediments. This variability is reflected in the complex character of the Bouguer gravity anomalies.
The north-central part of the region contains the southe part of the Middle Proterozoic Wolf River Batholith, which is composed primarily of granitic and syenitic rocks, with inclusions of quartz monzonite, anorthosite, and gabbro. The greater homogeneity of the Wolf River Batholith produces a smoother Bouguer gravity field than exists in the northwest section.
The southern and eastern areas of the map largely overly Paleozoic sedimentary rocks, possibly underlain at depth by an extension of the Wolf River Batholith (Allen, 1990). resulting in a Bouguer gravity field dominated by longer wavelength anomalies than in the rest of the survey region.
REFERENCES
Allen, D.J., 1990, The Wisconsin Gravity Minimum: Source and Implications, unpub. MS thesis, Purdue Univ., West Lafayette, IN., 183 p.
Ervin, C.P., 1983, Wisconsin Gravity Base Station Network, Wis Geol. & Mat. Hist. Sum. Misc. Pap. 83-1, 43 p.
DMAAC, 1974, World Relative Gravity Reference Network - North America, DMAAC Ref. Pub. No. 25, 1974 Supplement.
Nonmetallic Mineral Resources and Minor Metals Potentialof Northern Wisconsin
Bruce A. BrownWisconsin Geological and Natural History Survey
The Precambrian terranes of northern Wisconsin contain significantnonmetallic mineral resources and have the potential for other metal depositsin addition to volcanic—hosted massive sulfides. The region has a longhistory as a producer of building and ornamental stone and a variety ofcrushed stone products. Other products include weathered or "rotten" graniteand clay from weathered crystalline rocks and glacial deposits. NorthernWisconsin contains potentially economic occurrences of industrial minerals(talc, graphite, marble, kyanite, nepheline syenite, silica sand, and others)and rare metals (molybdenum, beryllium, lithium, and others). Significantdeposits of iron are also present but are not currently economical to produce.
In recent years, dimension stone production has maintained a low butsteady level of activity; the crushed stone industry has grown significantly.This growth has been in response to a growing demand for high quality railroadballast and changing construction specifications that reQuire higher qualityaggregates for base course and asphalt paving mixtures. Specializedoperations produce crushed metavolcanic rock for roofing granules andaggregates for terrazzo stone and synthetic granite production.
Although base and precious metal potential has attracted much attentionrecently, the nonmetallic minerals have long been and will continue to be amajor economic resource of northern Wisconsin . Important factors are a goodtransportation network and Wisconsin's favorable location in respect tomarkets in the upper Midwest. The future development of regulatory frameworkon the state and local Level will have important implications for fut uredeve 1 opmen t of t hi s industry-.
I i
I Nonmetallic Mineral Resources and Minor Metals Potential of Northern Wisconsin
I nice A. Brown :nL and Natural History Survey
, .: .. . .
I The Precambrian terranes of northern Wisconsin contain significant
nmetallic mineral resources and have the potential for other metal deposits addition to volcanic-hosted massive sulfides. The region has a long story as a producer of building and ornamental stone and a variety of
crushed stone products. Other products include weathered or "rotten" granite and clay from weathered crystalline rocks and glacial deposits. Northern Wisconsin contains potentially economic occurrences of industrial minerals (talc, graphite, marble,-kyanite, nepheline syenite, silica sand. and others) and rare metals (molybdenum, beryllium, lithium, and others). Significant deposits of iron are also present but are not currently economical to produce.
In recent years, dimension st-one production has maintained a low but steady level of activity; the. crushed stone industry has grown significantly. This growth ha? been in response to a growing demand for high quality railroad ballast and changing construction specifications that require higher quality aggregates for base course and asphalt paving mixtures. Specialized operations produce crushed metavolcanic rock for roof ing granules and 'aggregates for terrazzo stone and synthetic granite production.
Although base and precious metal potential has attracted much attention recently, the nonmetallic minerals have long been and will continue to be a major economic resource of northern Wisconsin. Important factors are a good transportat ion network and Wisconsin's favorable location in respect. to markets in the upper Midwest. The future development of regulatory framework on t:he state and local level will have* important inipl-i.cations for future development of this industry.
14
jJ
J
Preliminary Bedrock Geologic Map of Eat, Claire County, Wisconsin JB.A. Brown & U.S. Maass
Wisconsin Geological and Natural History Survey JEau Claire Cotwty is underlain by rocks ranging froni Early Proterozoic
and Archean (?) to Late Cambrian age. Precambrian rock is exposed inscattered outcrops along the Eau Claire River and tributaries in northeasternEau Claire County. The Precambrian consists of a suite of amphibolites andgranitic gneisses of possible Archean age, and what is probably a youngersequence of inetavolcanic and metasedimentary rocks and granitic intrusions of Jprobable Penokean (1850 Ma) age. Underformed mafic dikes cut older units andmay he as young as Kewanawan.
The Precambrian i.s overlain by Upper Cambrian clastic and carbonaterocks of the Elk Mound, Tunnel City, and Trempealeau Groups. The UpperCambrian contains two fining—upward sedimentary cycles. The lower cycleconsists of the basal Mount Simon sandstone that grades upward into theglauconitic Tunnel City Group and sandy dolomite of the St. LawrenceFormat ion.
Eau Claire County has no historical record of mineral production fromthe Precambrian. Residual clay is associated with the Cambrian/Precambrianunconformity, and the Wonewoc Format ion is a potential source of sil ica sand.The only Faleozoic unit potentially suit-able for aggregate is the St. Lawrencedolomite, which is present only on ridge tops in the southwestern area nf thecounty. Pleistocene alluvial deposits along the Chippewa and Eau ClaireRivers are the primary source of construction mat eriols. Principal aqui fersarc the alluvial deposits and the Mount Simon sandstone.
j
Preliminary Bedrock Geologic Map of Eau Claire County. Wisconsin
B.A. Brown & R.S. Maass Wisconsin Geological and Natural History Survey
Eau Claire County is underlain by rocks ranging from Early Proterozoic and Archean (? ) to Late Cambrian age. Precambrian rock is exposed in cattered outcrops along the Eau Claire River and tributaries in northeastern au Claire County. The Precambrian consists of a suite of amphibolites and granitic gneisses of possible Archean age, and what is probably a younger equence of metavolcanic and metasedimentary rocks and granitic intrusions of probable Penokean (1850 Ma) age. Underfonned mafic dikes cut older unit.s and may be as young as Kewanawan.
The Precambrian i.s overlain by Upper Cambrian clastic and carbonate rocks of the Elk Mound, Tunnel City, and Trempealeau Groups. The Upper Cambrian contains two fining-upward sedimentary cycles. The lower cycle consists of the basal Mount Simon sandstone that grades upward into the glauconitic Tunnel City Group and sandy dolomite of the St. Lawrence Format ion.
Eau Claire County has no historical record of mineral production from the Precambrian. Residual clay is associated with the Cambrian/Precambrian mconfotmi~y, and the Wonewoc Formation 1s a potential source of silica sand. The only Paleozoic unit potentially suitable for aggregate i.s the St. Lawrence dolomite, which is present. only on ridge tops in the southwestern area of the county. Pleistocene alluvial deposits along the Chippewa and Eau Claire Rivers are the primary source of construction materials. Principal aqui'fers are the alluvial deposits and the Mount Simon sandstone.
Collision Induced Ripoffs, Ancient and Modern:The Mideontinent Rift System and the Red Sea—Gulf of Adencompared.
F. William Cambray and Kazuya Fujita, Dept. of Geol. Sci.,Michigan State University, East Lansing MI 48824—1115;517—355—4626)
Slab pull is considered to be a major force in determiningthe rate and directions of plate motion. Irregular oroblique convergence will result in closure of an ocean atsome localities prior to others. Once a portion of thesubducting plate has sutured, the plate will slow down orstop. Adjacent, unclosed sections of oceanic crust,however, will continue subduct under the influence of slabpull( fig.1.l). This may result in tensional stresses ina portion the subducting plate. Since fractures are morelikely to develop in the weaker, continental part of thesubducting plate, a fragment of the plate may be detachedalong an intracontinental rift system (fig.1.2). Openingof this rift system would be limited to a small oceanbasin since continuing subduction will close the remainingportion of the original ocean basin. Closure of theoriginal ocean basin, could then result in compression inthe rift system (f ig.1.3). This sequence explains thenear synchronous association of extension and compression,and the subsequent deformation of the rift fill, as foundin many once extensional regions.
The Red Sea is a modern example of this process. Closureof the Tethys occurred first in the Mediterranean and lefta portion of the African plate, with attached oceaniccrust, still subducting under the Zagros region.Continuing slab pull in this region, combined with therestraint to the west, induced the detachment of theArabian Peninsula along the Red Sea and Gulf of Aden(fig.2). Extension in the Proterozoic Mid Continent Riftsystem of North America was coincident with the Grenvillecompressional event to the south and east. The riftexperienced only limited opening which we suggest was dueto slab—pull along an unclosed segment in the collisionzone of the Grenville Orogenic belt (fig.3).Subsequently, the entire collision zone closed and therift was pushed back into place giving rise to a complexpattern of folds and flower structures (fig.4).
15
Collision Induced Ripoffs, Ancient and Modern:
I The Midcontinent Rift System and the Red Sea-Gulf of Aden compared.
I F. William Cambray and Kazuya Fujita, Fpt. of Geol. Sci., - Michigan State University, East Lansing MI 48824-1115;
517-355-4626)
slab pull is considered to be a major force in determining fl ' the rate and directions of plate motion. Irregular or , oblique convergence will result in closure of an ocean at
1 some localities prior to others. Once a portion of the subducting plate has sutured, the plate will slow down or stop. Adjacent, unclosed sections of oceanic crust, however, will continue subduct under the influence of slab pull( fig.l.l). This may result in tensional stresses in a portion the subducting plate. Since fractures are more likely to develop in the weaker, continental part of the subducting plate, a fragment of the plate may be detached along an intracontinental rift system (fig.1.2). Opening of this rift system would be limited to a small ocean basin since continuing subduction will close the remaining portion of the original ocean basin. Closure of the original ocean basin, could then result in compression in the rift system (fig.1.3). This sequence explains the near synchronous association of extension and compression, and the subsequent deformation of the rift fill, as found in many once extensional regions.
The Red Sea is a modern example of this process. Closure of the Tethys occurred first in the Mediterranean and left a portion of the African plate, with attached oceanic crust, still subducting under the Zagros region. Continuing slab pull in this region, combined with the restraint to the west, induced the detachment of the Arabian Peninsula along the Red Sea and Gulf of Aden (fig.2). Extension in the Proterozoic Mid Continent Rift system of North America was coincident with the Grenville compressional event to the south and east. The rift experienced only limited opening which we suggest was due to slab-pull along an unclosed segment in the collision zone of the Grenville Orogenic belt (fig.3). Subsequently, the entire collision zone closed and the rift was pushed back into place giving rise to a complex pattern of folds and flower structures (fig.4).
Cambray and Fujita16
(1991)
jJ
4
—4
Fig. 2
-I
J
Fig. 3
Fig. 4
3. COMPLETE C L O S
Fio 1
Fig. 4
TURKEY GREECE
BITLIS
ARABIAN
N O R T H
1 ' /"ass? Early Miocene
Fig. 2
Fig. 3
Cambray and F u j i t a (1991)
DETACHMENT FAULTING AND THE ORIGIN OF THE ASYMMETRIC DEPOSITIONALPATTERN OF THE MARQUETTE TROUGH.F.wjlljam Cambray, Dept ofGeological Sciences, Michigan State University, East Lansing MI48824., Joseph J.Mancuso, Dept. of Geology, Bowling Green StateUniversity, Bowling Green, OH 43403., and Went Slitor, C.C.I.,504, Spruce Street, Ishpeming, MI 49849.
The Marquette Trough is one of several fault bound basinsL set into the Archean crust of northern Michigan which contain
tower Proterozoic metaseditnentary rocks. The pattern isinterpreted as deposition in rift basins associated with the
L development of a passive margin prior to the Penokean Orogeny.The orogeny was triggered by collision of an island arc to thesouth. Deposition in the trough began with the Siamo Formationwhich consists of alternating layers of pelite and arenite, the
L latter having turbidite characteristics. This was followed by athick sequence of banded iron formation, the Negaunee BIF. Theiron formation is interbedded with clastic lenses which thicken
L and become more frequent towards the southern margin suggestingprovenance from this direction, sole marks indicating a turbiditeorigin have been found at the base of these layers. The clasticlenses are somewhat irregular in distribution and disappear
L rather abruptly to the north. Changes in the pattern of clasticsis often associated with faults. This pattern is similar toasymmetric fault basins which have been describe in relation to
L major crustal detachment faults in the Cenozoic of the Western US(fig.l). The overall stratigraphic pattern suggests that such adetachment may be present. On the south side the stratigraphy istruncated adjacent to the margin which is consistent withcontinual displacement during sedimentation whereas thesuccession is more continuous across the margin of the trough(fig.l). The Palmer Gneiss is a candidate for the main detachment
L fault on the south side, outcrops of the gneiss close to thecontact with the Proterozoic rocks are broken and containchlorite, this lithology passes into a banded gneiss to the southnear the Tilden Mine. The banding consist of normal gneissictexture interspersed with mylonite. This association is similarto the description of detachment surfaces in the western US. Thechlorite rich material represents the high level breccia which
L formed in the brittle zone and the banded mylonite is the deeperductile portion of the detachment fault. The two were broughttogether by a large normal displacement. The Penokean orogeny
L complicated the picture, reactivating some of the normal faultsas reverse faults but the basic pattern is still recognizable(fig. 2) -
17
DETACHMENT FAULTING AND THE ORIGIN OF THE ASYMMETRIC DEPOSITIONAL PATTERN OF THE MARQUETTE TROUGH.P.William Cambray, Dept of Geological Sciences, Michigan State University, East Lansing MI 48824., Joseph J-Mancuso, Dept. of Geology, Bowling Green State University, Bowling Green, OH 43403., and Went Slitor, c.c.I., 504, Spruce Street, Ishpeming, MI 49849.
The Marquette Trough is one of several fault bound basins set into the Archean crust of northern Michigan which contain Lower Proterozoic metasedimentary rocks. The pattern is interpreted as deposition in rift basins associated with the development of a passive margin prior to the Penokean Orogeny. The orogeny was triggered by collision of an island arc to the south. Deposition in the trough began with the Siamo Formation which consists of alternating layers of pelite and arenite, the latter having turbidite characteristics. This was followed by a thick sequence of banded iron formation, the Negaunee BIF. The iron formation is interbedded with clastic lenses which thicken and become more frequent towards the southern margin suggesting provenance from this direction, sole marks indicating a turbidite origin have been found at the base of these layers. The clastic lenses are somewhat irregular in distribution and disappear rather abruptly to the north. Changes in the pattern of elastics is often associated with faults. This pattern is similar to asymmetric fault basins which have been describe in relation to major crustal detachment faults in the Cenozoic of the Western US (fig. 1) . The overall stratigraphic pattern suggests that such a detachment may be present. On the south side the stratiara~hv is truncated adjacent to the margin which is consistent with continual displacement during sedimentation whereas the succession is more continuous across the margin of the trough (fig.1). The Palmer Gneiss is a candidate for the main detachment fault on the south side. Outcrops of the gneiss close to the contact with the Proterozoic rocks are broken and contain chlorite, this lithology passes into a banded gneiss to the south near the Tilden Mine. The banding consist of normal gneissic texture interspersed with mylonite. This association is similar to the description of detachment surfaces in the western US. The chlorite rich material represents the high level breccia which formed in the brittle zone and the banded mylonite is the deeper ductile portion of the detachment fault. The two were brought together by a large normal displacement. The Penokean Orogeny complicated the picture, reactivating some of the normal faults as reverse faults but the basic pattern is still recognizable (fig. 2).
N
N 2R3UU ______ JflOIfD A'IO3oN3 ______ $31LW13 HIDI LIE flNflflN
(1661 •Osnotnj4 put ñt%V1Wt3) Irnouz 11nn0uw4 3HZ 10 MZXNNlsb
8I
I I
MCqUHEX M? UITH CLAStICS CHOCOLflY CROUP I
A LATE KEWEENAWAN THRUST ? MARQUETTE COUNTY, MICHIGAN.F.Wulliam Cambray Dept of Geological Sciences, Michigan StateUniversity, East Lansing Michigan 48824 and Glenn Scott, CallahanMining Co. Marquette, Michigan.
Records from a drill hole on the southern margin of theMarquette Trough ( Hole 1(30, sec. 17 T47N R2SW, Sands Quadrangle.Core made available by CCI Ishpeming.) describe a 'Cambrianclastic dike' occurring at a depth of 360 feet. The overlyingmaterial is deformed and metamorphosed Proterozoic KonaFormation. Re—examination of the 'dike' has shown that it is animmature, poorly sorted unmetamorphosed quartz arenite containingfragments of the Icona Formation. The lithology is very similar tothe local outcrops of Jacobsville Sandstone. Although theJacobsville has been described as Cambrian in the past it is nowwidely believed to be late Keweenawan in age. A re—examination ofthe core reveals that the sandstone bedding is vertical and thefragments of Kona Formation included in the sandstone are alignedparallel to this vertical bedding. All the exposures in theimmediate region are Lower Proterozoic rocks, mostly KonaFormation.
We interpret the pattern observed in the drill hole toindicate a late Keweenawan thrust placing the Kona Formation overthe Jacobsville (fig.2). During the Keweenawan widespread riftingproduced clastic filled basins throughout the Lake Superiorregion. Towards the end of the period the Jacobsville Sandstone,and the equivalent Bayfield Group formed a widespread blanketover much of the region. This was followed by a compressionalevent which gave rise to thrust faulting on the KeweenawPeninsula and several reverse faults in the Nidcontinent Riftsection of Wisconsin and Minnesota. Until now there has been norecord of this late compressional event east of the KeweenawPeninsula. The sequence of events proposed is as follows.
1) Extensional tectonics associated with the opening of theMidcontinent Rift System produced a rift basin above theMarquette Trough which filled with sediment derived from thesurrounding Archean Basement and the Proterozoicnetasediments (fig. 1) The sediments contain unweatheredmicrocline and fragments of mylonite from the gneisses.Pelite lenses mark the bedding surface and compactionstructures are clearly visible. Fragments of the KonaFormation up to 5 centimeters long by 2 centimeter wide arealigned parallel to the bedding. The poor sorting suggeststhat this locality was close to the fault scarp.2) The conpressional event produced in a thrust close to theoriginal rift margin and transported the adjacent KonaFormation over the Jacobsville basin, tilting the sandstoneto its present vertical position in the process (fig 2).
This pattern of deformation in the late Keweenawan is notsurprising considering the structures seen to the west. If it ismore widespread it adds another factor to be considered whenattempting to unravel the structure of the older Proterozoic inthe Great Lakes region.
19
A LATE KEWEENAWAN THRUST ? MARQUETTE COUNTY, MICHIGAN. '.William Cambray Dept of Geological Sciences, Michigan State University, East Lansing Michigan 48824 and Glenn Scott, Callahan Mining Co. Marquette, Michigan.
Records from a drill hole on the southern margin of the Marquette Trough ( Hole M30, sec. 17 T47N R25W, Sands Quadrangle. Core made available by CCI Ishpeming.) describe a 'Cambrian clastic dike' occurring at a depth of 360 feet. The overlying material is deformed and metamorphosed Proterozoic Kona Formation. Re-examination of the 'dike' has shown that it is an immature, poorly sorted unmetamorphosed quartz arenite containing fragments of the Kona Formation. The lithology is very similar to the local outcrops of Jacobsville Sandstone. Although the Jacobsville has been described as Cambrian in the past it is now widely believed to be late Keweenawan in age. A re-examination of the core reveals that the sandstone bedding is vertical and the fragments of Kona Formation included in the sandstone are aligned parallel to this vertical bedding. All the exposures in the immediate region are Lower Proterozoic rocks, mostly Kona Formation.
We interpret the pattern observed in the drill hole to indicate a late Keweenawan thrust placing the Kona Formation over the Jacobsville (fig.2). During the Keweenawan widespread rifting produced clastic filled basins throughout the Lake Superior region. Towards the end of the period the Jacobsville Sandstone, and the equivalent Bayfield Group formed a widespread blanket over much of the region. This was followed by a compressional event which gave rise to thrust faulting on the Keweenaw Peninsula and several reverse faults in the Midcontinent Rift section of Wisconsin and Minnesota. Until now there has been no record of this late compressional event east of the Keweenaw Peninsula. The sequence of events proposed is as follows.
1) Extensional tectonics associated with the opening of the Midcontinent Rift System produced a rift basin above the Marouette Trouqh which filled with sediment derived from the surounding kchean Basement and the ~roterozoic metasediments (Ciq. 1). The sediments contain unweathered microcline and fragments of mylonite from the gneisses. Pelite lenses mark the bedding surface and compaction structures are clearly visible. Fragments of the Kona Formation up to 5 centimeters long by 2 centimeter wide are aligned parallel to the bedding. The poor sorting suggests that this locality was close to the fault scarp. 2) The compressional event produced in a thrust close to the original rift margin and transported the adjacent Kona Formation over the Jacobsville basin, tilting the sandstone to its present vertical position in the process (fig 2).
This pattern of deformation in the late Keweenawan is not surprising considering the structures seen to the west. If it is more widespread it adds another factor to be considered when attempting to unravel the structure of the older Proterozoic in the Great Lakes region.
N) C
LAT
E R
EW
EE
IIAW
AN
TH
RU
ST
ING
, MA
RQ
UE
TT
E M
ICH
IGA
N
Car
ibra
y an
d S
cott
1991
. Jao
ob!v
llI! S
st.
coar
se le
nses
Kon
a F
M.
__
LL. E
'1
__ _
_ _
1L
I[J
L._
CT
rI
__
LL
rig.
1B
Ittin
g P
kr.
rIg. 2
CoM
pres
ilenJ
i Pkn
.
The Feasibility of Recovering Metals Fran a Hazardous Waste SiteA (Brief) Case Study
Frederick K. Campbell
Minnesota Pollution Control Agency520 Lafayette Road
St. Paul, Minnesota 55155
The National Lead (NL) fTaracorp/C3olden Site (Site) is located in St. LouisPark, Hennepin County, Minnesota. The Site is approximately 10 acres in sizeand is the former site of a secondary lead smelter. The Site is included in theU.S. Environmental Protection Agency's (EPA'S) National Priorities List (NPL)and is also present on the Minnesota Pollution Control Agency's (MPCA' 5)Permanent List of Priorities (PLP). The secondary smelter was operated by NLand Taracorp between 1940 and 1981, and was used to reprocess lead, lead oxideand lead sulfate fran batteries and scrap into various lead alloys. The mainsolid waste produced by the smelter was slag fran the blast furnace which wasdeposited on—site between 1940 arid 1962. The presence of the lead-bearing slagin the sub-surface constitutes a potential threat to the local surficial andbedrock aquifers, and is the main reason for the inclusion of the Site on theNPL and the PLP.
The slag, which is a major component of the fill materials on the Site, isheterogeneous in nature. Limited analytical data indicate that the slagcontains up to 45,900 part per million (4.5 percent) lead. Visual inspection ofslag samples indicates a high iron content in sane samples, while other sarripleshave a high lead (galena) content. Approximately 16,000 cubic yards of fillmaterials are present on the Site, and it is estimated that 25 to 90 percent ofthe fill is slag.
The current redy for the Site is an asphalt "cap, installed in 1988 whichwas designed to prevent infiltration of precipitation and thereby preventleaching of lead fran the slag. The EPA is currently engaged in a five-yearreview of the Site, which is required by the Superfund program (CctnprehensiveEnvironmental Response, Compensation, and Liability Act (CERCIA)). This reviewprocess will consider the status of the Site and the risks represented by thehazardous materials (slag) that retain on the Site. In this context, other rnrepermanent renedies such as encapsulation, fixation or excavation and ranoval maybe considered by the EPA and the MPCA. An alternative permanent raredy whichmerits consideration is the recovery of lead and other metals (Cd, P.s) from the
slag. Technical, regulatory and economic barriers must be overccne in order tomake the recovery of lead fran the slag a feasible alternative.
Technical barriers to the recovery of lead fran the slag revolve around theheterogeneous nature of the slag and the variable iron, lead and silica contentof the slag. The U.S. Bureau of Mines, through its Environmental TechnologyResearch Program, has investigated the possibility of recovering lead from milltailings in Missouri. Approaches such as an air sparged hydrocyclone showpromise and may also permit recovery of lead from slag. Another technique,utilizing a flame reactor process, has been evaluated by the EPA in itsSuperfund Innovate Technology Evaluation (SITh) Program.
21
The Feasibility of Recovering Metals From a Hazardous Waste Site A (Brief) Case Study
Frederick K. -11
Minnesota Pollution Control Agency 520 Lafayette Road
St. Paul, Minnesota 55155
)/Taracorp/Golden Site (Site) is located in St. Loui Park, Hennepin County, Minnesota. The Site is approximately 10 acres in size and is the former site of a secondary lead smelter. The Site is included in the U.S. Environmental Protection Agency's (EPA's) National Priorities List (NPL) and is also present on the Minnesota Pollution Control Agency's (MPCA'S) Permanent List of Priorities (PLP). The secondary smelter was operated by NL and Taracorp between 1940 and 1981, and was used to reprocess lead, lead oxide and lead sulfate tram batteries and scrap into various lead alloys. The main solid waste produced by the smelter was slag from the blast furnace which was deposited on-site between 1940 and 1962. the presence of the lead-bearing slag in the subsurface constitutes a potential threat to the local surficial and bedrock aquifers, and is the main reason for the inclusion of the Site on the NPL and the PLP.
The slag, which is a major component of the fill materials on the Site, is ..etercqeneous in nature. Limited analytical data indicate that the slag contains up to 45,900 part per million (4.5 percent) lead. Visual inspectxon of slag samples indicates a high iron content in some samples, while other sanples have a high lead (galena) content. Approximately 16,000 cubic yards of fill materials are present on the Site, and it is estimated that 25 to 90 percent of he fill is slag.
The current remedy for the Site is an asphalt "cap", installed in 1988 which was designed to prevent infiltration of precipitation and thereby prevent leaching of lead from the slag. The EPA is currently engaged in a five-year revied of the Site, which is required by the Suprfund program (Cmprehensive Environmental Response, Canpensation, and Liability Act (CERCLA)). This review process will consider the status of the Site and the risks represented by the hazardous materials (slag) that remain on the Site. In this context, other more permanent remedies such as encapsulation, fixation or excavation and ?33nOval may be considered by the EPA and the MPCA. An alternative permanent remedy which merits consideration is the recovery of lead and other metals (Cd, As) from the slag. Technical, regulatory and economic barriers must be overcane in order to make the recovery of lead from the slag a feasible alternative.
Technical barriers to the recovery of lead from the slag revolve around the iieterogeneous nature of the slag and the variable iron, lead and silica content of the slag. The U.S. Bureau of Mines, through its Environmental Technology Research Program, has investigated the possibility of recovering lead from mill tailings in Missouri. Ap~roaches such as an air spar- wra=yclone s h promise and may also permit recovery of lead from slag. Another technique, utilizing a flame reactor process, has been evaluated by the EPA in its Superfund Innovate Technology Evaluation (SITE) Program.
J
Regulatory barriers to the recovery of lead fran the slag are related to theinvolvanent of tvo major statutory authorities. The Resource Conservation andRecovery Act (ICRA) and CEIW may both regulate this type of rredial activityat the Site. Definitions of hazardous waste and cleanup standards are cauplexissues in this context and must be clarified in order to make recovery of leadfrau the slag a feasible permanent raedy.
Econcniic barriers to the recovery of lead fran the slag at the Site aresimilar to those that axe faced during the cievelopnent of an ore deposit. Thecosts for excavation, rancval and transportation of the slag to a smelter willprob&1y be substantial given the volume, distribution and heterogeneous natureof the material. The metallurgical problans in separating the lead fran theslag may be significant. The econanic incentive for conç'leting this type ofpermanent raTedial action is the prospect of industr±al/camercial developnentof the Site.
J
J
J
Li
Li
LI
Li
J22
I Regulatory barriers to the recovery of lead from the slag are related to the
involvement of two major statutory authorities. The Resource Conservation and Recovery Act (m) and CERCIA may both regulate this type of remedial activity at the Site. Definitions of hazardous waste and cleanup standards are complex issues in this context and mast be clarified in order to make recovery of lead from the slag a feasible permanent remedy. I
1- Econdc barriers to the recovery of lead from the slag at the Site are
shilar to those that axe faced during the -1-nt of an ore deposit. The 1 - costs for excavation, removal and transprtatim of the slag to a melter will probably be substantial given the volume, distribution and heterogeneous nature of the material. The metallurgical problems in separating the lead from the slag may be significant. The economic incentive for completing this type of permanent remedial action is the prospect of industrial/commercial development of the Site.
PALEOMAGNETISM OF CENTRAL WISCONSIN DIKE SWARM:CONSTRAINTS ON THERMOMECHANICAL MODEL OF MIDCONTINENT RIFT
Lung S. C/ianDepartment of Geology
University of Wisconsin - Eau Claire
Five mafic dikes in central Wisconsin were sampled for paleomagnetic determinations. The paleopolesobtained reveals more than one dike intrusion episode. The preliminary paleomagnetic results canbe divided into two groups. The mafic dikes in Marathon City, Big Falls Park, and Little Falls Parkin Eau Claire County yield a cluster of paleopoles that lie near the 1.1 Ga paleomagnetic pole of theNorth American craton. The paleopoles of the dikes in Lake Wissota and Jim Falls in ChippewaCounty are about 1.0 Ga. The lack of intermediate poles implies episodic dike intrusions. Similarly,Keweenawan dike swarms and volcanic units in the Lake Superior area also show an episodic nature.The determination of the intrusion ages is conducive to our understanding of the thermomechanicalstate of the midcontjnent rift. The dike swarm is located about 200 km from the rift axis. Therecurrence of dike intrusion activities at such a far distance from the rift axis implies oscillationbetween strong tensile stress during the intrusive stages and weak tensile or compressional stressduring the inter-intrusive stages. Such variation in stress likely resulted from fluctuation in magmaticactivity along the rift axis.
23
PALEOMAGNETISM OF CENTRAL WISCONSIN DIKE SWARM: CONSTRAINTS ON THERMOMECHANICAL MODEL OF MIDCONTINENT RIFT
Lung S. Chan Department of Geology
University of Wisconsin - Eau Claire
Five mafic dikes in central Wisconsin were sampled for paleomagnetic determinations. The palwpoles obtained reveals more than one dike intrusion episode. The preliminary palwmagnetic results can be divided into two groups. The mafic dikes in Marathon City, Big Falls Park, and Little Falls Park in Eau Claire County yield a cluster of palmpoles that lie near the 1.1 Ga paleomagnetic pole of the North American craton. The palwpoles of the dikes in Lake Wissota and Jim Falls in Chippewa County are about 1.0 Ga. The lack of intermediate poles implies episodic dike intrusions. Similarly, Keweenawan dike swarms and volcanic units in the Lake Superior area also show an episodic nature. The determination of the intrusion ages is conducive to our understanding of the thennomechanical state of the midcontinent rift. The dike swarm is located about 200 km from the rift axis. The recurrence of dike intrusion activities at such a far distance from the rift axis implies oscillation between strong tensile stress during the intrusive stages and weak tensile or compressional stress during the inter-intrusive stages. Such variation in stress likely resulted from fluctuation in magmatic activity along the rift axis.
AEROMAGNETIC SURVEYING PROGRAM IN MINNESOTA: PAST AND FUTUREPERSPECTIVES
CHANDLER, VAL W., Minnesota Geological Survey, 2642 University Avenue, St. Paul. MN 55114 jThe high-resolution aemmagnetic surveying program of the Minnesota Geological Survey (MGS),
which began in 1979 with support from the Legislative Commission on Minnesota Resources (LCMR). willbe completed in 1991. In addition to the LCMR-sponsored data, data have been contributed by the U.S.Geological Survey and the Geological Survey of Canada for north-central Minnesota and by USXCorporation for southwestern Minnesota When complete, the statewide aemmagnetic data base will be theonly one of its kind in North America to encompass a relatively large area with such detail. All surveying todate has maintained nearly the same specifications, the key ones being close flight-line spacing (400—500meters) and low terrain clearance (150—200 meters). In the present survey over southeastern Minnesota,some data can be flown at 10(X)-meter spacing, owing to burial-related smoothing of anomalies fromPaleozoic strata.
The data, which have already served a broad spectrum of economic and scientific interests, areavailable as 1:24,000-scale contour maps, 1:250,000-scale contour and color maps, and as digital tapes offlight-line and gridded data.
Use of the aeromagnetic data in conjunction with test drilling has led to several major revisions of thePrecambrian geology of Minnesota, most of which lies concealed beneath a thick mantle of Pleistoceneglacial materials. Inverse modeling of aeroinagnetic anomalies and Poisson analysis of gravity andmagnetic data have assisted significantly in a recent reinterpretation of the Early Proterozoic Penokeanorogen in east-central Minnesota. Aemmagnetic data have been consistently used in interpreting thepartially exposed Archean greenstone-granite tenane of north-central Minnesota. Derivative-enhancedaeromagnetic and gravity data have been used to interpret the geology of the poorly exposed central part ofthe Middle Protcrozoic Duluth Complex of northeastern Minnesota. In northern and central Minnesota,aeromagnetic data are being used to investigate a major dike swann of Early Pmterozoic age. The —j
Minnesota Geological Survey plans further interpretive wo& in the Archean greenstone-granite terrane ofnorthwestern Minnesota, the Archean gneiss terrane of southwestern Minnesota, and the Middle ProterozoicDuluth Complex.
Looking to the future, several major tasks related to aeromagnetic surveying remain. In a recentworkshop hosted by the MOS and sponsored by the L.CMR, several recommendations were made forfuture geophysical woit in the state. Among the recommendations pertinent to aeromagnetic surveyingwere (1)10 accelerate processing and interpretation of aeromagnetic data in conjunction with test drilling andgeologic mapping; (2) to make the digital aeromagnetic data available in more diverse media than presentlyavailable, with emphasis on PC-based technology; and (3) to expand the MGS physical-property data baseon Minnesota's bedrock and glacial sediments. The MGS plans to pursue these recommendationsaggressively.
In the broader perspective of the Midcontinent itgion, much remains to be done. In most areas theaeromagnetic coverage is simply inadequate, especially with regard to flight-line spacing versus depth tobasement. The tremendous success of the aemmagnetic surveying program in Minnesota presents a strongincentive for other state or federal agencies woiking in the Midcontinent region to pursue similar programs.
24
PERSPECTIVES
CHANDLER, VAL W., Minnesota Geological Survey, 2642 University Avenue, St. Paul, MN 551 14 I \ The high-resolution aemmagnetic surveying pmgram of the Minnesota Geological Survey (MGS),,
which began in 1979 with support from the Legislative Commission on Minnesota Resources (LCMR), will be completed in 1991. In addition to the LCMR-sponsored data, data have been contributed by the U.S. Geological Survey and the Geological Survey of Canada for north-central Minnesota and by USX Corporation for southwestern Minnesota When complete, the statewide aemmagnetic data base will be the only one of its kind in North America to encompass a relatively large area with such detail. All surveying to date has maintained nearly the same specifications, the key ones being close flight-line spacing (400-500 meters) and low terrain clearance (150-200 meters). In the present survey over southeastern Minnesota, some data can be flown at 1000-meter spacing, owing to burial-related smoothing of anomalies from Paleozoic strata. I
The data, which have already served a bmad spectiurn of economic and scientific interests, are available as 1:24,000-scale contour amps. 1:250,000-scale contour and color maps, and as digital tapes of flight-line and gridded data.
Use of the aeromagnetic data in conjunction with test drilling has led to several major revisions of the Precambrian geology of Minnesota, most of which lies concealed beneath a thick mantle of Pleistocene glacial materials. Inverse modeling of aeromagnetic anomalies and Poisson analysis of gravity and magnetic data have assisted significantly in a recent reinterpretation of the Early Proterozoic Penokean I omgen in east-central Minnesota. Aemmagnetic data have been consistently used in interpreting the , partially exposed Archean greenstone-granite terrane of north-central Minnesota. Derivative-enhanced aeromagnetic and gravity data have been used to interpret the geology of the poorly exposed central part of the Middle Proterozoic Duluth Complex of northeastern Minnesota. In northern and central Minnesota, aeromagnetic data are being used to investigate a major dike swarm of Early Protemzoic age. The Minnesota Geological Survey plans farther interpretive work in the Archean greenstone-granite terrane of northwestern Minnesota, the Archean gneiss terrene of southwestern Minnesota, and the Middle Proterozoic Duluth Complex.
I Looking to the future, several major tasks related to aeromagnetic surveying remain. In a recent , workshop hosted by the MGS and sponsored by the LCMR, several recommendations were made for future geophysical work in the state. Among the recommendations pertinent to aemmagnetic surveying were (1) to accelerate processing and interpretation of aeromagnetic data in conjunction with test drilling and geologic mapping; (2) to make the digital aeromagnetic data available in more diverse media than presently available, with emphasis on PC-based technology; and (3) to expand the MGS physical-property data base on Minnesota's bedrock and glacial sediments. The MGS plans to pursue these recommendations aggressively. I
In the broader perspective of the Midcontinent region, much remains to be done. In most areas the ' aemmagnetic coverage is simply inadequate, especially with regard to flight-line spacing versus depth to basement. The tremendous success of the aemmagnetic surveying pmgram in Minnesota presents a strong incentive for other state or federal agencies working in the Midcontinent region to pursue similar programs.
CONTINUOUS STRIKE-SLIP FAULT-EN ECHELON FRACTURE ARRAYS INDEFORMED ARCHEAN ROCXS: IMPLICATIONS FOR FAULT PROPAGATIONb CHAN IC $
John P. Craddock and Andrew Moshoian, Geology Department,Macalester College, St. Paul, MN 55105
AbstractA regional strike—slip and en echelon fracture array
(orientation: N—B, 90) is observed in three dimensions inArchean rocks in northern Minnesota, northern Michigan, andwestern Wisconsin. Distinct fault planes (Mode I) changealong strike into single or conjugate en echelon fracturearrays (Mode II) resulting in an undulating fault trace withan average amplitude of —5 cm and wavelength of —1 in. Bothdextral and sinistral displacements are found in the sameoutcrop, and fault displacements change along strike.Brittle en echelon shear sense indicators generallycompliment the observed fault displacement, but not always.
Mode I and II fractures cut and cross—cut each other andare thus contemporaneous features, indicating that the ModeI extensional fractures became strike—slip faults whereinitially the macimum principal compressive stress (Cl) wasparallel to the strike-slip fault. Mode II en echelon setscan be used as shear sense indicators for the strike—slipfaults however, displacements along individual Mode IIfracture planes are opposite that of the strike—slip fault.The undulating fault planes resulted from the interactionbetween propagating Mode I fractures and Mode II en echelonshear fracture pods, both of which are three dimensionallycontinuous and locally out—of—phase. Timing constraints onthese fault systems, which are regionally distributed inArchean rocks of the Great Lakes region, indicate a post—2.613? and pre-1.l BY age for this deformation.
25
CONTINUOUS STRIKE-SLIP FAULT-EN ECHELON FRACTURE ARRAYS IN DEFORMED ARCHEAN ROCKS: IMPLICATIONS FOR FAULT PROPAGATION MECHANICS
I John P. craddock and Andrew Moshoian, Geology Department, Macalester College, St. Paul, MN 55105
AbsmaGL A regional strike-slip and en echelon fracture array
(orientation: N-S, 90) is observed in three dimensions in Archean rocks in northern Minnesota, northern Michigan, and western Wisconsin. Distinct fault planes (Mode I) change along strike into single or conjugate en echelon fracture arrays (Mode 11) resulting in an undulating fault trace with an average amplitude of -5 cm and wavelength of -1 m. Both dextral and sinistral displacements are found in the same outcrop, and fault displacements change along strike. Brittle en echelon shear sense indicators generally compliment the observed fault displacement, but not always.
Mode I and I1 fractures cut and cross-cut each other and are thus contemporaneous features, indicating that the Mode I extensional fractures became strike-slip faults where initially the maximum principal compressive stress (0-1) was parallel to the strike-slip fault. Mode I1 en echelon sets can be used as shear sense indicators for the strike-slip faults however, displacements along individual Mode I1 fracture planes are opposite that of the strike-slip fault. The undulating fault planes resulted from the interaction between propagating Mode I fractures and Mode I1 en echelon shear fracture pods, both of which are three dimensionally continuous and locally out-of-phase. Timing constraints on these fault systems, which are regionally distributed in Archean rocks of the Great Lakes region, indicate a post-2.6 BY and pre-1.1 BY age for this deformation.
26
PRELIMINARY DRILL CORE STUDY OF TWO HOLES DRILLED ON THE CUYUNA IRONRANGE AND EMILY MANGANIFEROUS IRON FORMATION DISTRICT OF MINNESOTA
LINDA J. DAHL and SUSAN E. BRINK, Twin Cities Research Center, U.S. Bureau ofMines, 5629 Minnehaha Avenue South, Minneapolis, MN 55417
Two diamond drilled core holes were recently completed in manganiferous ironformations in Central Minnesota--one on the Cuyuna Range and one in the EmilyDistrict. The purposes for drilling were first to obtain drill core for geologiccharacterization and whole core leaching experiments, and subsequently to conductgeophysical logging and hydrologic flow testing. This drilling program was cost-shared by the U.S. Bureau of Mines and the University of Minnesota-MineralResources Research Center under a Bureau Cooperative Agreement and is part ofthe Bureau's ongoing research on in situ mining of critical and strategicminerals. The information gathered from these two holes will be used in theevaluation of whether or not in situ mining techniques can be utilized to recovermanganese from the manganiferous iron formations in Central Minnesota.
In situ mining involves the use of fluids to remotely solubilize andmobilize target metals in an ore deposit and transport them to the surface viarecovery wells where the fluids can be directly processed for metal recovery.The success of an in situ mining operation depends to a large degree on whetherthe target ore minerals can be selectively dissolved and whether the dissolvedtarget metal can be transported to the production wells via fluid flow channels.The degree of success is governed by the inherent chemical and physicalcharacteristics of the ore body such as the types of ore minerals, types andabundance of reactive gangue minerals, kinetics of ore and gangue mineraldissolution, ore zone permeability, proximity of ore minerals to the fluid path,and groundwater flow characteristics in both saturated and unsaturated deposits.At the microscale, the relationships between the ore and gangue minerals,particularly texture and composition, are very important. Low target metalrecoveries will occur if the leach solution cannot contact the target oreminerals regardless of how well the leach chemistry is optimized. For example,target ore minerals may be encapsulated by unreactive gangue or short circuitingof leach fluid may occur due to structural controls such as joints andmicrofractures. Also, fluid pathways may be plugged by clay swelling oraccumulation of mobilized particles. This type of information obtained througha petrographic study of preleach and postleach ore samples provides valuableguidance in predicting in situ mining metal recoveries and the feasibility ofin situ mining for a prospective ore body.
Bureau research is providing new insights into predicting in situ miningmetal recoveries. Potential problems and constraints are being identified andmodeled before field leach mining tests are initiated. As part of this research,the Bureau is conducting a geologic characterization study of the drill core fromthe Cuyuna Range and Emily District which includes: stratigraphy, gangue andore mineralogy, textural relationships, assays, detailed whole rock chemicalanalyses of selected samples, permeability and porosity, fracture analysis, andmagnetic susceptibility. A combination of laboratory geologic characterizationtechniques are being used on whole core samples to determine ore/gangue andstructural relationships of preleach and postleach material. These techniquesinclude petrographic analysis of polished thin sections, scanning electronmicroscope imaging, electron microprobe analyses, and X-ray diffraction.
PRELIMINARY DRILL CORE STUDY OF TWO HOLES DRILLED ON THE CUYUNA IRON RANGE AND EMILY MANGANIFEROUS IRON FORMATION DISTRICT OF MINNESOTA
LINDA J. DAHL and SUSAN E. BRINK, Twin Cities Research Center, U.S. Bureau of Mines, 5629 Minnehaha Avenue South, Minneapolis, MN 55417
Two diamond drill ed core holes were recently completed in manganiferous iron formations in Central Minnesota--one on the Cuyuna Range and one in the Emily District. The purposes for drilling were first to obtain drill core for geologic characterization and whole core leaching experiments, and subsequentlyto conduct geophysical logging and hydrologic flow testing. This drilling program was cost- shared by the U.S. Bureau of Mines and the University of Minnesota-Mineral Resources Research Center under a Bureau Cooperative Agreement and is part of the Bureau's ongoing research on in situ mining of critical and strategic minerals. The information gathered from these two holes will be used in the evaluation of whether or not in situ mining techniques can be utilized to recover manganese from the manganiferous iron formations in Central Minnesota.
In situ mining involves the use of fluids to remotely solubilize and mobilize target metals in an ore deposit and transport them to the surface via recovery wells where the fluids can be directly processed for metal recovery. The success of an in situ mining operation depends to a large degree on whether the target ore minerals can be selectively dissolved and whether the dissolved target metal can be transported to the production wells via fluid flow channels. The degree of sulccess is governed by the inherent chemical and physical characteristics of the ore body such as the types of ore minerals, types and abundance of reactive gangue minerals, kinetics of ore and gangue mineral dissolution, ore zone permeability, proximity of ore minerals to the fluid path, and groundwater flpw characteristics in both saturated and unsaturated deposits. At the microscale, the relationships between the ore and gangue minerals, particularly texture and composition, are very important. Low target metal recoveries will occur if the leach solution cannot contact the target ore minerals regardless of how well the leach chemistry is optimized. For example, target ore minerals may be encapsulated by unreactive gangue or short circuiting of leach fluid may occur due to structural controls such as joints and microfractures. Also, fluid pathways may be plugged by clay swelling or accumulation of mobilized particles. This type of information obtained through a petrographic study of preleach and postleach ore samples provides valuable guidance in predicting in situ mining metal recoveries and the feasibility of in situ mining for a prospective ore body.
Bureau research is providing new insights into predicting in situ mining metal recoveries. Potential problems and constraints are being identified and modeled before field leach mining tests are initiated. As part of this research, the Bureau is conducting a geologic characterization study of the drill core from the Cuyuna Range and Emily District which includes: stratigraphy, gangue and ore mineralogy, textural re1 ationships, assays, detailed whole rock chemical analyses of selected samples, permeability and porosity, fracture analysis, and magnetic susceptibility. A combination of laboratory geologic characterization techniques are being used on whole core samples to determine ore/gangue and structural relationships of preleach and postleach material. These techniques include petrographic analysis of polished thin sections, scanning electron microscope imaging, electron microprobe analyses, and X-ray diffraction.
Hole G-1 from the Cuyuna Range was drilled on the National SteelCorporation's former Gloria underground mine site near Ironton, MN, located inthe northwest 1/4 of the southeast 1/4 of the southeast 1/4 of Section 28,Township 47N, Range 29W, Crow Wing County. This hole was drilled to a depth(along the incline) of 1200 ft at an inclination of 55 degrees to the north-northwest. Hole E-1 was drilled on public land near Emily, MN, and located inthe southeast 1/4 of the southwest 1/4 of the northwest 1/4 of Section 21,Township 138N, Range 26W, Crow Wing County. This hole was vertically drilledto a depth of 787 ft.
The following descriptions of the stratigraphy from each hole are basedsolely upon hand sample observations. Detailed petrographic analyses arecurrently ongoing.
HOLE G-1
Depth, ft (along the incline)0 - 85 Glacial surficial material
85 - 202 Rabbit lake Formation - Dark to medium-dark gray carbonaceousargillite, locally mineralized with < 1-2% marcasite and pyrite,contains minor 0.2-mm to 15-cm thick clastic beds which are variablyhematitic.
202 - 1167 Trommald Iron Formation
202 - 438 Thick-bedded facies - Massive and mottled appearing beds(2-5 cm and up to 1.5 m) of intermixed goethite, limonite, andhematite with locally occurring thin-bedded clastics (< 1 mmthick) and massive to thin-bedded chert. Manganese minerali-zation begins at 243 ft.
438 - 659 Mixed thick- and thin-bedded facies - Massive to mottled tothin-bedded iron formation consisting of interbedded goethite,limonite, hematite, and manganese oxides with minor locally-occurring argillized clastics and oolitic chert.
659 - 1167 Thin-bedded facies - Oxidized interval (659-922 ft) consistsof alternating thin beds (0.2-5 mm and up to 1 cm thick) ofhematitic/goethitic iron formation, argillizedclastics,chert,and magnetite. The transition from oxidized to reduced ironformation occurs from 922-943 ft. Reduced iron formation (943-1167 ft) consists of very fine-grained, thinly-bedded (0.5mm -2 cm thick) alternating dark to light gray to dark greenishgray bands. The unit contains minor thin (0.5 mm - 3.5 cm)
chert and clastic interbeds. Magnetite beds are partiallyoxidized to hematite above 913 ft.
1167 - 1200 Mahnomen Formation - Medium to light gray, massive to weakly bedded,sericite-quartz semischist with � 1% dark mafic porphyroblasts thatare 0.25-1 mm in diameter.
Hole G-1 was drilled along cross section A-A" on Plate 3 described in"Geology and Ore Deposits of the Cuyuna North Range, Minnesota" by Robert GordonSchmidt, U.S. Geological Survey Professional Paper 407, 1963, 96 pp. Schmidtsuggested that the contact between the Trommald Iron Formation and the underlying
27
Hole G-1 from the Cuyuna Range was drilled on the National Steel Corporation's former Gloria underground mine site near Ironton, MN, located in the northwest 1/4 of the southeast 1/4 of the southeast 1/4 of Section 28, Township 47N, Range 29W, Crow Wing County. This hole was drilled to a depth (along the incline) of 1200 ft at an inclination of 55 degrees to the north- northwest. Hole E-1 was drilled on public land near Emily, MN, and located in the southeast 1/4 of the southwest 1/4 o f the northwest 1/4 of Section 21, Township 138N, Range 26W, Crow Wing County. This hole was vertically drilled to a depth of 787 ft.
The following descriptions of the stratigraphy from each hole are based solely upon hand sample observations. Detailed petrographic analyses are currently ongoing.
HOLE G-I
Deoth, ft (along the incline) 0 - 85 Glacial surficial material
85 - 202 Rabbit Lake Formation - Dark to medium-dark gray carbonaceous argillite, locally mineralized with < 1-2% marcasite and pyrite, contains minor 0.2-mm to 15-cm thick clastic beds which are variably hematitic.
202 - 1167 Trommald Iron Formation
202 - 438 Thick-bedded facies - Massive and mottled appearing beds (2-5 cm and up to 1.5 m) of intermixed goethite, limonite, and hematite with locally occurring thin-bedded clastics (< 1 mm thick) and massive to thin-bedded chert. Manganese minerali- zation begins at 243 ft.
q38 - 659 Mixed thick- and thin-bedded facies - Massive to mottled to thin-bedded iron formation consisting of interbedded goethite, limonite, hematite, and manganese oxides with minor locally- occurring argillized clastics and oolitic chert.
659 - 1167 Thin-bedded facies - Oxidized interval (659-922 ft) consists of alternating thin beds (0.2-5 mm and up to 1 cm thick) of hematitic/goethiticironformation,argillizedclastics,chert, and magnetite. The transition from oxidized to reduced iron formation occurs from922-943 ft. Reduced iron formation (943- 1167 ft) consists of very fine-grained, thinly-bedded (0.5 mm - 2 cm thick) alternating dark to light gray to dark greenish gray bands. The unit contains minor thin (0.5 mm - 3.5 cm) chert and clastic interbeds. Magnetite beds are partially oxidized to hematite above 913 ft.
1167 - 1200 Mahnomen Formation - Medium to light gray, massive to weakly bedded, sericite-quartz semischist with 7% dark mafic porphyroblasts that are 0.25-1 mm in diameter.
Hole G-1 was drilled along cross section A-A" on Plate 3 described in Geology and Ore Deposits of the Cuyuna North Range, Minnesota" by Robert Gordon Schmidt, U.S. Geological Survey Professional Paper 407, 1963, 96 pp. Schmidt suggested that the contact between the Trommald Iron Formation and the underlying
27
Mahnomen Formation was folded. Using this interpretation, hole 6-1 should havecontacted the Mahnomen Formation at approximately 760 ft depth along a 65-degreeincline. Hole 6-1 located the contact between these two formations at 1167 ftwhich is approximately 400 ft lower than proposed by Schmidt, thereby provingthat the proposed fold is not present. Also, the oxidized zone in the upper partof the Trommald Formation extends deeper (about 800 ft below bedrock surface)than previously described by Schmidt.
HOLE E-1
Depth, ft0 - 169 Glacial surficial material
169 - 321 Ferruginous graywacke - Light to medium-gray, well-bedded (1-5 mm)clastics consist of fine silt- to medium sand-sized grains of quartz,argillized feldspar, and disseminated hematite in a clay-rich matrix.Irregular, mottled-appearing iron oxide staining loosely followsbedding and fractures in 1 mm to 5 cm bands. Argillization variesfrom slight at the top of the bedrock (221-305 ft) to extreme at thebottom of the unit (310-321 ft).
321 - 437 Cherty iron formation - Consists of massive white chert with dis-seminated clots (1-5 mm diameter) of hematite and goethite inter-bedded with predominantly massive to thinly bedded hematitic ironformation. Towards the top of the unit, the chert is thinly andirregularly bedded (321-328 ft). Thin, argillized clastic beds(1-10 mm) are interbedded with iron formation from 337-350 ft.
437 - 734 Manganiferous iron formation - From 437-584 ft, the unit consists ofhematitic, manganiferous, thin-bedded to massive iron formation withminor oolitic to massive chert interbeds. Manganese oxides occur inzones intermixed with hematite and as beds and irregular lenses(1 mm - 3 cm thick). From 584-734 ft. the unit consists of thin tothickly (1 mm - 10 cm) interbedded hematitic and goethitic ironformation, chert, and manganese oxide-rich beds with intervals whereiron formation, chert, or manganese oxides predominate. J
734 - 761 Manganiferous chert - Oolitic granule-rich chert. Sooty manganeseoxides locally replace oolites and the matrix as irregular vuggypatches. A medium grey, massive chert bed (80 cm thick) occurs at765 ft.
761 - 763 Argillized thin-bedded slate - Argillized, finely bedded (1 mm - j2 cm) slate consists of medium- to fine-grained clastics withdisseminated hematite pseudomorphs after magnetite octahedra.Localized iron-oxide staining generally follows bedding. j
763 - 767 Manganiferous chert - Same as 734-761 ft.
767 - 787 Argillized thin-bedded slate - Same as 761-763 ft.
The stratigraphy of hole E-1 identified that the dip of the sedimentarylayers in this area flatten to the north. The information from this hole wasadded to the geologic database of holes previously drilled (approximately 40
years ago) to further define the stratigraphy of the Emily district.
28
Mahnomen Formation was folded. Using this interpretation, hole G-l should have contacted the Mahnomen Formation at approximately 760 ft depth along a 65-degree incline. Hole G-l located the contact between these two formations at 1167 ft which is approximately 400 ft lower than proposed by Schmidt, thereby proving that the proposed fold is not present. Also, the oxidized zone in the upper part of the Trommald Formation extends deeper (about 800 ft below bedrock surface) than nreviouslv described bv Schmidt.
Deoth. ft 0 - 169 Glacial surficial material
169 - 321 Ferruainous aravwacke - Light to medium-gray, well-bedded (1-5 mm) elastics consist of fine silt- to medium sand-sized grains of quartz, araill ized feldsoar. and disseminated hematite in a clay-rich matrix. irregular, mottled-appearing iron oxide staining loosely follows bedding and fractures in I mm to 5 cm bands. Argillization varies from slight at the top of the bedrock (221-305 ft) to extreme at the bottom of the unit (310-321 ft).
Chertv iron formation - Consists ot massive white chert wiin dis- seminated clots (1-5 mm diameter) of hematite and goethite inter- bedded with predominantly massive to thinly bedded hematitic iron formation. Towards the top of the unit, the chert is thinly and irregularly bedded (321-328 ft). Thin, argillized clastic beds (1-10 mm) are interbedded with iron formation from 337-350 ft
437 - 734 Manaaniferous iron formation - From 437-584 ft, the unit consists of hematitic, manganiferous, thin-bedded to massive iron formation with minor oolitic to massive chert interbeds. Manganese oxides occur in zones intermixed with hematite and as beds and irregular lenses (1 mm - 3 cm thick). From 584-734 ft, the unit consists of thin to thickly (1 mm - 10 cm) interbedded hematitic and goethitic iron formation, chert, and manganese oxide-rich beds with intervals where iron formation, chert, or manganese oxides predominate.
734 - 761 Manqaniferous chert - Oolitic granule-rich chert. Sooty manganese oxides locally replace oolites and the matrix as irregular vuggy patches. A medium grey, massive chert bed (80 cm thick) occurs at 765 ft.
761 - 763 Araillized thin-bedded slate - Argillized, finely bedded (1 mm - 2 cml slate consists of medium- to fine-drained elastics with - , - - ~- - ~~
, , disseminated hematite pseudomorphs aftermagnetite octahedra Localized iron-oxide staining generally follows bedding.
763 - 767 Manaaniferous chert - Same as 734-761 ft.
767 - 787 Araillized thin-bedded slate - Same as 761-763 ft. The stratigraphy of hole E-1 identified that the dip of the sedimentary
layers in this area flatten to the north. The information from this hole was added to the geologic database of holes previously drilled (approximate1 y 40 years ago) to further define the stratigraphy of the Emily district.
GEOLOGIC SETTING OF THE EARLY PROTEROZOICBASE- AND PRECIOUS-METAL-RICH METAVOLCANIC BELT
OF WISCONSIN
Theodore A. DeMatties, Ernest K. Lehmann & Associates, Inc., 430First Avenue North, Minneapolis, Minnesota 55401
M.G. Mudrey, Jr., Wisconsin Geological and Natural HistorySurvey, 3812 Mineral Point Road, Madison, Wisconsin 53705
Since the late 1960s, measured and inferred resources ofover 100 million short tons of volcanogenic massive—sulfidemineralization have been discovered in northern Wisconsin. The
largest deposit, near Crandon, was identified by Exxon Mineralsand contains approximately 75 million short tons of ore averaging5% zinc, 1.1% copper, and 0.4% lead. The most recent discoverywas made by Noranda Exploration Inc. at Lynne, in Oneida County.One of their discovery holes contained a 128—foot interceptgrading 22.7% zinc, 0.64% copper, and 2.95% lead. Significantprecious metal values are associated with many of the knowndeposits.
The deposits occur within the Early Proterozoic Penokeanfold belt (1850—1900 Ma) which has been divided by Greenberg andBrown (1983) into two major terranes: the northern terranesupported by a supra—crustal sequence deposited on the Archeanbasement (Sims' Continental Margin assemblage; Sims and others,1989) and a southern terrane composed of volcanogenic rock andEarly Proterozoic granite intrusives. This southern terrane, the
Wisconsin Maginatic Terrane (also known as the Wisconsin Penokeanvolcanic belt) , is characterized by an island—arc basinassemblage containing abundant calc—alkaline metavolcanic unitsand associated lesser amounts of metasedimentary rock. Sims and
others (1989) have further divided the Wisconsin Magnetic Terraneinto two volcanic arc terranes on the basis of lithology andstructure; they include the northern Pembine—Wausau terrane and
29
a
GEOLOGIC SETTING OF THE EARLY PROTEROZOIC I BASE- AND PRECIOUS-METAL-RICH METAVOLCANIC BELT
OF WISCONSIN . 3
Theodore A. DeMatties, Ernest K. Lehmann & Associates, Inc., 430 First Avenue North, Minneapolis, Minnesota 55401 I M.G. Mudrey, Jr., Wisconsin Geological and Natural History Survey, 3812 Mineral Point Road, Madison, Wisconsin 53705
.. , "... . t d . . . . Since the late 1960s, measured and inferred resources ofF
over 100 million short tons of volcanogenic massive-sulfide'
mineralization have been discovered in northern Wisconsin. -%he largest deposit, near Crandon, was identified by Exxon Minerals
and contains approximately 75 million short tons of ore averaging
5% zinc, 1.1% copper, and 0.4% lead. The most recent discovery
was made by Noranda Exploration Inc. at Lynne, in Oneida County.
One of their discovery holes contained a 128-foot intercept
grading 22.7% zinc, 0.64% copper, and 2.95% lead. Significant
precious metal values are associated with many of the known
deposits.
I The deposits occur within the Early Proterozoic Penokean
fold belt (1850-1900 Ma) which has been divided by Greenberg and
Brown (1983) into two major terranes: the northern terrane
supported by a supra-crustal sequence deposited on the ~ r c h e a n
basement (Sims' Continental Margin assemblage; Sims and others
1989) and a southern terrane composed of volcanogenic rock and 1
Early Proterozoic granite intrusives. This southern terrane, the
Wisconsin Magmatic Terrane (also known as the Wisconsin Penokean
volcanic belt), is characterized by an island-arc basin
assemblage containing abundant calc-alkaline metavolcanic units
and associated lesser amounts of metasedimentary rock. Sims and
others (1989) have further divided the Wisconsin Magnetic Terrane
into two volcanic arc terranes on the basis of lithology and
structure; they include the northern Pembine-Wausau terrane and
J
the Marshfield terrane to the south. The three terranes are
believed to be separated from one another by major paleosuture
zones (Niagara Fault, Eau Pleine Shear Zone)
The Pembine—Wausau arc sequence is the focus of base— and
precious—metal exploration activity that has resulted in the
discovery of a number of massive—sulfide deposits and occur-
rences. The dominant volcanic complex in this terrane, based on
regional gravity and magnetic data, has been referred to
informally as the "Ladysmith—Rhinelander greenstone belt". At
its western end, the complex can be further subdivided into three
major regional units, based on geophysics and known lithology.
They include a central volcanic—arc sequence that defines the
structural core of the complex, a marginal back—arc basin
sequence, and a number of major felsic centers (DeMatties, 1989).
These units may correlate with LaBerge and Myers' (1984)
amphibole—greenschist succession in the Wausau and Eau ClaireJareas.
Within this geologic framework, three distinct geologic
environments appear to host massive—sulfide mineralization: (1)
syngenetic strata—bound and stratiform sulfide mineralization Jwithin, along the flanks of, or near the stratigraphic top of the
felsic centers; syngenetic strata—bound and stratiform
massive—sulfide mineralization associated with cherty magnetic
iron-formation and located in the main volcanic arc sequence; and
epigenetic stringer—sulfide mineralization and syngenetic
strata—bound and stratiform massive—sulfide mineralization
associated with mafic piles within the back—arc basin sequence.
Based on our present knowledge of the region, the three defined
host environments and their association with meta-argillite jformations appear to be major regional features controlling the
localization of metal—bearing massive—sulfide mineralization inj
30
the Marshfield
believed to be
zones (Niagara
terrane to the south. The three terranes are 1 separated from one another by major paleosuture 1 Fault, Eau Pleine Shear Zone).
The Pembine-Wausau arc sequence is the focus of base- and
precious-metal exploration activity that has resulted in the
discovery of a number of massive-sulfide deposits and occur-
rences. The dominant volcanic complex in this terrane, based on
regional gravity and magnetic data, has been referred to 1 informally as the "Ladysmith-Rhinelander greenstone belt". At 1 its western end, the complex can be further subdivided into three
major regional units, based on geophysics and known lithology.
They include a central volcanic-arc sequence that defines the
structural core of the complex, a marginal back-arc basin
sequence, and a number of major felsic centers (DeMatties, 1989).
These units may correlate with LaBerge and Myers' (1984)
amphibole-greenschist succession in the Wausau and Eau Clail
areas.
Within this geologic framework, three distinct geologi(
environments appear to host massive-sulfide mineralization: (1)
syngenetic strata-bound and stratiform sulfide mineralization
within, along the flanks of, or near the stratigraphic top of the
felsic centers; syngenetic strata-bound and stratiform
massive-sulfide mineralization associated with cherty magnetic 1 iron-formation and located in the main volcanic arc sequence; and
epigenetic stringer-sulfide mineralization and syngenetic - * 1 strata-bound and stratiform massive-sulfide mineralization
associated with mafic piles within the back-arc basin sequence.
Based on our present knowledge of the region, the three defined
host environments and their association with meta-argillite
formations appear to be major regional features controlling the
localization of metal-bearing massive-sulfide mineralization in I
the western part of the complex. Other host environments willundoubtedly be identified as exploration continues in thecomplex.
References:
DeMatties, Theodore A.,massive sulfide depositsEconomic Geology, v. 84,
1989, A proposed geologic frameworkin the Wisconsin Penokean volcanicp. 946—952.
Zone: A Lowersulfide depositp. 1908—1916.
Sins, P.R., Van Schmus, W.R., Schulz, R.J., and Peterman, Z.E.,1989, Tectono—stratigraphic evolution of the Early ProterozoicWisconsin magmatic terranes of the Penokean Orogen: CanadianJournal Earth Science, v. 26, p. 2145—2158.
Mudrey, M.G., Jr., Evans, T.J., Babcock,Jr., Eisenbrey, E.H., and LaBerge, G.L.,metallic mineral exploration in WisconsinHistories of Mineral Discoveries, v. 3, p.
R.C., Cummings, M.L.,1991, Case history of1955—1990: AIME, Case117—132.
LaBerge, C.L., and Myers, P.E., 1984, Two Early Proterozoicsuccessions in central Wisconsin and their tectonic significance:Geological Society of America Bulletin, v. 95, p. 246-253.
31
______________
1960, The Ritchie Creek MainProterozoic copper—gold volcanogenic massivenorthern Wisconsin: Economic Geology v. 85,
forbelt:
in
.. ~ . . .
the e complex. Other host environments will
undoubtedly be identified as exploration continues in the
Â¥ complex.
References:
DeMatties, Theodore A., 1989, A proposed geologic framework for massive sulfide deposits in the Wisconsin Penokean volcanic belt: Economic Geology, v. 84, p. 946-952.
, 1960, The Ritchie Creek Main Zone: A Lower Proterozoic copper-gold volcanogenic massive sulfide deposit in northern Wisconsin: Economic Geology v. 85, p. 1908-1916.
Sims, P.K., Van Schmus, W.R., Schulz, K.J., and Peterman, Z.E., 1989, Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen: Canadian Journal Earth Science, v. 26, p. 2145-2158.
Mudrey, M.G., Jr., Evans, T.J., Babcock, R.C., Cummings, M.L., Jr., Eisenbrey, E.H., and LaBerge, G.L., 1991, Case history of metallic mineral exploration in Wisconsin 1955-1990: AIME, Case' Histories of Mineral Discoveries, v. 3, p. 117-132.
LaBerge, G.L., and Myers, P.E., 1984, Two Early Proterozoic I successions in central Wisconsin and their tectonic significance: Geological Society of America Bulletin, v. 95, p. 246-253.
I.- Phillips \LVI4N?
I
/ THORNAPPLE /'1I Ladysmilh
5 II I FLAMBEAU RITCHIE
_*._______BENDI '_#— tMe c/ford
—.
RhinelanderPELJCAN RIVER
' IZ'CRANDONWOLF— —RIVER
CrandonDistrict
Major massive-sulfidedeposit or occurrence
50 miles
32
Index Map of Wisconsin Mineral Districts'-I
U
U
U
Li
Li
U
J
-J
Area of regionalgeologic map
SomoDistrict
Iron
ONTARIO
ThunderBay
EauClaire
Wausau
LadysmithDistrict
Chicago
S Population center
Green Bay•
(j
Wisconsin massive-sulfide districts
50km
:.. . . Ã Ironwood - , , . .
ONTARIO I
s
Population center
Â¥ Major massive-sulfide .- . deposit or occurrence
(1, Wisconsin massive-sulfide districts
I 50 mites
I___________1
' 50km '
, ,. .. , .- ' , Index Map of Wisconsin Mineral Districts . . .~ . ~ ..
. ~
Eflf
lflrr
flflfl
rcrr
nrr-
rrr—
',,
Sed
imen
tary
Roc
k U
nits
——
Con
tact
, bas
ed o
n ai
rbor
ne
Und
iffer
entia
ted
Cam
bria
n sa
ndst
one
lorm
atio
ns; t
hin
(.c
SO
ft) s
ands
tone
mag
netic
dat
a
— —
:.J u
nits
loca
lly c
over
ing
base
men
t met
avol
cani
c un
its n
ot s
how
n—
— P
roje
cted
or
inte
rred
fauf
t
Pb:
Hj L
ower
Pro
tero
zoic
Bar
ron
Oua
rtzi
teS
hear
zon
e
Low
er P
rote
rozo
ic (
?) in
trus
ive
Roc
k U
nits
QP
OS
ctM
etag
rani
te, q
uart
z m
etad
iorit
e,M
etag
abbr
o, a
ltere
d ul
tram
atic
AD
epos
it w
ith d
efin
ed r
eser
ves
LrLr
sJ m
etad
iolit
e an
d m
etas
yeni
tein
trus
ives
, sye
nodi
orile
.2•
Rev
erse
and
nor
mal
mag
-2
Low
er P
rote
rozo
ic M
etav
olca
nic
and
Rel
ated
floc
ksne
tized
maf
ic d
ikes
Mai
n V
olca
nic
Arc
Seq
uenc
eB
ack-
Arc
Bas
in S
eque
nce
Fei
sic
Cen
ter(
s)(K
ewee
naw
an a
ge)
____
0S
niie
s°
Pm
v —
maf
ic 1
0 ul
tram
afic
vol
cani
c-P
vs —
dom
inan
tly tu
tlace
ous
met
a-
____
__
Nv
— d
omin
antly
tels
ic
____
____
__
L\
intr
usiv
e co
mpl
ex; i
nclu
des
met
avol
-se
dim
ents
; inc
lude
s m
etag
rayw
acke
,m
elav
olca
nic
tutls
/lapi
lliI
IN
cani
c flo
ws,
inte
ritow
tufts
and
sed
i-re
wor
ked
met
atuf
is, a
nd a
ssoc
iate
dm
etat
ults
(lil
hici
crys
tal),
05k
mm
ents
, and
che
rty
iron
torm
atio
nch
emic
al m
etas
edim
ents
cher
ly m
etal
ults
, and
ass
o-I
.ci
ated
che
mic
al m
etas
edi-
LYN
NE
PR
OS
PE
CT
Piv
— d
omin
antly
inte
rmed
iate
to m
atic
Pm
vl —
dom
inan
tly in
term
edia
te to
mat
icm
ents
(m
etac
hert
)(2
'.i P
b 4
/m
etav
olca
nic
tlow
s an
d in
terb
edde
d m
eta-
\m
etav
olca
nic
flow
s an
d su
bvol
cani
c in
trus
ives
\\\\\
\\\''
9/
tufts
and
tulta
ceou
s m
etas
edim
ents
Pm
s —
gra
phiti
c, s
ulfid
e-be
arin
g m
eta-
argi
tlite
s\\
\\ C —
CLE
AR
CR
EE
KS
.. A
Y E
R .\
'#"
I-
ziV
£55±
____
_
C H
IP
P E
W A
PR
OS
PE
CT
Pm
vE
ND
PR
OJE
CT
,'Ce,
Au)
-J
Reg
iona
l geo
logi
c m
ap
J
BEND, A LOWER PROTEROZOIC, COPPER- AND GOLD-ENRICHEDVOLCANOGENIC MASSIVE-SULFIDE DEPOSIT IN
TAYLOR COUNTY, WISCONSIN
Theodore A. DeMatties and William F. Rowell, Ernest K. Lehmann &Associates, Inc., 430 First Avenue North, Minneapolis, Minnesota 55401
Exploration work conducted between 1976 and 1991 by the Jump River JJoint Venture (Wisconsin Mineral Resources Incorporated and Chevron USAInc.) has identified a potentially economic copper—gold deposit within the
I
Chequamegon National Forest in Taylor County. Wisconsin. Geophysicalfollow-up and evaluation by diamond drilling (over 30,000 feet in 25 holes) ofa single-line AEM (INPUT) anomaly identified in 1977-78 resulted in thediscovery of the Bend volcanogenic massive—sulfide deposit.
Geologically the project area lies within the west—central part of theLadysmith—Rhinelander Volcanic Complex, a member of the Lower Proterozoic
UPenokeari volcanic belt of northern Wisconsin. The copper—gold mineralizationoccurs within an upright, steeply dipping, thick (up to 2000 feet). andrelatively undistrubed felsic sequence dominated by rhyolite—rhyodacite flows,flow breccias, and fine to coarse pyroclastic tuffs and subordinate sedimen-tary rocks. At depth, this flow—pyroclastic section is intercalated with orintruded by a thick (nearly 1000 feet) series of silicic rhyolite flows whichmay be part of a larger domal structure. The felsic succession or center has LIdeveloped along the flanks of a larger volcanic complex.
Ore reserves are hosted by an altered quartz crystal rhyolite tuff(quartz—sericite schist) and have been di'C'ided into a copper—rich hanging-wallhorizon (HW) and a gold-rich footwall (FW) zone.
Two stacked stratiform massive (50—90%) to semimassive (greater than30%) sulfide lenses (the upper lens and the middle lens) at or near thestratigraphic top of the quartz crystal tuff constitute the hanging—wall Jhorizon. These lenses range from 5 to over 40 feet true thickness and
J34j
BEND, A LOWER PROTEROZOIC, COPPER- AND GOLD-ENRICHED ,.
VOLCANOGENIC MASSIVE-SULFIDE DEPOSIT IN
TAYLOR COUNTY. WISCONSIN
Theodore A. DeMatties and William F. Rowell. Ernest K . Lehrnann &
Associates, Inc. , 430 First Avenue North, Minneapolis, Minnesota 55401
Exploration work conducted between 1976 and 1991 by the Jump River
Joint Venture (Wisconsin Mineral Resources Incorporated and Chevron USA
Inc.) has identified a potentially economic copper-gold deposit within the
Chequarnegon National Forest in Taylor County, Wisconsin. Geophysical
follow-up and evaluation by diamond drilling (over 30,000 feet in 25 holes) of
a single-line AEM (INPUT) anomaly identified in 1977-78 resulted in the
discovery of the Bend volcanogenic rnassive-sulfide deposit.
Geologically the project area lies within the west-central part of the
Ladysrnith-Rhinelander Volcanic Complex, a member of the Lower Proterozoic
Penokean volcanic belt of northern Wisconsin. The copper-gold mineralizatk
occurs within an upright, steeply dipping, thick (up to 2000 feet), and
relatively undistrubed felsic sequence dominated by rhyolite-rhyodacite flow
flow breccias, and fine to coarse pyroclastic tuffs and subordinate sedimen-
tary rocks. A t depth, this flow-pyroclastic section is Intercalated with or
intruded by a thick (nearly 1000 feet) series of silicic rhyolite flows which
may be part of a larger domal structure. The felsic succession or center has
developed along the flanks of a larger volcanic complex.
Ore reserves are hosted by an altered quartz crystal rhyolite tuff
(quartz-sericite schist) and have been divided into a copper-rich hanging-wall
horizon (HW1 and a =old-rich footwall (FW) zone.
TWO stacked stratiform massive (50-YU%) to semimassive igreazer tnan
30%) sulfide lenses (the upper lens and the middle lens) at or near the
stratigraphic top of the quartz crystal tuff constitute the hanging-wall
horizon. These lenses range from 5 to over 40 feet true thickness and
contain mostly fine- to very fine-grained, granular. pyrite with varyingamounts of interstitial chalcopyrite ± tetrahedrite ± bornite ± goldtellurides. Gangue minerals include quartz, carbonate (calcite) • and sericite.Petrographic analysis indicates that the majority of the pyrite grains containvery fine to submicroscopic inclusions of chalcopyrite and bornite. Individualbeds within the lens may be fragment-bearing (altered and unaltered quartz-crystal tuff fragments) and exhibit vague to well—developed bedding/lamina-tions. Sedimentary features such as graded bedding have been observed indrill core, suggesting a syngenetic origin. Both lenses extend to subcropand are overlain by about 120 feet of glacial moraine. Near the subcropthere is evidence of oxidation and supergene enrichment. Supergene mineralsinclude chalcocite and bornite.
Each lens is partially underlain stratigraphically by a serniconformable,poorly developed stockwork—stringer or stringer like zone consisting of finecross-cutting chalcopyrite veinlets and/or pyrite ± chalcopyrite anastomasingveinlets-veins and/or fine pyrite matrix supporting altered (silicified) mediumto coarse—sized, subrounded—subangular quartz crystal tuff fragments.Locally wispy chalcopyrite stringers may overprint bedded massive sulfides.The stockwork stringer mineralization may be accompanied by weak to strongsilicification and generally weak, wispy chlorite alteration. No well—developedalteration pipe has been recognized to date. Locally these zones may carryore—grade gold values -
Both lenses are enveloped by a pyritic stockwork sulfide halo (up to 30%sulfides) which extends throughout the stratigraphic upper portions of thecrystal tuff unit and along strike an undetermined distance. The stockworkhalo consists of pyrite disseminations, bands, laminations, discontinuouscross—cutting and parallel (to foliation and bedding) veinlets and is associatedwith widespread pervasive sericitization. Stockwork—stringer mineralizationdeveloped stratigraphically below both lenses grades both vertically andlaterally into the halo.
The footwall gold zone is comprised of at least two stratiform ore—grade(plus 0.1 oz/ton) gold assay subzones ranging from less than 10 feet to over
35
contain mostly fine- to very fine-grained, granular, pyrite with varying
amounts of interstitial chalcopyrite 2 tetrahedrite 2 bornite  gold
tellurides. Gangue minerals include quartz, carbonate (calcite), and sericite.
Petrographic analysis indicates that the majority of the pyrite grains contain
very fine to submicroscopic inclusions of chalcopyrite and bornite. Individual
beds within the lens may be fragment-bearing (altered and unaltered quartz-
crystal tuff fragments) and exhibit vague to well-developed beddingllamina-
tions. Sedimentary features such as graded bedding have been observed in
drill core, suggesting a syngenetic origin. Both lenses extend to subcrop,
and are overlain by about 120 feet of glacial moraine. Near the subcrop
there is evidence of oxidation and supergene enrichment. Supergene minerals
include chalcocite and bornite.
Each lens is partially underlain stratigraphically by a semiconformable,
poorly developed stockwork-stringer or stringer like zone consisting of fine
cross-cutting chalcopyrite veinlets andlor pyrite 2 chalcopyrite anastomasing
veinlets-veins andlor fine pyrite matrix supporting altered (silicified) medium
to coarse-sized, subrounded-subangular quartz crystal tuff fragments.
Locally wispy chalcopyrite stringers may overprint bedded massive sulfides.
The stockwork stringer mineralization may be accompanied by weak to strong
silicification and generally weak, wispy chlorite alteration. No well-developed
alteration pipe has been recognized to date. Locally these zones may carry
ore-grade gold values.
Both lenses are enveloped by a pyritic stockwork sulfide halo (up to 30%
sulfides) which extends throughout the stratigraphic upper portions of the
crystal tuff unit and along strike an undetermined distance. The stockwork
halo consists of pyrite disseminations, bands, laminations, discontinuous
cross-cutting and parallel (to foliation and bedding) veinlets and is associate"
with widespread pervasive sericitization. Stockwork-stringer mineralization
developed stratigraphically below both lenses grades both vertically and
laterally into the halo.
The footwall gold zone i s comprised of at least two stratiform ore-grade
(plus 0.1 ozlton) gold assay subzones ranging from less than 10 feet to over
36
20 feet true thickness. One designated as the "tuck under" subzone isstratigraphically below the upper lens and hosted by stockwork sulfides. A
second occurs within, above and along strike of a thin lower massive sulfidelens (or group of lenses) below the 600—foot elevation. Both assay subzonesappear to have continuity along strike and down dip. Primary Cu, As, Bi,and Sb dispersion patterns developed in the stockwork halo indicate ageochemical continuity between the lower sulfide lens and the lower gold zone Jand suggest that it may widen at depth. Additional drilling is needed tofully delineate these gold subzones, Hypogene gold tellurides present include Jcalaverite (AuTe), petzite (Ag3AuTe2), and krennerite (AuAgTe4).
Widespread gold values greater than 0.01 oz/ton have been foundthroughout the stockwork sulfide halo. Higher values can form poorlydeveloped strataform assay subzones of limited down dip or lateral extent.
The geologic and geochemical data obtained thus far indicate that the JBend deposit exhibits characteristic hypogene copper—gold zoning patterns,i.e. development of copper-rich massive-sulfide lenses accompanied by several Jprominent parallel gold subzones. The isopach data indicate that the locus ofthe sulfide systems was both within and along a paleotopographic highdeveloped by a thickening of the quartz crystal tuff unit. The massive—
sulfide mineralization tends to be more fragment-bearing and disrupted nearthe top of the pile than toward its margins, where generally well—developedlaminations and beds are evident. The sulfide system is believed to beplunging in two directions. This may be the result of paleotopographiccontrol on sulfide deposition.
The presence of tetrahedrite explains the unusual geochemistry of themineralization which includes anomalous concentrations of arsenic, bismuth,and antimony. Concentrations of tellurium are also anomalously high owing tothe tellurides present.
A geologic reserve base estimate as of March 1, 1991, of drill—indicatedand -inferred reserves includes approximately 2.2 million short tons of 2.77% Jcopper, 0.05 oz/ton gold, and 0.43 oz/ton silver in the stratigraphically
j
20 feet true thickness. One designated as the Yuck undern subzone is
stratigraphically below the upper lens and hosted by stockwork sulfides. A
second occurs within, above and along strike of a thin lower massive sulfide
lens (or group of lenses) below the 600-foot elevation. Both assay subzones
appear to have continuity along strike and down dip. Primary Cu, A s , Bi,
and Sb dispersion patterns developed in the stockwork halo indicate a
geochemical continuity between the lower sulfide lens and the lower gold zone
and suggest that it may widen at depth. Additional drilling is needed to
fully delineate these gold subzones. Hypogene gold tellurides present include
calaverite (AuTe) , petzite (Ag3AuTe2), and krennerite (AuAg Te4).
Widespread gold values greater than 0 .01 ozlton have been found
throughout the stockwork sulfide halo. Higher values can form poorly
developed strataform assay subzones of limited down dip or lateral extent.
The geologic and geochemical data obtained thus far indicate that the
Bend deposit exhibits characteristic hypogene copper-gold zoning patterns,
i.e. development of copper-rich massive-sulfide lenses accompanied by several
prominent parallel gold subzones. The isopach data indicate that the locus of
the sulfide systems was both within and along a paleotopographic high
developed by a thickening of the quartz crystal tuff unit. The massive-
sulfide mineralization tends to be more fragment-bearing and disrupted near
the top of the pile than toward its margins, where generally well-developed
laminations and beds are evident. The sulfide system is believed to be
plunging in two directions. This may be the result of paleotopographic
control on sulfide deposition.
The presence of tetrahedrite explains the unusual geochemistry of the
mineralization which includes anomalous concentrations of arsenic, bismuth,
and antimony. Concentrations of tellurium are also anomalously high owing to
the tellurides present.
A geologic reserve base estimate as of March 1, 1991, of drill-indicated
and -inferred reserves includes approximately 2.2 million short tons of 2.77%
copper, 0.05 ozlton gold, and 0.43 ozlton silver in the stratigraphically
upper hanging-wall copper-rich horizon and an additional 1.5 million shorttons of 0.14 oz/ton gold, 0.26% copper, and 0.11 az/ton silver in the footwallgold subzones.
Exploration for additional reserves is continuing down—plunge and alongstrike of the productive mineralized unit; at least five high-prioritygeophysical-geologic targets have been identified thus far, two of which havebeen partially drill tested.
upper hanging-wall copper-rich horizon and an additional 1.5 million short
I tons of 0.14 ozlton gold, 0.26% copper, and 0.11 ozlton silver in the footwa
gold subzones.
I Exploration for additional reserves is continuing down-plunge and
I e of the productive mineralized unit; at least five high-priority
hysical-geologic targets have been identified thus far, two of which hav
— Phillips, 'S •IHORNAPPLE / 1
Ladysmfth 5ti SFLAMBEAU (RITCHIE
1CREEK'
BENDS '.1Med!ord 7 Major massive-sutfide
deposit or occurrence
Index Map of Wisconsin Mineral DistrictsjJ
J
-I
jJ
jjjJ
jj
Area of regionalgeoiogic map
Marquelle
SomoDistrict
ONTARIOJ
IronRiver • Crystal Fafl's J
.Eat,C/afro
Wausau
Chicago
-A
LadysmithDistrict
I'CrandonDistrict
S Population centerj
(j Wisconsin massive-sulfide districts-A
50 miles
50km
j
 Population center i y Major massive-sulfide I
deposit or occurrence I (-,Wisconsin massive-sulfide districts -
&j - - ' 50km '
Index Map of Wisconsin Mineral Districts
50 miles .
!
r r—
iri
r ci
r—
ci r
- r
r— r
r—
ci r
- rT
r—
rC
gS
edim
enta
ry R
ock
Uni
ts—
—C
onta
ct, b
ased
on
airb
orne
Und
iffer
entia
ted
Cam
bhan
san
dsto
ne fo
rmat
ions
; thi
n (<
50 It
) sa
ndst
one
mag
netic
dat
a
— —
units
loca
lly c
over
ing
base
men
t met
avoi
cani
c un
its n
ot s
how
n—
— P
roje
cted
or
infe
rred
fauf
t
Low
er P
rote
rozo
ic B
arro
n O
uart
zite
. ,...
She
ar z
one
Low
er P
rote
rozo
ic (
7) In
trus
Ive
Roc
k U
nits
0P
rosp
ect
Met
agra
nite
, qua
rtz
met
adio
rite,
.4.4
.M
etag
abbr
o, a
ltere
d ul
tram
afic
AD
epos
it w
ith d
efin
ed r
eser
ves
met
adio
rite
and
rnet
asye
nite
intn
jsiv
es, s
yeno
dior
ite2
•R
ever
se a
nd n
orm
al m
ag-
2Lo
wer
Pro
tero
zoic
Met
avol
cani
c an
d R
elat
ed R
ocks
netiz
ed m
ac d
ikes
Mai
n V
olca
nic
Arc
Seq
uenc
eB
ack-
Arc
Bas
in S
eque
nce
Fei
sic
Cen
ter(
s)(K
ewee
naw
an a
ge)
____
oS
mile
se
'p\N
Pm
v —
maf
ic to
uftr
amaf
rc v
olca
nic-
IP
vs —
dom
inan
tly tu
fface
ous
met
a-
____
___
Pfv
— d
omin
antly
fels
ic
____
____
___
2'in
trus
ive
com
plex
; inc
lude
s m
etav
ol-
sedi
men
ts: i
nclu
des
met
agra
ywac
ke.
met
avol
cani
c tu
ffs/ta
pilli
I
cani
c flo
ws,
inte
iflow
tufts
and
sed
i-re
wor
ked
met
atuf
fs, a
nd a
ssoc
iate
dm
etat
uffs
(lit
hicl
crys
tat)
,0
5km
N m
ents
, and
che
rty
iron
form
atio
nch
emic
al r
neta
sedi
men
tsch
erly
net
atut
fs, a
nd a
sso-
I-
ciat
ed c
hem
ical
nie
tase
di-
LYN
NE
PR
OS
PE
CT
men
ts (
met
ache
n)P
AA
9)/
tufts
and
tuffa
ceou
s m
etas
edim
ents
Pm
s —
gra
phiti
c, s
ulfid
e'an
g m
eta-
argi
llite
s-
-,C
TH
UN
DE
RR
IVE
' EC
LEA
R C
RE
EK
SA
W Y
E R
Pm
v•'#
J\'P
R0S
PE
CT
.\\4
W\
L
___
____
___
_ __
,,
\pm
vP
vszv
it/_
7I
--:
,4(\'f
Pvs
"' / I
cD
'fl-"in
i" / i
'/5
-g' "
'a'
.-,
/P
nwf
ø'fl
IJ\
\\\',1
/X'-\
__'
jS
PIR
ITJ
::Css
: \J-
\-.c
yt,'
I\\\'
V\'A
c.')-
7-V
c, '/
\P
RO
SP
EC
Ta
-
-:-:
/<i
Pvs
(',','
''' ',I
JK'\
'i;I
I''—
,., •
.—.
••_2
'
____
__ _
____
_ __
____
____
__¼
L,,
'-'P
ry'?
': ;"
'—_.
-'.—
'.—' .4.7
,c-y
--(Z
n,P
b,C
u,A
u) 0
____
___
_-
PbL
C '#
t(',
'.'—
\)--
_...
..4.
,2
(LLS
CH
OO
LHO
US
Ep
FM
-<
,f4R
—
C H
IP
P E
W A
PR
OS
PE
CT
Pm
vj>
..P
RO
JEC
T(c
u,A
a)-J
Reg
iona
l geo
logi
c m
ap
0
Pi
A
Depo
sit w
ith d
elin
etlff
lser
ves
Back
-Arc
Bas
in S
eque
nn
.. Fe
lslc
Cen
ler(s
)
Pmv - mn
k to
ultra
mat
ic vo
lcanic
- in
tiusi
ve co
mple
x: in
clude
s meta
vo!-
maa
tufls
(rih
iisl
al),
c@
.Bows
. in
terf
low
lulls
and sedi
. m
enls
. and
dÈit
iron
form
atio
n ch
ew m
etal
ulls
, and
asso
-
-dom
inan
tly
int6
fmed
iate
to m
sftc
da
ted
chem
ical m
ewed
- - d
offlln
mnl
y inte
rmaf
ale
@ m
afc
kani
c flo
ws an
d. su
bvok
anic
inlw
sive
s m
ents
(met
actie
rt)
niot
svol
csni
c flow
s and
inte
rtwdd
cd m
ela-
tuf
fs and
mffa
ceou
s mala
sedi
men
ts
Pms - gra
phitic
. sul
lide-
bear
ing
mel
a-am
ilite
s
. . .
. . .
. .
. . .
. . ,
. .
. . . .
. . .
. ,
. . .
. . .
.
Regi
onal
geolo
gic m
ap
MA
RC
H 1
991
Long
itudi
nal P
roje
ctio
nof
Min
eral
izat
ion,
Ben
dP
roje
ct
Tay
lor
Cou
nty,
Wis
cons
in
L__
LL.
L_.
L.
___
L_ L
L..
LL_
IL
LL
[ L
LI
I
a 0A
ssay
Dat
a
DD
H#
Tru
e%
ozlto
noz
fton
Thi
ckne
ssC
uA
uA
g
7.4,
5.6
5.2
21.1
'
0.10
0.09
0.08
0.08
0.09
30.
072
0.47
60.
058
0.05
0.02
0.22
0.02
FW
FW
EW
FW
29.9
'(7
1')
5.2'
6.1'
0.11
0.13
0.01
0.02
90-1
40.7
'(1
4.1'
)(2
6.6)
2.66
5.98
0.90
0.07
60.
091
0.06
8
0.61
1.51
0.13
HW
FW
90-2
14.7
'12
.3'
3.88
0.10
0.02
40.
119
0.35
0.09
NW
FW
90-3
21.8
'2.
220.
023
0.29
HW
90-4
90-5
90-6
69.5
'2.
460.
038
0.38
NW
91-7
5.1'
16.5
'11
.9'
1.28
0.20
0.04
0.04
10.
085
0.05
5
1.12
0.02
0.02
HW
FW
FW
91-8
61.4
'
5.7'
1.87
0.10
0.08
9 (c
ut)
0.19
0 (u
ncut
)0.
139
0.35
0.10
NW
FW
HW
-H
angi
ng w
ail z
one
FW
. Foo
twai
l zon
e
0.04
10.
058
0.05
20.
067
FW
0.03
0.06
0.01
0.01
FW
FW
WS
W
ENE
€L
E A
SL
1400
I
90-4
0
- S
EA
L
WE
I 0
16
<
"
-100
0 I
0
= W
eakJ
y m
iner
aliz
ed
1986
-198
9 dr
illha
les
1990
-1 99
1 dr
illho
les
Ass
ay D
ata
DO
H#
True
%
oz
non
ozno
n Th
lckn
ese
Cu
Au
Ag
1 .87
0.
089
(cut
) 0.
35
HW
0.
190
(um
ut)
0.10
0.
139
0.10
FW
HW
- Han
ging
wal
l zon
e FW
- Foo
twal
l zon
e MA
RC
H 1
991
Long
itudi
nal P
roje
ctio
n of
Min
eral
izat
ion,
Ben
d P
roje
ct
Tayl
or C
ount
y, W
isco
nsin
rrnr
rrrn
rr-f
lrcrn
r-r-
rN
ZW
Wt H
tF-t PrFID
P1 HID
ftN
mp•
lo'fl
(D0
ii
1H
N D
iD)
P•ft
:imtdO
(A çtI In ::rO
Fl (DO P•
Dl C'Q 0 0
(A p.pa.fl)
p.H0 DiH
fti-'ct
10- (D'<N
0QiC
(Am
-C
CP
Pi-I
D-•
0rt
ftH
ii—
ID H
CD
)i-rtHrfN CA
I 0Ci
H rt(DrtO
mct O
<: X
ID
(DID
Hi-
ftH
tQQ,i-
(D0W0
awo
Di
p.mo 1 OH
i-i,ftC
rtct
HH
,m
—lID
'N
HW
IDN
mD
iC(A
1O
Iftft
fti-
rD
INl-.
m C
DDi
p.00
(tH
ctfr
t,P1
ftID
'—O
ftC
rfi-'1D Di
tOWi H
DiiID
(DH
nj'a
rtID
'<(:
1 Nt
OlD
Di
ON
'lC
DI
P0
ftC01
oom
aD
iWO
OW
ftHo
H ID
a'<
a N
ft'<
N•
rtC
(0
Dii-
(DID
ID r
P)rt
H I
i Z e
muD
aHtO
P1
rtt'
0J 0
ID00
'-,,O
CD
Hi-
HID
NP-
tID
IDH
(Q0D
iNC
Cdf
tCC
O<
frD
iH
H"
WN
ciD
Nftf
tIDftH
<ID
DiC
OC
DH
IDN
0CC
OID
IDou
rtC
(AC
DiC
D tD
iH-0
HQ
i(D
CC
IDW
HD
i0N
'<O
ftH
"
tO a
0'N
W (
DW
FtID
HD
i-ftD
iID
l-'1W
Dft
tlrr'l
Oft
ftw
ui0I
'tQa)
Dio
0tm
D'
Wrt
IDH
Di w
0Nct
(AH
NC
N O
OC
I-N
rtW
H,
OW
•HIDLqOi-CD'<H0tWH'
10-
'jIQID
tt(fl
ft ID00Q0
Di
0k-
OH
ICO
OO
IC(D
F-'H
i-C
wC
O0-
'rhO
DiN
0.H
0WH
ftoa
mC
ftID
(0to
mW
ftftD
iC'<
NftIDHNftSCP0P
ftrt rtftDiNW0DCOON
NIDDiW0Wft0
N•t5
i-
IDID
ftH
IDft
(D
IlD
iOt
NdO
Nartifi
-"DIDNIDID
ftWCO(D
OiC
il-
-d1Q
Qi
0.D
ftN
ftD
iam
Dio
IDD
irnC
HH
,CD
i(1
NH
"ID
i14<
ON
ON
D)F
iDift
ID'awC(DNDiID00
ft
i-'ID
P•ft
IDID
NiD
WO
Diff
ltfto
nmW
aOW
JOC
C O
0ID
HO
CO
Hffl
IDui
—.D
iDi
WO
Ja a
ftID
P.0
J-C
OtO
tOtO
ImO
)_h
ftI
p-D
iNlft
NiQ
thb
(DID
F--
tOP
N'Q
IID
rtID
CN
H (
AID
(AH
0ft
ftrF
00
r1<H"C
(DIDNXD
flhrtOf N'i mi-'-o
t1 r
rp'rt
0flQ
i —'c
taF
CC
0Di0Q' IDftWLOIH
QlI
0'
0DiNI-
IDO
OC
OftC
DiO
-NrH
"H"0
NC
'<C
Dt
IDtr
nrC
ooJD
HD
O(D
X H
IDID
HH
Ht'f
tID
WU
<N
01DItOrtID
IDQaH"HIDHID
W1NtNHC
OH
XH
rtU
MH
CD
'<W
HH
ItH"f
t'ijD
irtC
OC
H,
r14<
CO
1001
3'<
001
Dft
rtH
H"D
iH
NIA
CIO
ti-Q
IDH
OID
Q4I
HD
IH"
CN
0ffl
NO
HH
OO(DWDiftHNItDi
ID
IDOftlDCO
ft1ft
IDO
ID0'
<O
frfI
IDID
H-c
t<(D
tO O
WID
:HO
OH
"0N
Nt0
Di
0N
(AfflH,H,HWIDIDft00)00"
N1Y
H"0
HD
iOP
IDO
(DD
)tH
DiU
IftID
Di
rt0
Nt
HQADIDWO
Cna
ftmID
ft<D
'D
IN'.ONN
H.IDNrbYP<
—'
WD
lWD
itNN
ftON
I-c.
IDO
Wft
CCHND10H"OCOQ)<
DH
"l-C
IDID
NH
O<
CP
•ID
OflO
'O0
IDD
iftC
tftC
'NI
ctaHH"ftaftDiftON
HO
IC(D
HH
'<H
"H"I
DPf
tDp-
C)
ctW
oDIr
tID O
HH
IOD
) O
H (
D'<
frtt<
rtO
WC
(D
000
aO
Ct
(N
Qi
Qi'
'0
H C
O c
OD
N C
cit H
0p)WH.CCHDIH
NID
NO
HtCCO
C$D
iH"c
tft
0'O
DH
"ft O
Wt-
0rtO
tci
-aai
-"W
tit
H-f
tID 0
0H
ZO
0cl'<
ID0I
D0'
H,D
IID
HO
)N
H"
'CN
CO
ftC
O
HC
Ot
o0-.
Nff
ltftD
ift0
HH
WH
NID
X 0
WN
OW
•ft.
DiH
"CO
IDC
OH
-ID
N(t
IDH
" tO
ftH•¼
oHO
ftO0)
OH-l• (ON
0)
tOOtN
• O
(D
OftHIDOID
HO
—fti
-CO
ftQi
OH
0O
C'<
IDn0
DiH
tn0H
IDft
Dim
<OIOID
H(D
DIP
ID'<
CD
(D,Q
ft'<(Od
to
ft am
H.orMtOIDOH"aCOH"Nrt
Otm
N m
H-N
omm
0ft
DID
lftD
iOH
"DitODlOO1
'-CD
xftDidIDDaCODi
CHOP-H" IDIBO
IDD
iooa
OC
Oft
rittO
0>ct
IDC
O H
H,tt
WO
OW
Nt'<
C 1
ftH
XN
HO
'Nftf
tDl'<
N'<
H"W
IDO
)t D
)ID
OW
NO
ma
NH
WC
O ID
IDID
Ol0
It(A
OID
DiH
"ID
ftC
Oit
CO
CO
IDO
'<ftQ•
OftI
Dft
QN
WH
"O
H"H
OD
iH"W
DiN
Oft
OW
OF
-H
,OH
"NH
0H
NH
ftO
i-C
A tQ
o\°
O(D
OD
i 100
ftCftc
tCD
tON
(DO
WO
O-m
a ot
ct0
OC
Otft
NC
CH
rtO
NftO
ID 0
bH
" 0'
N H
ifitO
0 0
>fta
-tt<
ftWD
OH
"i- X
(DC
dO'
H"O
ftHnH
Dit
DcC,-'.DlOft(DdCOH"
F-4J0
(DC
OO
O•
IDW
ftID
Oo\
0WA
H(O
HH
CO
ftID
WO
HQ
lF-'f
t jp
DID
NO
OrQ
10rh
QH
CH
ft'<
HaI
Dfr
tlCtID
t0'<
trt
Dm
tDrt
tID
H<
I- <
ftW
—O
WD
)tD
IC
O N
HID
N 0
>0
rUfl
0) N
ftHID
0 H
O o
\°C
Ot
(I)
N it
Ood
aoO
aC
Opi
0)N
id tQtft
UH.ftftclH
ftID
00W
H-O
H0"
'Hftm
nIH
mw
CN
'<O
CO
HQ
HC
DO
IDft
0)01
0O
0(0
'Oft
D)r
ttO
CH
,H
OC
IDp0
)0
H-
ft
0C1N
<N
M10
OH
CO
ft01
Oft
0 *(0
OiO
CW
OW
HH
HO
OD
Sw
OdC
rtt'-
MD
IftC
-D
'CO
ftN(D
NA
(U(D
IDftW
ON
O O
OH
a>N
NC
AO
N C
CA
XZ
dCO
H"d
WP
-d -
OZ
P-
00)
a A
OH
.NH
H p
-HD
i0 N
OH
HH
IDC
dtof
tO0C
CID
aID
tO O
'(Dd
Z0
i Cs—
'a
H tD
Hft
•0<
'00(0
ftHn<Di
ttOP
00O
H 0
CM
CH
OC
)ID
Cft
NJW
C tQ
Hft0
P-C
A-
Crt
WH
Ift.
Q'ii
ftNOD'H
DiIDmCH-C
HID
awH
"O
ftDJI
DID
MC
Oft
IDN
ftH
ftN
IDZ
OID
• D
io'o
)ab(
DP
CD
)XID
OID
HOC>
rtWN
OQ
i O(D
tOd
10(O
dID
IDfti
0)Q
iOH
W C
O (
A F
C a
—<
i-
DWDi'OIUIDft
10
'—'0
aftCOrtONO
-t-.
- 0
OD
'H"I
D<
' H"Q
)HC
Hoa
ID(A
IDC
CC
IDft
ftl
1101
(A—
4HH
"C
Owd
tb'IDOCOi
cHOP'
0)P
'OW
C-'O
ftpC
DD
id0i
Di
01<
aaN
NiC
OQ
IDQ
i H ID
CO
CO
SN
a.p
.ftID
'<0
DI P-P•
ctdoH"rt-
hl,W
0(D
ftDIO
IDd
-it
mS<ft
CO
NiD
ifi t
OH
-HoD
i H0
OID
H"C
QJO
CID
P&
OC
AC
O(D
Ofta
art-
4ID
> O
'1D
iaf
tWO
adO
ltftft
(DdI
UN
itNZ
CIDH"ID
N011CCO
H-H
-ftH
,Qj
Qi(ANt OO H"
ON
IDC
OdN
O<
IDuC
d0D
iQi0
WW
HID
-0<
(0 tt
CNftO
Oft
O tU
ftLQ
IDO
)OP
IDH
i-W
(D'<
PH
OO
dM <
Na
ID
'<H
"rt I
JID
IDC
DI
OftD
)H
ND
IP-P
-ffl
01H
DIU
1OdO
ftt
0)0
0D
IH"D
'V
fflftWH-dQi01O
Oft
d-'i1
1 aI
Dw
H,
,i-
Cao
H"
CO
ftN
OH
"pt
flU)Ø
H"D
IWC
Oft
aCN
JOC
AH
"dd
0N
-H
Qi
ftCOlt
NC
OO
S0
<0
00t'
IDO
N1<
ft-Z
CO
0 0 F<
CH"bl
za•
:ri
DIN
IZ
alt
000
CH
< C
-CA-
tO
•d
0ID
P1C4 ND)
HN
i-CO
C'
0-0
HN
CA
0
OC
NO
H"C
OO
dw CH,
ZWOtID
'C
,-'-O
OW
d NCO
0-C
OH
"0"1
H-
ON
1.51
:1N
CS
ti-O
—10
H,
(DPI
OlD
C0
0)0
P•0
0H
(AID
000
Z (AZ
tO
OH
HH
H.
:10
Zn
0C
OtD
00'
H"<
ZH
INP1
t0)
010<
t1 C:1
aft
QiODID
03:1
0z
CU
)Z
Dl
P.
-ft
0C
CN
-0
Di
CH
CC
)
:1 H.
H"
C[A
CO
ID
ftN
n0
(51
0N
H-
C'C
ft
Z'C
H IC
0-
S.0
SEOLOGY AND PETROGRAPHY OF THE AMNICON PLUTON, DOUGLAS COUNTY, WISCONSIN
Albert B. Dickas, Professor of Geology/Extension, University of Wisconsin-Superior, Superlor, Wlsconsln 54880
M. G. Mudrey, 3r.< Wisconsin Geological and Natural History Survey, Madlson Wisconsin, 53705
The s ~ a l l , sill-like Amnicon kntruskon located,southeast of Su erlor &Douglas Count ) , Wlsconsln, identifled by D+ckas avd otEers (i9 9 and ~ e n g e y (1970), has noy been mapped 1n detcll The Amnlcon biuton exceeds 6 square km. ln outcrop area and 1s named for exposures alon the bank of the Amnicon River in sec. 32, T. 48 N., R. 12 W. ?t is poorly exposed in sec 31 and 3'2, T. 48 N., R. 12 w., and sec. 5 and 6 T. 47 N-, R. i2 w. ( ~ i g u r e 1). N? contact? were observed, and the qutcrop ext9nt c o ~ n c ~ d e s
roxlmatel wlth the map ed magnetlc signature (Dlckas and. :&?ers, 1969y Field relagions lndlcate lntruslve contact wlth surrounding ckengwatana basalt and interbedded volcaniclastic units is well constrained to the north, and moderately constra+ned to the east and west. The southern margin appears to be a chllled contact.
The Amnicon intrusion is chemical1 structurally, and petrographically similar to $he otikr ma or K=weenawan intrusive suite? assoc4ated with the Mldcontinen? &ift ln the western Lake Su erlor reglon, the Duluth Igneous Sulte to the north and the ~ e y l e n Intruslye Suite to the east (Table 1), and probabl represents a mxnor, high-level differentiated gabbroic bozy.
42
Figure 1. Areal distribution of the Amnicon Pluton and itsrelation to regional, Middle Proterozoic (Keweenawan) geology ofthe Nidcontinent Rift System within Douglas County, Wisconsin.
Figure 2. Line diagram interpretation of the upper two and one-half seconds of reflection seismology, magnetics, and gravity ascollected along highways 2—53 and 53, Douglas County, Wisconsin.Display presented here courtesy of Halliburton GeophysicalServices, Inc., Texas.
• •%%_,•_INi I
NI mile
OR1ENTA SANDSTONE
cIIENGWAANA
IENTA SANDSTONE
Falls1
-Illgl,'.ay 2
VOLCANICS-
' -jitE:::-1
PLUTON
CIIENGWATANA -
4,
fcSE
61K 0 5000Feet NW
+80
Bayf.eliI Noritotall
.Amnicon Norm
Ilicloway 5)—
E_48N-12W
.t48N-ISW ——
I
________________
HIgt.ny2andSS
.80
GIo.cioolCuaar*
NgS -—
p BAYFIELD.OR0NTOU')
I S. 1W1
CLASTICSR.tl0..OII Dii, 3-to SE
-j
J
jjLi
-j
jJ
-j
2 Stc t'vr
4S0-t(
Figure 1. Areal distribution of the Amnicon Pluton and its relation to regional, Middle Proterozoic (Keweenawan) geology or the Midcontinent Rift System within Douglas County, Wisconsin.
Figure 2. Line diagram interpretation of the upper two ar half seconds of reflection seismology, magnetics, and grak collected along highways 2-53 and 53, Douglas County, Wisc Display presented here courtesy of Halliburton Geophysical Services, Inc., Texas.
ne- as in.
rrrr
rrrr
r---
-rO
1CtZ
¼00
DN
OO
P-C
DC
. CD
PflJ
P)F1
(DH
QJC
DC
U'c
ip.
na_
pjic
mf-
cict
CD
01f-
im
ztr
n-tm
<o
WD
DN
Jr0)
NC
DT
'(DN
OH
Om
itW
CD
• H
DD
Uff)
O—
'<01
0(0
'—'--
'CD
DN
rtH
P-0
)N
JN
F--
i—-
DD
HO
OD
It'D
CD
•—C
Dqa
a_U
PH
"N
-H
-OH
rp-
mito
t',. '
<c
ro'x
F-'-
O,
10tll
DtC
D•
0D0
WN
DO
0)-
'--'b
itN
1Z
000
0110
D(A
<H
NH
•--
00-
CD
-p•
-0• Y
rt-
DD
-D
(0(0
--
CU
)0)
ctH
D(O
H(t
H0-
kb:—
Jf-
tOrt
DC
D
Hçt
Q(B
OO
)0-
HD
Qa
O'D
CO
-aN
J•-
OW
ND
H(0
kbtr
tH
Hot
-NZ
Qzj
—J
:t:Q
-'t
0)'--
'f--N
JtO
)rt
o,Z
CW
-•D
-E
-'WD
0)10
(DH
NIf
lO01
'<D
)H-O
)N
IF-'t
Qit
Hffl
rtH
0)0
,--'c
r to
WN
DC
-ct
NO
-C0
CD
0—C
t000
)-0 H
CD NH
0D
F-'o
0)ht
,0)
0'-0
(1)
01
00-m
mo-
C0)
rt(0
CIO
CO
rtrh
N,O
WO
dD0
ifiD
iN
q t<
'ci-
rtN
rl-P
-CN
OH
OC
D CD
OtO
CD
••C
N0)
H0
CD
Hi
0100
Df-
-H-
-tO
fl'-C
HC
OH
0100
)N
rtk-
rt-'•
QD
X-
Crt
Crt
00--
rtZ
-'<f-
00(t
HC
DO
0fl)
Q.i.
D0
DH
DZ
Ort
H(0
P-k
b0 'ID
O'
DU
H D
D10
Zct
moC
(OtO
N10
—]
CitO
DW
OR
D'
Hht
O'C
C00
3C
DN
O'
OD
E'-
rtD
F-'-
H0
100—
-tO
tC
Dct
OP
-WI-
'-N
D<
Dct
O1S
CD
DC
DC
D0)
0 100
010
DD
(0 (OC
tO
dD-'D
'N
}-ID
OO
ON
O'iO
O0)
NH
ZC
Hi
D'O
Qiç
tk.-
CD
0)o'
CD
'.Q-G
Da
0(0
CD
H(O
I-'-f
--H
--t -
O'N
OIN
CD
DN
OF-
"O)
rHO
CD
OD
10P
-tQ
b'C
O)t
OC
Ot
tOO
OH
000
10tD
P-
HO
D•
DC
100
I-'-
CD
tOO
CD
P-
I-'-W
U'-O
0110
DN
0)0
NH
Dct
O01
0H
DO
)H
CD
OJ
Yrt
fflN
ftC-"
OH
Nt
I0
DO
CD
DD
OO
CD
P-O
D03
00O
H,
Nf-
---'-
CD
DO
D'
NJ1
t'tC
DP•
it00
0)ct
rtff
l•
(0it
0<
PC
DC
tON
O0
rt-
DN
NO
tOD
CH
WD
'C
D
'<H
-H
HC
DY
IP-
CD
I-'-
N-
O0)
rtp-
ftC
D(0
ituD
w<
CD
kt-
.tO
'C -t
1<0
(OC
tO
'P
••
-In
1!)
D'
c0
CD
DD
UN
JH '—
OO
NJN
JM
QD
(OH
H-I
-'-<
(FH
OO
NJ0
000N
JOO
I-'-
CD
'01
0O
NflJ
0,H
'—3
CD
'tot—
"d01
CD
NO
I'lO
—s
NO
W0)
(0O
Clfl
tflLO
-'C
Dt H
1tP
Db'
CD
a_(t
—H
N H
0)O
CD
DC
DH
tON
NW
CD
tt0
CD
0)C
DP
)H
UN
DN
HN
0H
01rt
W(O
HC
D00
00U
lOtO
03H
O0
DD
CD
DH
tOH
rtD
it0)
WA
G\N
JkbH
NJt
OU
0)A
P.tS
Ow
rtD
—]D
HD
ON
JOJD
OJ
(DD H
-H
r'Q r
hOU
DN
Dit
Ditt
O)
GO
ND
CIf
i
0O
P-fl
'0H
C\
0)
—]H
ouN
Jooa
How
D0<
00'
DC
DD
0O
HH
NJO
WH
U3-
J'U
D'U
'CIli
tIl 1
1111
10)
0010
HA
H W
CW
000-
U)H
0001
NJ0
H N
'CP
+'-P
00
(0•
tbH
HU
1-3O
3HH
H01
CD
t-'-C
DP-
-10
O—
'OU
)
DD
'—
0 Hi O
Ofr
tü-'H
,'.0
CD
NZ
C'C
0'O
f-tO
'0H
H.N
CD
DN
Dit
tOIO
ON
JO0)
000D
I-"0
H0
rt-O
'1
H'U
itw
pNJc
okbH
Ho1
A--
JC
D
'H
O1<
01kb
ND
'03
—C
D0—
]'-C
-'
N-<
-'0
(00
D'
'UN
'0H
HH
A0)
C.
—]IO
ON
JHO
1000
IH-]
C(0
0H
-IC
CD
HC
-'DW
INJU
1AN
JUN
JO1a
W'-)
Hflo
utC
D'N
JNit
DC
H(0
I_I
P•—
'0
NJ
D—Amnicon Mellen (1) Duluth (2)
Oxide ( 8 ) Troctol~te Granophyre (3)
sio2 : . .-., .,, 50.4 69-8-71. 47.7 Ti02. $ . , 1.8 0.3- 0. 1.3 A1203 18.3 11-7-12. 18.4 FeO 9.9 4.3- 5. 10.6 MnO 0.2 0-1- 0.1 0.1 0.2
%% 5.1 0.3- 0.8 6.1 5.3 9.9 2.0- 1.7 9.9 11 - 2
Na20 3.2 3.2- 3.5 2.9 2.4 K20 0.6 4.1- 4.1 0.6 0.5 P205 0.4 0.1-<O. 1 0.2 0.2 LO1 0.8 1.9- 0.9 --- - - Totals 100.6 97.8-100.7 99-8 97.8
(1) Possible Earent ma ma ~ompos~tion of Potato River Intrusion Eastern ellem 1n?rusLve SuLte) (Klewln 1987, p. 152)
(2) bossible parent ma ma composition of ~orth Shore Volcanic Group (Green, 19727
'(3) Percentage range of two analyses of granophyre phase
Table 1. Chemical anal ses of the two etrographic phases within the Amnicon Flvton, witg comparison of the Amnicon trqctolice phase with slmllar phases from the Mellen and Duluth lntruslves.
References :
Dickas, A. B. et al., 1969 Relationships of regional magnetics to the bedrock geolo y of the South Range Quadrangle, Douglas CoUnCY Wlsconsln: ~gstracts, 15th Inst. Lake Super lor Geology, p . 12-i4.
Green, J. C., 1972 North Shore volcanic Group, in Sims,.P. K. and More G. B. Ieds), Geology of Minnesota: A centennial volume ( Z : M. ~cliwartz volume), Minnesota Geological survey, p. 294-332-
Klewin, K. W. 1987< The petrology and zeochemistry,o the Keweenawan potato Rlver Intryion, nort ern W+scqnsIn; un ubllshed Ph. D. dissertation, Northern Illlno~s Un~versity, 357 p .
Mengel, J. T., 1970 Geology of the western Lake Superior region, privately printed, $6 p.
Tabet, D. E., and Mangham, J. R., 1978, The geology of the eastern,Mellen Intrusive Complex, Wisconsin: Geosclence Wisconsin, v. 3, p. 1-19.
Weiblen, P. W., 1982, Keweenawan intrusive igneous rocks in R- J. Wold and W. J. Hinze (eds): Geology and tectonics of &he Lake Superlor Basln, Geol. SOC. America Memolr 156, p. 57-82.
44
REGULATING METALLIC MINERAL DEVELOPMENTIN WISCONSIN
Thomas J. Evans, Wisconsin Geological and Natural History Survey
Despite the significant potential for metallic mineral deposits in theLadysmith/Rhinelander volcanic complex of northern Wisconsin, only one miningproject has received permits for actual mineral development. In part, theregulatory framework for metallic mining has been responsible for the slowpace of mine development.
The statutes and administrative rules governing metallic mineraldevelopment in Wisconsin form a comprehensive, broad-based regulatoryframework. In addition to extensive environmental regulations focusing ongroundwater, surface water, air, water-supply wells, and wastewater dischargeactivities, Wisconsin has adopted a net proceeds tax on the occupation ofmetalliferous mineral mining.
The regulations affecting metallic mining in Wisconsin include athorough assessment of environmental impacts and, together with the extensivepermitting requirements, result in a lengthy process from mineral discovery toeventual mine start-up. However, the process does provide a mechanism thatwill lead to positive decisions on the necessary permits, as long as theenvironmental standards can be addressed.
The net proceeds tax provides a reasonable tax basis for mineraldevelopment. This tax, often misunderstood or mischaracterized by the miningindustry in the early years after its adéption in 1977, was modified in 1981to clarify and expand certain allowable deductions and to reduce the overalltax rate so that it was more closely tied to the profitability of miningventures. Although the tax is an added burden or "cost of doing business" inWisconsin, it does not have a significant impact on the overall potential formining in Wisconsin.
The modified net proceeds tax, a "workable" mine-permitting process, andrecent decisions that favor mineral leasing create a positive climate formining; however, the small but vociferous anti-mining activity and recentlegislation that would severely limit mining in the state do much to obscurethat positive climate. It is important to remember the "upside" when you areup to your ears in the "downside".
REGULATING HETALLIC HINEPAL DEVELOPHENT IN WISCONSIN I Thomas J. Evans, Wisconsin Geological and Natural History Survey 1
I
Despite the significant potential for metallic mineral deposits in the Ladysmith/Rhinelander volcanic complex of northern Wisconsin, only one mining project has received permits for actual mineral development. In part, the regulatory framework for metallic mining has been responsible for the slow pace of mine development.
The statutes and administrative rules governing metallic mineral development in Wisconsin form a comprehensive, broad-based regulatory framework. In addition to extensive environmental regulations focusing on groundwater, surface water, air, water-supply wells, and wastewater discharge activities, Wisconsin has adopted a net proceeds tax on the occupation of metalliferous mineral mining.
The regulations affecting metallic mining in Wisconsin include a thorough assessment of environmental impacts and, together with the extensive permitting requirements, result in a lengthy process from mineral discovery to eventual mine start-up. However, the process does provide a mechanism that
" will lead to positive decisions on the necessary permits, as long as the environmental standards can be addressed.
I I I ;
The net proceeds tax provides a reasonable tax basis for mineral development. This tax, often misunderstood or mischaracterized by the mining industry in the early years after its adoption in 1977, was modified in 1981 to clarify and expand certain allowable deductions and to reduce the overall tax rate so that it was more closely tied to the profitability of mining ventures. Although the tax is an added burden or "cost of doing business" in Wisconsin, it does not have a significant impact on the overall potential for mining in Wisconsin.
The modified net proceeds tax, a "workable" mine-permitting process, and recent decisions that favor mineral leasing create a positive climate for mining; however, the small but vociferous anti-mining activity and recent 1 legislation that would severely limit mining in the state do much to obscure that positive climate. It is important to remember the "upside" when you are up to your ears in the "downsiden. 1
I
GEOLOGY OF PLATINUM GROUP ELEMENT - ENRICHED HORIZONSWITHIN THE DUNKA ROAD COPPER-NICKEL PROSPECT
ST. LOUIS COUNTY, MINNESOTA
Stephen D. GeertsNatural Resources Research Institute,
University of Minnesota, Duluth
The Dunka Road Cu-Ni prospect is located within what is informallyknown as the Partridge River intrusion (T. 60 W., R. 13 W.), which is partof the Duluth Complex, 1.1 b.y. (Keweenawan) in age (Figure 1). Seven majorlithologic units, along with several internal ultramafic subunits, have beenidentified and are correlatable over the deposit. The ultramafic subunits(layers of picrite to dunite), exhibit relative uniform thicknesses and arepresent at the same relative position within the major lithologic units.The major lithologic units are the same as delineated by Severson and Hauck(1990), and upward from the basal contact are (Figure 2): Unit I, a fine-to coarse-grained sulfide-bearing anorthositic troctolite to pyroxenetroctolite (450 ft. thick) with associated ultramafic layers 1(a), 1(b), and1(c); Unit II, a medium- to coarse-grained troctolite to pyroxenetroctolite (200 ft. thick) with a basal ultramafic layer 11(a); Unit III,a fine-grained, mottled textured troctolitic anorthosite to anorthositictroctolite (200 ft. thick); Unit IV, a coarse-grained anorthositictroctolite to pyroxene troctolite (400 ft. thick); Unit V, a coarse-grainedanorthositic troctolite (300 ft. thick); Unit VI, a fine- to coarse-grainedtroctolitic anorthosite to troctolite (400 ft. thick) with basal ultramaficlayer VI(a); and Unit VII, a coarse-grained troctolitic anorthosite toanorthositic troctolite (400.4- ft. thick) with basal ultramafic layer VII(a).
Most sulfide mineralization occurs within Unit I. The sulfidemineralization is both interstitial and widespread, but variable in modalpercentage (rare to 5%), continuity, and thickness (few inches to tens offeet). Sulfide mineralization is generally related with proximity to:hornfels inclusions, basal contact with the footwall Virginia Formation, andsome of the internal ultramafic layers within Unit I. Primary sulfidemineralization includes chalcopyrite, pyrrhotite, pentlandite and cubanit.Minor amounts of bornite, native copper, talnakhite and mackinawite orvalleriite have also been identified in preliminary petrographicobservations. Pt+Pd values range from 100 to >2400 ppb over 10 footintervals, and these occur as isolated values or along stratigraphichorizons in the upper 3/4 of Unit I.
Several Cu/PGE-enriched horizons have been identified, and occurlaterally throughout the prospect. The most continuous horizon, (REDHorizon) is found directly beneath ultramafic layer 11(a), within theuppermost portion of Unit I. This horizon ranges from 10 to 30 feet thickand contains average values of 0.65% Cu and 1200 ppb Pt+Pd. Two otherhorizons, (ORANGE Horizon and YELLOW Horizon) occur roughly at 100 and 200feet beneath Red Horizon, respectively. These are less continuous horizonsthat range from 10 to 50 feet thick and contain average values of 0.70% Cuand 1000 ppb Pt+Pd. Only one PGE-enriched horizon has been identifiedoutside of Unit I. It occurs in Unit VI (MAGENTA Horizon), directly beneathultramafic layer VII(a). Although it has been identified in only six drillholes to date, it ranges from 10 to 30 feet thick and contains averagevalues of 0.90% Cu and 1875 ppb Pt+Pd.
45
GEOLOGY OF PLATINUM GROUP ELEMENT - ENRICHED HORIZ WITHIN THE DUNKA ROAD COPPER-NICKEL PROSPECT
ST. LOUIS COUNTY, MINNESOTA
Stephen 0. Geerts a t u r a l Resources Research I n s t i t u t e ,
U n i v e r s i t y o f Minnesota, Duluth
The Dunka Road Cu-Ni prospect i s l oca ted w i t h i n what i s i n f o r m a l l y known as t h e Par t r i dge R ive r i n t r u s i o n (T. 60 W., R. 13 W.), which i s p a r t of t h e Du lu th Complex, 1.1 b.y. (Keweenawan) i n age (F igure 1) . Seven major 1 i t h o l o g i c un i t s , along w i t h several i n t e r n a l u l t r a m a f i c subuni ts , have been i d e n t i f i e d and are c o r r e l a t a b l e over t h e depos i t . The u l t r a m a f i c subuni ts ( l aye rs of p i c r i t e t o dun i te ) , e x h i b i t r e l a t i v e uniform th icknesses and are present a t t h e same r e l a t i v e p o s i t i o n w i t h i n t h e major l i t h o l o g i c u n i t s . The major l i t h o l o g i c u n i t s are t h e same as de l ineated by Severson and Hauck (1990), and upward from t h e basal contact are (F igure 2) : U n i t I, a f i n e - t o coarse-grained su l f i de -bea r ing a n o r t h o s i t i c t r o c t o l i t e t o pyroxene t r o c t o l i t e (450 ft. t h i c k ) w i t h associated u l t r a m a f i c l a y e r s I ( a ) , 1 (b), and I ( c ) ; U n i t 11, a medium- t o coarse-grained t r o c t o l i t e t o pyroxene t r o c t o l i t e (200 ft. t h i c k ) w i t h a basal u l t r a m a f i c l a y e r I I ( a ) ; U n i t 111, a f ine-gra ined, mo t t l ed tex tu red t r o c t o l i t i c ano r thos i te t o a n o r t h o s i t i c t r o c t o l i t e (200 ft. t h i c k ) ; U n i t I V , a coarse-grained a n o r t h o s i t i c t r o c t o l i t e t o pyroxene t r o c t o l i t e (400 ft. t h i c k ) ; U n i t V, a coarse-grained a n o r t h o s i t i c t r o c t o l i t e (300 ft. t h i c k ) ; U n i t V I , a f i n e - t o coarse-grained t r o c t o l i t i c ano r thos i te t o t r o c t o l i t e (400 ft. t h i c k ) w i t h basal u l t r a m a f i c l a y e r V I (a ) ; and U n i t V I I , a coarse-grained t r o c t o l i t i c ano r thos i te t o a n o r t h o s i t i c t r o c t o l i t e (400+ ft. t h i c k ) w i t h basal u l t r a m a f i c l a y e r V I I ( a ) .
Most s u l f i d e m i n e r a l i z a t i o n occurs w i t h i n U n i t I. The s u l f i d e m i n e r a l i z a t i o n i s bo th i n t e r s t i t i a l and widespread, b u t v a r i a b l e i n modal percentage ( ra re t o 5%), c o n t i n u i t y , and th ickness (few inches t o tens o f f e e t ) . Su l f i de m i n e r a l i z a t i o n i s genera l l y r e l a t e d w i t h p r o x i m i t y to : ho rn fe l s i nc lus ions , basal contac t w i t h t h e f o o t w a l l V i r g i n i a Formation, and some o f t h e i n t e r n a l u l t ramaf i c l a y e r s w i t h i n U n i t I. Primary s u l f i d e m i n e r a l i z a t i o n inc ludes cha lcopy r i t e , p y r r h o t i t e , p e n t l a n d i t e and cubani t e . Minor amounts o f bo rn i t e , n a t i v e copper, t a l n a k h i t e and mackinawite o r v a l l e r i i t e have a l so been i d e n t i f i e d i n p r e l i m i n a r y pe t rograph ic observat ions. Pt+Pd values ranae from 100 t o >2400 ~ o b over 10 foot i n t e r v a l s , and these occur as i s o l a t e d values o r along s t r a t i g r a p h i c hor izons i n the upper 3/4 of U n i t I.
Several Cu/PGE-enriched hor izons have been i d e n t i f i e d , and occur l a t e r a l l y throughout t h e prospect. The most cont inuous hor izon, (RED Horizon) i s found d i r e c t l y beneath u l t r a m a f i c l a y e r I I ( a ) , w i t h i n t h e uppermost p o r t i o n of U n i t I. This hor izon ranges from 10 t o 30 f e e t t h i c k and conta ins average values o f 0.65% Cu and 1200 ppb Pt+Pd. Two o the r hor izons, (ORANGE Horizon and YELLOW Horizon) occur rough ly a t 100 and 200 fee t beneath Red Horizon, respec t i ve l y . These are l e s s cont inuous hor izons t h a t range from 10 t o 50 f e e t t h i c k and con ta in average values o f 0.70% Cu and 1000 ppb Pt+Pd. Only one PGE-enriched ho r i zon has been i d e n t i f i e d ou ts ide o f U n i t I. It occurs i n U n i t V I (MAGENTA Horizon), d i r e c t l y beneath u l t r a m a f i c l a y e r V I I ( a ) . Al though i t has been i d e n t i f i e d i n on l y s i x d r i l l ho les t o date. i t ranges from 10 t o 30 f e e t t h i c k and conta ins average values o f 0.90% Cu and 1875 ppb Pt+Pd.
j.The predominant host rock for these Cu/PGE-enriched horizons is
coarse-grained anorthositic troctolite, which may exhibit some subtlefracturing accompanied with minor alteration. The alteration assemblagewithin these mineralized zones is serpentine, uralite and saussurite. Thistype of alteration assemblage has also been observed throughout the entireprospect, but is not always associated with mineralization. Although themajority of sulfide mineralization is believed to be primary, mineralizedzones that are intersected by fractured/altered zones can contain secondarysulfides and textures, suggesting local enrichment. The majority of thesulfide itself is coarse-grained (5 mm) and commonly rimmed by secondaryred-brown biotite. Ilmenite occurs in two habits within these zones, aseuhedral to subhedral laths throughout the host rock, and as "bleb-like"black shiny droplets within the sulfides. This second ilmenite habit hasonly been identified in sulfide zones that are enriched in Pd and/or Pt.
References:
Morton, P., and Hauck, S. A., 1987, PGE, Au and Ag contents of Cu-Nisulfides found at the base of the Duluth Complex, northeasternMinnesota: Natural Resources Research Institute, Technical ReportNRRI/GMIN-TR-87-04, 85 pp.
Morton, P., and Hauck, S. A., 1989, Precious metals in the copper-nickeldeposits of the Duluth Complex: Minn. Geol. Survey, Inf. Circ. 30,pp. 47-48.
Severson, M. J., and Hauck, S. A., 1990, Geology, geochemistry, andstratigraphy of a portion of the Partridge River Intrusion,northeastern Minnesota: Natural Resources Research Institute,Technical Report, NRRI/GMIN-TR-89-11, Duluth, Minnesota, 240 pp.
_1
j-J
'H1'
j-A
46
tMIT VII=UNIT VI
CENERAUZED IGNEOUS$TRATCRAPHIC COLUMN
IMIT V
UNIT IV
UNIT fil
UNIT I——K —UNIT I
Insow ..,.— r_AL_.r*
qRNIA FOlIaTiON
BIWASIC ON-FOlMAT1ON
a a,cn. zn oa s,.n—c a,...IIGC II WOAL C flc PRNIIGL PlYtI INTmITD
The predominant hos t rock f o r these Cu/PGE-enriched hor izons i s coarse-gra ined a n o r t h o s i t i c t r o c t o l i t e , which may e x h i b i t some s u b t l e f r a c t u r i n g accompanied w i t h minor a1 t e r a t i o n . The a1 t e r a t i o n assemblage w i t h i n these m ine ra l i zed zones i s serpent ine, u r a l i t e and saussur i te . Th i s type o f a l t e r a t i o n assemblage has a l so been observed throughout t h e e n t i r e prospect, b u t i s n o t always associated w i t h m ine ra l i za t i on . Although t h e m a j o r i t y o f s u l f i d e m i n e r a l i z a t i o n i s be l ieved t o be primary, m ine ra l i zed zones t h a t a re i n t e r s e c t e d by f rac tu red /a l t e red zones can conta in secondary s u l f i d e s and tex tures , suggest ing l o c a l enrichment. The m a j o r i t y o f the s u l f i d e i t s e l f i s coarse-grained (5 mm) and commonly rimmed by secondary red-brown b i o t i t e . I l m e n i t e occurs i n two hab i t s w i t h i n these zones, as euhedral t o subhedral l a t h s throughout t h e host rock, and as " b l e b - l i k e " b lack shiny d r o p l e t s w i t h i n t h e s u l f i d e s . This second i l m e n i t e h a b i t has o n l y been i d e n t i f i e d i n s u l f i d e zones t h a t are enriched i n Pd and/or P t .
References:
Morton, P., and Hauck, S. A., 1987, PGE, Au and Ag contents o f Cu-Ni s u l f i d e s found a t t h e base o f t h e Duluth Complex, nor theas tern Minnesota: Natura l Resources Research I n s t i t u t e , Technical Report NRRI/GMIN-TR-67-04, 85 pp.
Morton, P., and Hauck, S. A., 1989, Precious metals i n t h e copper-n ickel depos i t s o f t h e Du lu th Complex: Minn. Geol. Survey, I n f . C i r c . 30. pp. 47-48.
Severson, M. J., and Hauck, S. A., 1990, Geology, geochemistry, and s t r a t i g r a p h y o f a p o r t i o n o f t h e Par t r idge R ive r I n t r u s i o n , - . - nor theas tern ~ i n n e s o t a : Natura l Resources R e s e a r c h ~ n s t i t u t e ; Technical Report, NRRI/GMIN-TR-89-11, Duluth, Minnesota, 240 pp.
GENERALIZED IGNEOUS STRATIGRAPHIC COLUMN
I_____I UNIT 111
DRIFT PEBBLE LITUOLOGY OF THE TOMAHAWK ROAD AREA, LAKECOUNTY, MINNESOTA, USED TO HELP INFER LOCAL BEDROCK
John C. Green, Univ. of Minnesota, Duluth; Ed. Venzke & Tom Lawler, D.N.R. 1-libbing
Abstract
In 1989 a multi-media, regional, geochemical orientation and reconnaissance survey was completed fora portion of Lake County, Minnesota. The results of this survey suggest that three anomalous localities existacross the project area, It was also concluded that the geochemical survey reflects bedrock lithologies, despitevariable gladal overburden thickness of several deposit types and a complex depositional history, (Buchheit etal,, 1989, p. 1). These results were related to the geology as mapped by Green on the 1:250,000 scale, 'GeologicMap of Minnesota, Two Harbors Sheet', (1982) but the complex pattern of geochemical results made obviousthe need for a more detailed geologic map to assess this relationship.
Traditional geologic maps are a reflection of the quantity and quality of available geologic information,which did not provide the needed detail. Improved quality of geophysical surveys and sophisticated computerenhancement techniques now provide the means to generate inferred geologic maps. For accuracy and detailthese maps require high resolution aeromagnetic and regional gravity surveys, provided in Minnesota by anaeromagnetic survey funded by the Legislative Commission on Minnesota Resources (LCMR) and a gravitysurvey by the Minnesota Geologic Survey (MGS). Geologic interpretations made from remote measurementsof physical properties are known as nseudo-Qeoloic mans.
As a pilot test of the practicality of these maps, the Department of Natural Resources (DNR) contractedwith private consultants to interpret geophysical data and make maps in two areas. One of the areas selectedwas a four township block in the McDougal Lakes Area of Lake County, in the interior of the Duluth Complex,a Proterozoic layered maCic intrusive complex. This area was mapped by Robert J. Ferderer, Eagan, Minnesota.Ferderer's pseudo-geologic map is much more detailed than its predecessor The Two Harbors geologic map.Lithologic units and structural features are confirmed by verification procedures wherever tested. In additionto lithologic units and structural features this map has three dimensional aspects shown by depth to magneticsource calculations and forward modeling profiles. A portion of this map in the pebble count area is shown onPlate 3.
In this area glacial cover is 0 to 65 feet deep, and there are some outcrops, but prior to this programthere were only three drill holes. To verify the pseudo-geologic map it was tested with: 1) Six drill holes, withlithologic logging using assays and thin section studies; 2) Geophysical measurements on the core; 3) Groundgeophysical traverses over selected features; and 4) Pebble counts of glacial till and outcrop studies that relatelithologic observations to geophysical parameters. The pebble count, thin section, outcrop studies, and drill corelogging were done by, or under the direction of, John C. Green (University of Minnesota - Duluth).
The nebble countine technique used by Green and Venake (1990) was to determine if glacial drift pebblecomposition can be used to determine the lithology of the underlying bedrock in drift-covered areas. A strip ofsections was chosen along the Tomahawk Road where, with additional outcrop mapping and drill hole logging,an improved geologic map could be made to evaluate the results of the pebble counts and the pseudo-geologicmap. Although some contacts are poorly constrained due to gaps in outcrop and drill hole coverage. Bedrockunits of anorthosite, olivine gabbro and troctolite of the Bald Eagle Intrusion, and one or two troctolite unitswere mapped. Based on counts of the 50 largest pebbles in each sample, four lithologic drift units were mapped:1) Troctolitic; 2) Transition Zone; 3) Anorthositic; and 4) Mixed Volcanic (Plate 5).
In general, the most abundant pebble type in these samples corresponds to the underlying bedrock type,suggesting that this technique can be useful for 'remotely sensing' bedrock types in covered areas. However,in the eastern 1/4 of the area the drift is dominated by llthologies (Archean, Animikie, Keweenawan lavas,granophyre) that have been transported for long distances (several 10's of kin) from the E, ENE, or ESE. Thismust have been carried by the Superior Lobe and is clearly not basal till (directly overlying bedrock). Elsewherein the study area, ice transport has produced some gradations or transition zones in the drift pebble assemblages,compared to the bedrock contacts. Also, since glacial transport in the Rainy Lobe (dominant here) was primarilyroughly parallel to the main bedrock contact (anorthositey troctolite), the pebble assemblage at any sample sitemay have come largely from a few km up-ice. Thus the technique will be most successful when the drift is
47
DRIFT PEBBLE LITHOLOGY OF THE TOMAHAWK ROAD AREA, LAKE COUNTY, MINNESOTA. USED TO HELP INFER LOCAL BEDROCK
John C. Green, Univ. of Minnesota, Duluth; Ed. Venzke & Tom Lawler, D.N.R. H i b b i i
Abstract
In 1989 a multi-media, regional, geochemical orientation and rewmaissance survey was completed for a portion of Lake County, Minnesota. The results of this survey suggest that three anomalous localities exist across the project area. It was also concluded that the geochemical survey reflects bedrock lithologies, despite variable glacial overburden thickness of several deposit types and a wmplex depositional history, (Buchheit et al., 1989, p. 1). These results were related to the geology as mapped by Green on the 1:250,000 scale, "Geologic Map of Minnesota, Two Harbors Sheet", (1982) but the complex pattern of geochemical results made obvious the need for a more detailed geologic map to assess this relationship.
Traditional geologic maps are a reflection of the quantity and quality of available geologic information, which did not provide the needed detail. Improved quality of geophysical surveys and sophisticated computer enhancement techniques now provide the means to generate inferred geologic maps. For accuracy and detail these maps require high resolution aeromagnetic and regional gravity surveys, provided in Minnesota by an aeromagnetic survey funded by the Legislative Commission on Minnesota Resources (LCMR) and a gravity survey by the Minnesota Geologic Survey (MGS). Geologic interpretations made from remote measurements of physical properties are known as pseudo-eeolo-ic mans.
As a pilot test of the practicality of these maps, the Department of Natural Resources (DNR) contracted with private consultants to interpret geophysical data and make maps in two areas. One of the areas selected was a four township block in the McDougal Lakes Area of Lake County, in the interior of the Duluth Complex, a Proterozoic layered mafic intrusive complex. This area was mapped by Robert J. Ferderer, Eagan, Minnesota. Ferderer's pseudo-geologic map is much more detailed than its predecessor The Two Harbors geologic map. Lithologic units and structural features are confirmed by verification procedures wherever tested. In addition to lithologic units and structural features this map has three dimensional aspects shown by depth to magnetic source calculations and forward modeling profiles. A portion of this map in the pebble count area is shown on Plate 3.
In this area glacial cover is 0 to 65 feet deep, and there are some outcrops, but prior to this program there were only three drill holes. To verify the pseudo-geologic map it was tested with: 1) Six drill holes, with lithologic logging using assays and thin section studies; 2) Geophysical measurements on the core; 3) Ground geophysical traverses over selected features: and 4) Pebble wunts of olacial till and outcroo studies that relate h h & g k observations to geophysical parameters. The pebble count, thin section, outcrop studies, and drill core logging were done by, or under the direction of, John C. Green (University of Minnesota - Duluth).
The -used 1 by Green and Venzke ( 1 W ) was to determine ifglacial drih pebble composition can be used to determine the litholoey of the underlying bedrock in drift-covered areas. A strip of sections was chosen along the Tomahawk Road &here, with addLtioLa1 outcrop mapping and drill hole log&& an improved geologic map could be made to evaluate the results of the pebble wunts and the pseudo-geologic map. Although some contacts are poorly constrained due to gaps in outcrop and drill hole coverage. Bedrock units of anorthosite, olivine gabbro and troctolite of the Bald Eagle Intrusion, and one or two troctolite units were map~ed. Based on counts of the 50 largest nebbles in each sample. four litholodc drift units were maoned: . . 1) ~roct&tic; 2) Transition Zone; 3) ~northositic; and 4) Mixed ~o l&nic (Plate 5).
In general, the most abundant pebble type in these samples corresponds to the underlying bedrock rvoc. suggesting that this technique can be useful for"remotely sensing" bedrock types in coveredareas. owe&; in the eastern 1/4 of the area the drift is dominated by lithologies (Archean, Animikie, Keweenawan lavas, granophyrc) that have been transported for long distances (several 10's of km) from the E, ENE, or ESE. This must have been carried by the Superior Lobe and is clearly not basal till (directly overlying bedrock). Elsewhere 1 the study area. ice transoort has oroduced some madations or transition zones in the drift nebble assemblaees. compared t o thebedrock contacts.-, since glacial transport in the Rainy Lobe (dominanthere) was rouehly ~aral lel to the main bedrock contact fanorthosite vs troctoliteV the nebble assemblaee at anv sarnole site - . - may have come largely from a few km ~ ~ - i c e . Thus thetechnique'wiU be most successful when the drift is
47
relatively thin and its stratigraphy is known well enough to exclude the existence of an upper drift sheet that isnot in contact with local bedrock, and where rock boundaries are at large angles to ice transport direction,(Green and Venzke 1990).
Comparing geochemical results with Fcrdcrer's map there is a correlative relationship between the westcontact area of his sb2 unit and high geochemical assays. This unit i5 similar in areal extent to the dg unit onthc Two Harbors Sheet. Anomalous geochemical values also correlate with Ferderer's fault zones.
Buchheit, R.L., Malmquist, K.L. and Niebuhr, J.R., 1989, "Glacial Drift Geochemistry for Strategic Minerals;Duluth Complex, Lake County, Minnesota", Minnesota Dept. of Natural Resources, Division ofMinerals, Report 262.
Green J.C. and Venzke, ES., 1990, "DriFt Pebble Lithology of the Tomahawk Road Area, Lake County,Minnesota: Can it be Used to infer Local Bedrock?", Minnesota Dept. of Natural Resources, Divisionof Minerals, part of Report 290.
I. —.•ofral,_fl•!t'oI— a.—. — oo•'. .4 ' — V.. S.'!—. OOq S.. a_a.,,a'.., —a. .4 V.. w Ca.!"V.' (aDIS.' •—'.°.—— •a" a' I.. SO!
a... ... ot'a. .-..t
—-—"-.0-I..,, ,'w—......, •.•..,..a'•t.593 t.. .v..l•,
-I
Hj
1
-I
jJ
J
J
J
J
* a 00 ::::: -
1"
Plot. 3. "op of Outcrop. one DcliiHal.. a,,d 'hair R.lolion.hlp I, haA..o'aootI.c l'.l.rp..Iol,or. •dap *tI (1*09)C*O •t (AV.,fl. - 'no
1"Plate S. Wop of Sompl.. She—in2the.. £saign.'a.n? IC Glacial DrillUnit..
48
1s9
CO So tao..... ISIC
relatively thin and its stratigraphy is known well enough to exclude the existence of an upper drift sheet that is not in contact with local bedrock, and where rock boundaries arc at large angles to ice transport direction, (Green and Venzke 1990).
Comparing geochemical results with Fcrdcrcr's map there is a corrclativc relationship between the west contact area of his lb2 unit and high geochemical assays. This unit is similar in areal extent to the dg unit on the Two Harbors Sheet. Anomalous geochemical values also correlate with Fcrderer's fault zones.
Buchhcit, R.L., Malmquist, K.L. and Nicbuhr, J.R., 1989, "Glacial Drift Geochemistry for Strategic Minerals; Duluth Complex, Lake County, Minnesota", Minnesota Dcpt. of Natural Resources, Division of Minerals, Report 262.
Green J.C. and Vcnzke, EA., 1990, "Drift Pebble Lithology of the Tomahawk Road Area, Lake County, Minnesota: Can it be Used to Infer Local Bedrock?". Minnesota Dent. of Natural Resources, Division of Minerals, part of Report 290.
GROUND GEOPHYSICAL SURVEYS LEADING TO THEBEND COPPER-GOLD DISCOVERY
Jay C. Hanson, Ernest K * Lehmann & Associates, Inc., 430 First AvenueNorth, Minneapolis, Minnesota 55401
SUMMARYThe initial ground geophysical surveys were intended to follow up a 1977INPUT airborne electromagnetic (AEM) anomaly located in the ChequamegonNational Forest in Taylor County • Wisconsin. The anomaly was delineated ononly one flight line. The ground geophysics consisted of horizontal-loopfrequency electrornagnetics (HLEM) and total-field magnetics profiling. Fivenorth—south survey lines, spaced 400 feet apart, were placed over theanomaly. A strong nonrnagnetic conductor was encountered on two of thecentral lines, suggesting the occurrence of a steeply dipping massive sulfidelens with short strike length. Subsequent drilling, which began with twocored holes drilled on each of the anomalous HLEM survey lines, verified theconductive source as being due to massive chalcopyrite mineralization.Assays of core samples indicated the presence of ore—grade gold as well. Theimportance of ground geophysical follow-up over single-line or poorly definedAEM anomalies cannot be overestimated.
INTRODUCTIONNorth—central Wisconsin has been a focal point for massive—sulfide explorationsince Kennecott Copper initiated a large exploration program in the late1950s. They were originally led to the area because of favorable outcrops(primarily in Rusk County), new volcanogenic models, and improved AEMsystems. The AEM surveys in particular were responsible for the initialdiscoveries of the economic Flambeau deposit and, to a major extent, thesubeconomic Thornapple deposit, both located near Ladysmith. Further to theeast, AEM exploration programs by the Central Wisconsin Joint Venture (GettyMinerals and Ernest K. Lehmann & Associates) • Noranda, and Exxon led tothe discoveries at Ritchie Creek, Pelican River, and Crandon in the mid— tolate—1970s. More recently, an exploration program funded by the Jump RiverJoint Venture (Chevron USA and Wisconsin Mineral Resources), althoughflown much earlier by the Getty-Lehmann joint venture (CWJV), led to the
49
I GROUND GEOPHYSICAL SURVEYS LEADING TO THE
I BEND COPPER-GOLD DISCOVERY
I Jay C. Hanson, Ernest K . Lehmann & Associates, Inc., 430 First Avenue
North, Minneapolis, Minnesota 55401
I ,..,.,< SUMMARY ' ' . . . . ,. . ... .~,.. ,~
The initial ground geophysical surveys were intended to follow up a 1977
I INPUT airborne electromagnetic (AEM) anomaly located in the Chequamegon
National Forest in Taylor County, Wisconsin. The anomaly was delineated on
I only one flight line. The ground geophysics consisted of horizontal-loop
frequency electromagnetics (HLEM) and total-field magnetics profiling. Five
I north-south survey lines, spaced 400 feet apart, were placed over the
anomaly. A strong nonmagnetic conductor was encountered on two of the
central lines, suggesting the occurrence of a steeply dipping massive sulfide
I lens with short strike length. Subsequent drilling, which began with two
cored holes drilled on each of the anomalous HLEM survey lines, verified the
I conductive source as being due to massive chalcopyrite mineralization.
Assays of core samples indicated the presence of ore-grade gold as well. The
I importance of ground geophysical follow-up over single-line or poorly defined
AEM anomalies cannot be overestimated.
I INTRODUCTION North-central Wisconsin has been a focal point for massive-sulfide exploration
I since Kennecott Copper initiated a large exploration program in the late
1950s. They were originally led to the area because of favorable outcrops
I (primarily in Rusk County), new volcanogenic models, and improved AEM
systems. The AEM surveys in particular were responsible for the initial
I discoveries of the economic Flambeau deposit and. to a major extent, the
subeconomic Thornapple deposit, both located near Ladysmith. Further to the
east, AEM exploration programs by the Central Wisconsin Joint Venture (Getty
I Minerals and Ernest K . Lehmann & Associates). Noranda, and Exxon led to - the discoveries at Ritchie Creek. Pelican River, and Crandon in the mid- to
I late-1970s. More recently, an exploration program funded by the Jump River
Joint Venture (Chevron USA and Wisconsin Mineral Resources), although
I flown much earlier by the Getty-Lehmann joint venture (CWJV), led to the
50
geophysical discovery of the Bend massive sulfide in 1986. In 1990, Norandaannounced its discovery of the zinc—rich Lynne deposit, also originallydelineated with AEM. This paper will discuss the sequence of geophysicalevents that ultimately led to the Bend copper—gold discovery.
DISCUSSION
The Bend deposit is located in the Chequamegon National Forest in TaylorCounty, about 12 miles northwest of Medford. The airborne anomaly mayhave been recognized at least as early as 1974 when National Lead Industriesconducted an AEM (INPUT) survey over the area. However, no groundfollow-up was done, probably because of the poor quality of the anomaly. At
about the same time, Kerr-McGee Corporation also flew an AEM survey overthe area but, again, did not perform ground geophysics.
Later, in 1977, adjacent surveys were flown simultaneously by AMAX andCWJV. Both surveys detected the Bend massive sulfide. AMAX obtained
prospecting permits in the area but, because of difficulties in securing a landposition, did not pursue exploration efforts and left the area in 1978. The
Getty—Lehmann joint venture dissolved in 1983 when Getty ceased mineralexploration operations. Chevron Resources, which also had an interest in thejoint venture, became the major partner and continued the exploration workthrough a new joint venture (JRJV). Its ground geophysical program beganwith HLEM and ground magnetics surveys in February, 1986,
Five north—south geophysical lines, each approximately 5000 feet long, werepositioned over the AiM anomaly. Ground magnetics were run with a Geo-metries proton precession magnetometer at 50- and 100-foot intervals. No
residual anomaly could be discerned in the vicinity of the AEM response.
MaxMin II HL.EM profiling was also undertaken. A constant 600—foot
separation was maintained between the transmitter coil and receiver coil, -resulting in a penetration depth of about 300-400 feet. In-phase and quad-rature components of the vertical secondary field were recorded for threefrequencies at 100—foot intervals. The 444 Hertz data are shown in profileform in Figure 1. A strong bedrock conductor (up to 93 mhos) was defined -J
on lines 4W and 8W and formed the basis for the initial drilling. The first
geophysical discovery of the Bend massive sulfide in 1986. In 1990, Noranda
announced its discovery of the zinc-rich Lynne deposit, also originally
delineated with AEM. This paper will discuss the sequence of geophysical
events that ultimately led to the Bend copper-gold discovery.
DISCUSSION
The Bend deposit i s located in the Chequamegon National Forest in Taylor
County, about 12 miles northwest of Medford. The airborne anomaly may
have been recognized at least as early as 1974 when National Lead Industries
conducted an AEM (INPUT) survey over the area. However, no ground
follow-up was done, probably because of the poor quality of the anomaly. At
about the same time, Kerr-McGee Corporation also flew an AEM survey over
the area but , again, did not perform ground geophysics.
Later, in 1977, adjacent surveys were flown simultaneously by AMAX and
CWJV. Both surveys detected the Bend massive sulfide. AMAX obtained
prospecting permits in the area but, because of difficulties in securing a land
position, did not 'pursue exploration efforts and left the area in 1978. The
Getty-Lehmann joint venture dissolved in 1983 when Getty ceased mineral
exploration operations. Chevron Resources, which also had an interest in the
joint venture, became the major partner and continued the exploration work
through a new joint venture (JRJV). I t s ground geophysical program began
with HLEM and ground magnetics surveys in February, 1986.
Five north-south geophysical lines, each approximately 5000 feet long, were
positioned over the AEM anomaly. Ground magnetics were run with a Geo-
metr ic~ proton precession magnetometer at 50- and 100-foot intervals. No
residual anomaly could be discerned in the vicinity of the AEM response.
MaxMin I1 HLEM profiling was also undertaken. A constant 600-foot
separation was maintained between the transmitter coil and receiver coil,
resulting in a penetration depth of about 300-400 feet. In-phase and quad-
rature components of the vertical secondary field were recorded for three
frequencies at 100-foot intervals. The 444 Hertz data are shown in profile
form in Figure 1. A strong bedrock conductor (up to 93 mhos) was defined
on lines 4W and 8W and formed the basis for the initial drilling. The first
r r
r. r
r r
i rr—
r-
r r
n r
r— r
— r
— r
FIG
UR
E 1
I0 5J oJ%
PrI
n..r
p
.5J
10J
BE
ND
MA
XM
IN P
RO
FIL
ES
-444
HZ
HLE
M C
oil S
epar
atio
n •
600
Fee
t
L 14
W
LI2W
L8W
L4W
LOO
NO
RT
HS
OU
TH
In—
Pha
se
——
——
. Out
-of-
Pha
se
2OO
i.0
200f
l.
FIG
URE
1
BEND
MAX
MIN
PR
OFI
LES
- 444
HZ
HLE
M
Coil
Sepo
rotlo
n . 60
0 Fe
et
---
SOUT
H NO
RTH
-.-.
In-P
hase
two holes, drilled in October 1986, were collared on lines 4W and 8W andintersected a thick massive-sulfide conductor, now known as the Benddeposit. To date, 25 cored holes have delineated 3.7 million tons of copper- Jgold mineralization in two zones. Depending on permit requirements, miningoperations could begin as early as 1995. jIn order to investigate the possibility of the occurrence of deep mineralizationalong strike, a large—loop time—domain EM-37 survey was performed. Large—
loop electromagnetic systems are often used for exploring deep ore deposits.At Bend, several additional survey lines were added, east and west of theoriginal five-line grid, and surveyed with the EM—37 system. The dataindicate deep conductors both east and west of the Bend deposit, very nearlyon strike with the known mineralization. At this time drilling has notconfirmed the results of the EM—37 survey. jIt is important to note that the Bend AEM anomaly was detected by a singleflight line during the 1977 survey. The nominal line spacing used during thesurvey was 1/4 mile (1320 feet). Considering that the high—amplitude HLEManomaly has a short strike length at the subcrop (400+ feet), it is fortunatethat the deposit was detected at all. If the flight line had been displacedslightly to the east or west, the response would have been greatly diminish-ed, reducing its attractiveness as an exploration target. To complicatematters, several other formational conductors are located to the north, north-east, and southwest. One of these was drilled shortly after the initialdiscovery of Bend and found to be conductive graphitic argillite. Several
holes, too shallow to test the weak EM—37 responses along strike with theBend conductor, were drilled and found to contain little or no sulfidemineralization. However, the geologic section containing the host felsic tuffunit persists.
Geophysical work is continuing along the Bend volcanic trend in an attempt tolocate sister orebodies. Recent drilling southwest of Bend has shown that thefelsic tuff host rock does strike for several thousand feet at least. Ground
geophysical work to the northeast suggests similar trends in that area. Small
volcanogenic massive-sulfide deposits such as Bend usually occur in clustersand continued exploration will undoubtedly result in additional discoveries.
52
two holes, drilled in October 1986, were collared on lines 4 W and 8W and
intersected a thick massive-sulfide conductor, now known as the Bend
deposit. To date, 25 cored holes have delineated 3.7 million tons of copper-
gold mineralization in two zones. Depending on permit requirements, mining
operations could begin as early as 1995.
In order to investigate the possibility of the occurrence of deep mineralization
along strike, a large-loop time-domain EM-37 survey was performed. Large-
loop electromagnetic systems are often used for exploring deep ore deposits.
At Bend, several additional survey lines were added, east and west of the
original five-line grid, and surveyed with the EM-37 system. The data
indicate deep conductors both east and west of the Bend deposit, very nearly
on strike with the known mineralization. At this time drilling has not
confirmed the results of the EM-37 survey.
It is important to note that the Bend AEM anomaly was detected by a single
flight line during the 1977 survey. The nominal line spacing used during the
survey was 114 mile (1320 feet). Considering that the high-amplitude HLEM
anomaly has a short strike length at the subcrop (400+ feet), it is fortunate
that the deposit was detected at all. If the flight line had been displaced
slightly to the east or west, the response would have been greatly diminish-
ed, reducing its attractiveness as an exploration target. To complicate
matters, several other formational conductors are located to the north, north-
east, and southwest. One of these was drilled shortly after the initial
discovery of Bend and found to be conductive graphitic argillite. Several
holes, too shallow to test the weak EM-37 responses along strike with the
Bend conductor, were drilled and found to contain little or no sulfide
mineralization. However, the geologic section containing the host felsic tuff
unit persists.
Geophysical work i s continuing along the Bend volcanic trend in a n attempt to
locate sister orebodies. Recent drilling southwest of Bend has shown that the
felsic tuff host rock does strike for several thousand feet at least. Ground
geophysical work to the northeast suggests similar trends in that area. Small
volcanogenic massive-sulfide deposits such as Bend usually occur in clusters
and continued exploration will undoubtedly result in additional discoveries.
CONCLUSION
In north-central Wisconsin, it is likely that most massive—sulfide deposits willbe small, averaging only a few million tons and will be geophysically express-ed as short—strike—length anomalies. This may be a result of (1) primarysulfide deposition, (2) structural complications, (3) conductive overburden,(4) depth of burial, and/or (5) flight—line orientation and location. There-fore, most AEM surveys should be flown with l/8-mile (660 feet) spacing. Allsingle—line anomalies and subtle anomalies should be investigated with appro—pilate ground techniques, if possible.
Over the years several AEM exploration programs were undertaken in oraround the Bend area by different companies. For various reasons the BendAEM anomaly was not investigated, not recognized for its importance, not welldefined or, perhaps, not detected at all. Whatever the reason, the discoveryillustrates the need for persistence and thoroughness in exploration, even ifan area has been looked at before. Weak or poorly defined AEM anomalies oranomalies without direct magnetic support should always be considered forfurther evaluation if geologic conditions warrant.
53
CONCLUSION
In north-central Wisconsin, it is likely that most massive-sulfide deposits will
be small, averaging only a few million tons and will be geophysically express-
ed as short-strike-length anomalies. This may be a result of (1) primary
sulfide deposition, ( 2 ) structural complications. (3 ) conductive overburden,
( 4 ) depth of burial, andlor ( 5 ) flight-line orientation and location. There-
fore. most AEM surveys should be flown with 118-mile (660 feet) spacing. All
single-line anomalies and subtle anomalies should be investigated with appro-
priate ground techniques, if possible.
El ,,-Over the years several AEM exploration programs were undertaken in or
-^around the Bend area by different companies. For various reasons the Bend
AEM anomaly was not investigated. not recognized for i ts importance, not well
defined or , perhaps, not detected at all. Whatever the reason, the discovery
illustrates the need for persistence and thoroughness in exploration, even if
an area has been looked at before. Weak or poorly defined AEM anomalies or
anomalies without direct magnetic support should always be considered for
further evaluation if geologic conditions warrant.
54
GEOLOGY OF THE MERIDIAN AGGREGATES QUARRY AND THE SURROUNDING AREA, ST.CLOUD, MINNESOTA: A STUDY OF THE BEDROCK INFLUENCES ON THE PRE-LATE
CRETACEOUS WEATHERING PROFILE.j
John J. Heine, Tom A. lath, and Steven A. Hauck
Natural Resources Research Institute JUniversity of Minnesota-Duluth Duluth, MN 55811
George W. Shurrj
Department of Earth ScienceSt. Cloud State University St. Cloud, Mn. 56301
jThe Meridian Aggregates Quarry in St. Cloud, Minnesota contains a
complex group of metamorphic and igneous rocks. Residual clays overlie thebedrock in some areas of the quarry. The focus of the study is to relate thecomposition and physical characteristics of the parent rocks to the overlyingresidual clay deposits. The quarry has been mapped at a scale of 1:1200, inorder to give sufficient control of the geology and structuralcharacteristics. This information has been linked with information fromregional studies to better understand the controls on clay distribution inthe St. Cloud area.
Because the composition of the parent rock is a critical factor in thecomposition of the clays that form during weathering, detailed mapping of therock types in the quarry was critical. Five major rock types have beenmapped: 1) a dark metamorphic rock type, 2) gray reformatory granite, 3) St.Cloud Granite, 4) mafic dikes, and 5) a porphyritic rhyolite dike. Thedominate rock type in the quarry is the dark gray metamorphic. All of therock types can be found in the western quarry wall where residual clays havedeveloped on them.
The most important physical characteristic related to the formation ofthe residual clays is the presence of faults throughout the quarry. Many ofthe residual clay occurrences in the St. Cloud area are found in linearbedrock lows. Work in the Meridian Aggregates Quarry shows that the residualclays are found in bedrock lows along fault traces. This conclusion is basedon bedrock and drill hole information from the quarry. In the quarry area,glacial deposits rest directly on bedrock or the residuum. Any overlyingresidual clays that may have developed in other areas of the quarry, wouldhave been removed prior to or during glaciation, leaving pockets of residuumin bedrock lows. Three major structural features in the quarry were outlinedby mapping: 1) an east-west trending fault zone on the north wall of thequarry; 2) a central, northeast trending fault zone; and 3) a western,northwest trending fault zone. The deepest clay formation in the quarry isassociated the northwest fault zone. Drill hole information, provided byMeridian Aggregates outlines the extension of this zone outside the activequarry area. In addition, the drill hole information locates the deepestclay development near where the northwest fault zone an the east-west faultzone are thought to intersect. Other northwestern trending, residual clayfilled bedrock lows, have been identified to the northeast and southwest,which correspond with linear features visible on high altitude photos.
GEOLOGY OF THE MERIDIAN AGGREGATES QUARRY AN0 THE SURROUNDING AREA, ST CLOUD, MINNESOTA: A STUDY OF THE BEDROCK INFLUENCES ON THE PRE-LATE
CRETACEOUS WEATHERING PROFILE.
John J. Heine, Tom A. Toth, and Steven A. Hauck
Natural Resources Research Institute Minnesota-Duluth Duluth, MN 55811
George W. Shurr
. . rtment of Earth Sciences St. Cloud State University St. Cloud, Mn. 56301
The Meridian Aggregates Quarry in St. Cloud, Minnesota contains a complex group of metamorphic and igneous rocks. Residual clays overlie the bedrock in some areas of the quarry. The focus of the study is to relate the composition and physical characteristics of the parent rocks to the overlying residual clay deposits. The quarry has been mapped at a scale of 1:1200, in order to give sufficient control of the geology and structural characteristics. This information has been linked with information from regional studies to better understand the controls on clay distribution in the St. Cloud area.
Because the composition of the parent rock is a critical factor in the composition of the clays that form during weathering, detailed mapping of the rock types in the quarry was critical. Five major rock types have been mapped: 1) a dark metamorphic rock type, 2) gray reformatory granite, 3) St. Cloud Granite, 4) mafic dikes, and 5) a porphyritic rhyolite dike. The dominate rock type in the quarry is the dark gray metamorphic. All of the rock types can be found i n the western quarry wall where residual clays have developed on them.
The most important physical characteristic related to the formation of the residual clays is the presence of faults throughout the quarry. Many of the residual clay occurrences in the St. Cloud area are found in linear bedrock lows. Work in the Meridian Aggregates Quarry shows that the residual clays are found in bedrock lows along fault traces. This conclusion is based on bedrock and drill hole information from the quarry. In the quarry area, glacial deposits rest directly on bedrock or the residuum. Any overlying residual clays that may have developed in other areas of the quarry, would have been removed prior to or during glaciation, leaving pockets of residuum in bedrock lows. Three major structural features in the quarry were out1 ined by mapping: 1) an east-west trending fault zone on the north wall of the quarry; 2) a central, northeast trending fault zone; and 3) a western, northwest trending fault zone. The deepest clay formation in the quarry is associated the northwest fault zone. Drill hole information, provided by Meridian Aggregates outlines the extension of this zone outside the active quarry area. In addition, the drill hole information locates the deepest clay development near where the northwest fault zone an the east-west fault zone are thought to intersect. Other northwestern trending, residual clay filled bedrock lows, have been identified to the northeast and southwest, which correspond with linear features visible on high altitude photos.
The residual clay deposits in the Meridian Aggregates Quarry vary ingrade both vertically and horizontally. The residual clays vary fromkaolinite-rich to chiorite-illite-rich horizontally. This variation in claygrade appears to be controlled by the distribution of the bedrockcomposition. The kaolinite-rich residual clays are located above the St.Cloud Granite and the porphyritic rhyolite dike. The amount of chlorite andillite increases in the dark metamorphic rocks and the gray reformatorygranite, and is dominate in the residual material above the mafic dikes. Thedepth of weathering also varies horizontally, and is controlled by thenorthwest trending fault zone. The residual clays are deepest in the centerof the fault zone, and thin to the north and south, away form the fault zone.The vertical variation is increase quartz feldspar, and biotite, ±hornblendewith depth in residuum above the granitic and metamorphic rocks. Theincrease in these minerals is consistent with decreasing chemical weatheringeffects with depth.
55
sidual clay deposits in the Meridian Aggregates Quarry vary in rade both vertically and horizontally. The residual clays vary from aolinite-rich to chlorite-illite-rich horizontally. This variation in clay rade appears to be controlled by the distribution of the bedrock omposition. The kaolinite-rich residual clays are located above the St. loud Granite and the porphyritic rhyolite dike. The amount of chlorite and llite increases in the dark metamorphic rocks and the gray reformatory ranite. and is dominate in the residual material above the mafic dikes. The epth of weathering also varies horizontally, and is controlled by the orthwest trending fault zone. The residual d a y s are deepest in the center f the fault zone, and thin to the north and south, away form the fault zone. he vertical variation is increase quartz feldspar, and biotite, +hornblende
with depth in residuum above the granitic and metamorphic rocks. The increase in these minerals is consistent with decreasing chemical weathering effects with depth.
56
Isoclinal Slump-folds in the Lower Pokegarna Ouartzite: Evidence for Seismicity and SlopeInstability During Deposition of the Animikie Group
Hemming, S.; Hanson, G.N.: McLennan, S.M.; Sharp. W.D., State University of New York at StonyBrook, Stony Brook, NY 11794
The Early Proterozoic Pokegama Quartzlte overlies Archean basement of the Superior Province inthe northwestern part of the Animikie Basin, Minnesota. Its age is constrained by geologicrelationships [1,2,3] and Pb isotope data on detrital and cross-cutting quartz [4] as between 2.17and 1.95 Ga. The depositional environment of the very fine grained, lower Pokegama Ouartzite hasbeen interpreted as tidal flat, based on the nature of crossbedding in the sandy middle PokegamaQuartzite and suggested correlation and similarity to the better exposed Palms Formation inWisconsin [51. The soft sediment deformation we recognize in the lower Pokegama, mostly tight toisociinal folds and refolded folds, may provide important information concerning tectonic activity inthe Animikie Basin during this time interval.
Abundant evidence for slump-f olds can be found in the only well-exposed outcrop in the lowerPokegama Quartzite (Figs. 1-6). It is located immediately north of Eveieth, MN. The outcrop isapproximately 500 ft long and 5-8 ft high. It is characterized by interbedded siltstones and argilliteswith numerous thin, fine sandstone layers (up to several inches thick). Folding is generally verytight to isoclinal, and recumbent (Fig. 7), and these characteristics are suggested by Farrel andEaton [6] to indicate:1) long distant transport of the slumps and2) a southeasterly paleoslope, consistent with previous interpretations of shoreline position 15.7]-Figures 1-6 are line drawings, from field photographs, of some of the fold noses. These foldscorrespond to type 3 refolded folds, defined by Ramsay [8,6,9]. In addition to the numerousisoclinal fold noses, shear surfaces and at least one sheath fold indicate substantial horizontaldisplacement [6.10].
The Pokegama Quartzite, as well as the Biwabik and Gunflint Iron-formations, are here interpretedto have been deposited in a tectonically active basin. This interpretation is based on:1) evidence for soft-sediment deformation in the lower Pokegama Quartzite (this study):2) evidence for soft-sediment deformation, observed in the Biwabik Iron-formation:3) presence of early brecciation [11] and isoclinal folds in the Gunflint Iron-formation in Ontario:4) great variation in facies of the Pokegama just below the contact with the Biwabik, in the easternMesabi district (Ron Graver, per. comm., 1990): and -
5) sudden change in thickness of the Pokegama Quartzite and Biwabik Iron-formation across a Jpresumed fault in the eastern Mesabi district [12).Strike-slip faulting in the region north of the Animikie Basin at about 1.95 Ga has been documentedby Peterman and Day [13] using Rb-Sr dating of pseudotachylite in the Rainy Lake-Sein River faultsystem. They interpret the faulting to be a result of stresses associated with one of the EarlyProterozoic, circum-Superior orogens, including the Penokean, as described by Hoffman [14]. Theshales and turbidites of the Virginia and Thompson Formations, which form the upper part of theAnimikie Group, are interpreted to have been deposited in a foreland basin associated with thePenokean Orogen [1]. Understanding the tectonic setting of the Animikie Basin prior to turbiditedeposition clearly has important implications for understanding the Early Proterozoic history in theGreat Lakes Region. The location of the Animikie Basin across the Great Lakes tectonic zone 1151, .Jand the location of Penokean magmatism generally between the Superior Province and the j
Isoclinal Slump-folds in the Lower Pokegama Quartzite: Evidence for Seismicily and Slope Instability During Deposition of the Animikie Group
Hemming. S.; Hanson, G.N.; McLennan. S.M.; Sharp, W.D., State University of New York at Ston Brook. Stony Brook, NY 11794
The Early Proterozoic Pokegama Quartzite overlies Archean basement of the Superior Province in the northwestern part of the Animikie Basin, Minnesota. Its age is constrained by geologic relationships [I ,2,3] and Pb isotope data on detrital and cross-cutting quartz [4] as between 2.17 and 1.95 Ga. The depositional environment of the very fine grained, lower Pokegama Quartzite ha been Interpreted as tidal flat. based on the nature of crossbedding in the sandy middle Pokegama Quartzite and suggested correlation and similarity to the better exposed Palms Formation in Wisconsin [5]. The soft sediment deformation we recognize in the lower Pokegama. mostly tight t isoclinal folds and refolded folds, may provide important information concerning tectonic activity il the Animikie Basin during this time interval
Abundant evidence for slump-folds can be found In the only well-exposed outcrop in the lower Pokegama Quartzite (Figs. 1-6). It is located immediately north of Eveleth, MN. The outcrop is approximately 500 ft long and 5-8 ft high. It is characterized by interbedded siltstones and argillites with numerous thin, fine sandstone layers (up to several inches thick). Folding is generally very tight to Isoclinal, and recumbent (Fig. 7). and these characteristics are suggested by Farrel and Eaton [6] to indicate: 1) long distant transport of the slumps and 2) a southeasterly paleoslope, consistent with previous interpretations of shoreline position [5,7]. Figures 1-6 are line drawings, from field photographs, of some of the fold noses. These folds correspond to type 3 refolded folds, defined by Ramsay [8,6,9]. In addition to the numerous isoclinal fold noses, shear surfaces and at least one sheath fold indicate substantial horizontal displacement [6,10].
The Pokegama Quartzite, as well as the BIwabik and Gunflint Iron-formations, are here interpreted to have been deposited in a tectonically active basin. This interpretation is based on: 1) evidence for soft-sediment deformation in the lower Pokegama Quartzite (this study); 2) evidence for soft-sediment deformation, observed in the Wwabik iron-formation; 3) presence of early brecciatton [ l l ] and isoclinal folds in the Gunflint Iron-formation in Ontario; 4) great variation in facies of the Pokegama just below the contact with the Biwabik, in the easterr Mesabi district (Ron Graver, per. comm.. 1990); and 5) sudden change in thickness of the Pokegama Quartzite and Wwabik Iron-formation across a presumed fault in the eastern Mesabi district [12]. Strike-slip faulting in the region north of the Animikie Basin at about 1.95 Ga has been documents by Peterman and Day [I31 using Rb-Sr dating of pseudotachylite in the Rainy Lake-Sein River fau system. They interpret the faulting to be a result of stresses associated with one of the Early Proterozoic, circum-Superior orwens. including the Penokean. as described by Hoffman [14]. The shales and turbidites of the Virginia and Thompson Formations, which form the upper part of the Animlkie Group, are Interpreted l o have been deposited In a foreland basin associated with the Penokean Oroaen 111. Understandina the tectonic senina of the Animikie Basin prior to turbidite - . . - - deposition clearly has important implications for understanding the Early Proterozoic history in the Great Lakes Region. The location of the Animikie Basin across the Great Lakes tectonic zone 1151, and the location of Penokean magmatism generally between the Superior Province and the
Minnesota River Valley Province makes the sedimentary rocks within the Aninilkle basin importantfor evaluating the nature and location of the pre-Penolce,an continental margin. An obliqueextensional setting, either drtven by or associated with, strike sup faulting Is suggested as a woiidngmodel for early Animikie Group deposition. Such a setting could be associated wfth the passivemargin side of a back arc basin (such as seen between Japan and China, and consistent with theinterpretation of Van Schmus [161 and Southwlck at at [1)). The time Interval for tectonic actMtyduring deposition of the Pokegama Ouartzite may be constrained between 2.17 and 1.95 Ga. The1.95 Ga Pb-Pb age is from quartz gash veins In the upper Pokegama Quartzite. Additional ageconstraints for pseudotach,lltes and quartz veins in the basement and sedimentary rocks of theregion, in conjunction with an understanding of their kinematic significance, may lead to importantfurther constraints on the early evolution of the Animikie Basin.
References: (1) Southwlck, D.L etat, 1988. MN Geol. Sur., Report of Investigations 37; [2)Southwlck, 0.L, and Day. W.C.. 1983, Can. Jour. Earth Sci..v.20.p.622; (31 Hanson, G.N., andMalhotra, R., 1971, Geot. Soc. America Bull., v.82.p.1107: [4) Hemming et al, 1990, EOS,v71p.654; 151 Ojakangas, R.W..1983. Geol. Soc. America Mom. 160. p.49; [6] Farrel, S.G., andEaton, 5,1987, Geoi. Soc. Spec. Pub, no 29; [7) Morey, G.B.,1972, MN Geol. Sur. Centennial Vol;[SI Ramsay, J.G.,1967, Folding and Fracturing of Rocks. McGraw-Hill, New York; [9) Rarnsay, J.G.,and Huber, M.i., 1987, The Techniques of Modem Structural Geology, v.2, Folds and Fractures.Academic Press; 110) Allen, J.R.L, 1982, Sedimentary Structures, Their Character and PhysicalBasis, 0ev. in Sod 306, Elsevier; till Franklin, J.M. eta!, 1972, IGC Field Excursion 034; [12)Gundersen, J.N., and Schwartz, G.M.,1952, MN God. Sur. Bull. 43; (Peterman, Z.E. and Day, W,1989. Geology. v.17,p.1089; [14) Hoffman, P.F., 1988, Ann. Rev. Earth and Plan. Sd, v.16,p. 543;[15) Sims, P,K et ai.. 1980, Geol. Soc. America Bull., v.91,p.690; [16) Van Schrnus, 1976, Phd.Trans. R. Soc. Lond..v280,p.605.
Fig. 1 Relatively large area of converging fold features. Heart-shaped sand body and elongate refolded fold are Interpreted astype 3 refolded folds of Ramsey [81. Note the fanning cleavage inthe argiilfte, between the refolded folds. Hammer for scale.
57
- —E:c
C -- a
— 'a.- -a
Minnesota River Valley Province makes the sedimentary rocks within the AnImIMe bask) Important for evaluating the nature and location of the pra-Penokean continental margin. An oblique extenstonal setting, either driven by or associated wfth, strike slip faulting Is suggested as a working model for eaW Anhlkle Group deposition. Such a setting could be associated with the passive margin side of a back arc basin (such as seen between Japan and China, and consistent with the Interpretation of Van Schmus (161 and Southwick et a/. (11). The time Interval for tectonic activity during deposition d the Pokegama Quartzite may be constrained between 2.17 and 1.35 Ga. The 1.95 Ga Pb-Pb age Is from quartz gash veins in the upper Pokagama Quartzite. Additional age constraints for pseudotachylites and quartz veins In the basement and sedimentary rocks of the region. In conjunction with an understanding of their kinematic significance, may lead l o Important further constraints on the early evolution of the Animikle Basin '
References: (11 Southwick, D L eta/.. 1988, MN Geol. Sur., Re&; of lnve Southwick. D.L. and Day, W.C.. 1983. Can. Jour. Earth Scl.,v.20,p.622; [3] Malhotra. R., 1971. Ged. Soc. America Bull.. v.82.p.1107- 141 Hemming et at., 1990. EOS. v.71 .p.654; (51 Ojakangas. R.W..1983. Geol. Soc. America Mem. 160. p.49; (61 Farrel, S.G., and Eaton. S..1987. Ged. Soc. Spec. Pub. no 28; [71 Morey, GA.1972, MN Geol. Sur. Centennial Vol: 181 Ramsay. J.G..1967. Folding and Fracturing of Rocks. McGraw-Hill. New York; (91 Ramsay, J.G.. and Huber, M.I.. 1987. The Techniques of Modem Structural Geology. v.2. Folds and Fractures, Academic Press; [lo) Alien. J.R.L. 1982. Sedimentary Structures. Their Character and Physical Basis, Dev. in Sed. 306. Elsevier; (1 1) Franklin, J.M. era/, 1972. IGC Field Excursion C34; [I21 Gundersen. J.N.. and Schwartz, G.M.,1%2. MN Geol. Sur. Bull. 43; [Peterman. Z.E. and Day, W.,
Geology. v.17.p.1089; (14) Hoffman. P.F.. 1888. Ann. Rev. Earth and Plan. Scl. v.16.p. 543; ims. P.K et al.. 1980, Ged. Soc. America Bull.. v.91.p.690; [I61 Van Schmus, 1976. PM. . R. Soc. Lond..v280.p.605.
Fig. 1 Relatively large area of converging fold features. Heart- shaped sand body and elongate refolded fold are Interpreted as type 3 refolded folds of Ramsay (81. Note the fanning cleavage in the argillite. between the refolded folds. Hammer for scale.
Fig. 2 A pair of fold noses. Hinge one (labeled) clearly visible onthe nose to the left. It is not possible to tell if the nose on the rightis refolded. Note scale bar at lower left of drawing.
Fig. 3 Fold nose with several sandy layers that are thickened inthe hinge and thinned to boudinaged on the limbs. The boxdrawn on this figure is the approximate location of Fig. 4. Non.....scale bar at lower left of drawing.
——j
Fig. 4 Detail of Fig. 3. Note thinning to separation of the limbs Fig. 5 An early tight fold refolded by a later tight recumbant fold.and thickening in the hinge of the sandier layer. Note scale bar at lower of drawing:
Fig. 6 Recumbent isoclinal fold. Lower limb is thinned andbedding is quite disrupted. Note scale bar at lower left ofdrawing.
_
- —a- —C— — ---- -. — — --—s——,-- t=_ — — - -a — —= —4
J
--2CM
58
Fig. 7 Stereoplot of fold hinges in the Lower Pokegama. Theplane representing the approximate attitude of bedding at theoutcrop is also plotted (1].
Fig. 2 A pair of fold noses. Hinge one (labeled) clearly visible on Fi9.3 Fold nose with several sandy layers that are thickened the nose to the left. It is not possible to tell if the nose on the right the hinge and thinned to boudinaged on the limbs. The box is refolded. Note scale bar at lower left of drawing. drawn on this figure is the approximate location of Fig. 4. N<
scale bar at lower left of drawing.
Fig. 4 Detail of Fig. 3. Note thinning to separation of the limbs and thickening in the hinge of the sandier layer.
Fig. 5 An early tight fold refolded by a later tight recumbant f Note scale bar at lower of drawing.
Fig. 6 Recumbent isoclinal fold. Lower limb is thinned and ~ i g , 7 Stereoplot of fold hinges in the Lower Pokqama. The bedding is quite disrupted. Note scale bar at lower left of @ane representing the approximate attiiude of bedding at th6 drawing. outcrop is also plotted [I].
Magmatic Evolution in the Midcontinent Rift: Evidence fromHypabyssal Rocks of the North Shore of Lake superior
Eric A. Jerde
Department of Earth arid Space Sciences, University of California, Los Angeles, CA 90024
The hypabyssal dikes and sills of northeastern Minnesota associated with the 1.1 GaMidcontinent Rift are a suite of rocks distinct from the flows of the North Shore VolcanicGroup, and although the dikes are far less significant volumetrically, they provide valuable
information about processes operating during rifting.In general, the diabase is ophitic, with pyroxenes 1-3 cm across containing varying
amounts of enclosed plagioclase and olivine chadacrysts. Exsolution features in pyroxene arecommon, but do not dominate the grain population. In a few samples, coarse symplectites arepresent, with blebby oxides within the pyroxene. Some of these symplectites displayexsolution lamellac in the pyroxene. This is evidence that the oxides and pyroxene formed
comagmatically, and not simply as an alteration of a pre-existing phase such as olivine sinceexsolution features are unlikely to form in alteration products.
Plagioclase is present mainly as laths and fragments formed during rapid cooling,although a small number of phenociysts are usually seen. These are not large (usually <2
mm), and many display prominent pits and dusty regions, interpreted as resorption featuresresulting from rapid decompression. Such an origin has been suggested for similar features in
plagioclase from the Rio Grande rift (Lipman, 1969), and the southern Rocky Mountains(Doe et aL, 1969). Plagioclase compositions range from laths and chill fragments of An5n,to phenocryst cores up to An. The phenoczysts often have rims of the "chill" composition,and a few are complexly zoned.
Olivine occurrs as fine disseminated grains, glomerociysts up to 3 mm, and occasionaloikocrysts up to 1 cm. It is generally unzoned, with compositions mainly between Fo andFop. Oxides make up a few percent of all samples, most being <1 mm across. Both ilmeniteand spinel phases are present, and most grains display some oxyexsolution features.
Mineral thermometiy (Table 1) performed on plagioclase and pyroxene gave crystallizationtemperatures of 1000- 1150°C. Oxides yielded temperatures between 850°C and 1000°C,and oxygen fugacities approximately one log unit below the QEM buffer, which are consistentwith results from the Portage Lake Volcanics (Paces, 1988). Oxide phases re-equilibrate
rapidly with a cooling liquid, and thus the temperatures obtained for the oxides are probablybest interpreted as near-solidus temperatures. The oxygen fugacities indicated are consistent
also with the modelling by Miller et al. (1990) for the evolution of the Sonju Lake intnision,which produced the best results when using oxygen fugacities one log unit below QEM.
The chilled margins were analyzed by INAA and microprobe fused bead techniques forbulk chemistry. Modelling of fractionation was done using the CHAOS program (a more
59
Magmatic Evolution in the Midcontinent Rift: Evidence from Hypabyssal Rocks of the North Shore of Lake Superior
Eric A. Jerde
Department of Earth and Space Sciences, University of California, Los Angeles, CA 90024
The hypabyssal dikes and sills of northeastern Minnesota associated with the 1.1 Ga Midcontinent Rift are a suite of rocks distinct from the flows of the North Shore Volcanic Group, and although the dikes are far less significant volurnetrically, they provide valuable
information about processes operating during rifting. In general, the diabase is ophitic, with pyroxenes 1-3 cm across containing varying
amounts of enclosed plagioclase and olivine chadacrysts. Exsolution features in pyroxene are common, but do not dominate the grain population. In a few samples, coarse symplectites are present, with blebby oxides within the pyroxene. Some of these symplectites display exsolution lamellae in the pyroxene. This is evidence that the oxides and pyroxene formed comagrnatically, and not simply as an alteration of a pre-existing phase such as olivine since exsolution features are unlikely to form in alteration products.
Plagioclase is present mainly as laths and fragments formed during rapid cooling, although a small number of phenocrysts are usually seen. These are not large (usually <2
mm), and many display prominent pits and dusty regions, interpreted as resorption features resulting from rapid decompression. Such an origin has been suggested for similar features in
plagioclase from the Rio Grande rift (Lipman, 1969), and the southern Rocky Mountains (Doe et al., 1969). Plagioclase compositions range from laths and chill fragments of Ans<,.;, to phenocryst cores up to Anon. The phenocrysts often have rims of the "chill" composition, and a few are complexly zoned.
Olivine occwrs as fine disseminated grains, glomerocrysts up to 3 rnm, and occasional " oikocrysts up to 1 cm. It is generally unzoned, with compositions mainly between Fom and
PoTT). Oxides make up a few percent of all samples, most being <1 mm across. Both ilmenite and spinel phases are present, and most grains display some oxyexsolution features.
Mineral thermometry (Table 1) performed on plagioclase and pyroxene gave crystallization temperatures of 1000 - 1 150°C Oxides yielded temperatures between 850eC and 1000°C and oxygen fugacities approximately one log unit below the QFM buffer, which are consistent with results from the Portage Lake klcanics (Paces, 1988). Oxide phases re-equilibrate
rapidly with a cooling liquid, and thus the temperatures obtained for the oxides are probably best interpreted as near-solidus temperatures. The oxygen fugacities indicated are consistent also with the modelling by Miller et al. (1990) for the evolution of the Sonju Lake intrusion, which produced the best results when using oxygen fugacities one log unit below QFM.
The chilled margins were analyzed by INAA and microprobe fused bead techniques for bulk chemistry. Modelling of fractionation was done using the CHAOS program (a more
0T
ale
i.M
iner
alge
oth.
riao
m.te
rre
sults
.
23
45
67
89
1011
1213
MgO
(%
)F
ig.
2. D
ingt
nmsh
owin
g hy
paby
ssal
mat
eria
l mat
chin
g I-
atm
mod
el.
(Silv
er R
ay in
tnis
ions
from
Sha
nk.
1990
; 1.
Mill
er, p
ers.
Com
m.;
47th
Ave
nue
SiU
from
K. S
eife
rt, p
ert C
omm
.
0•
Thi
sstu
dy-
aO
tlier
dike
sI3
RD
0-0-
0-o
00°
BR
D.5
-o..5
-•
BR
D.5
..2-.
51,
1,1,
11,
111
L1Iii
23
45
67
89
101!
1213
MgO
(%)
Fig
.1.
Din
gnim
sho
win
g I-
aim
mod
els
and
hypa
byss
al r
ocks
.('O
ther
dik
es"
from
Gre
en e
t al.,
198
7)
12 II
tVlg
O (
%)
Fig.
3.
Dia
gram
sho
win
g po
ssib
le fr
nciio
nnlio
n pn
lhs
for
nil
hypa
hyss
al r
ocks
.
LJ L
LL.
r
__
Eli
C L
LL
.L.
..I
__
L_[
LL
L_L
12 11
•a
I''I'
.0 0
sj
10
tsijt
j&df
lPi
agio
ciae
ea3
oTtJ
Pyro
x.n.
Oxi
de
1-1o
2
SB—
ico
re12
1110
0911
00 ±
5099
0—
12.3
gene
1192
1065
P.R
—i
core
1194
1064
1050
± 5
085
3—
14.8
qthn
a11
5610
15
BR
—i
1192
1066
1000
(115
0 9
5kb)
HP—
ico
re12
4511
3811
50 ±
50
gdm
s12
3611
26
CL
—i
961
—13
.0
P—S
860
—15
.0
9
00
C
a Cl
Là
0
0S
00
000
7
oa°
0
6 S
Tem
pera
ture
s in
°C
."g
dm&
' —gr
ound
mae
apl
agio
cias
e
12 11 10
a Cl
C)
BR
D
+
+.
4
9 S 7 6 5
l0 9
+ 0 .
C Cl
LàLe
ster
Riv
er S
ill
47th
Ave
nue
Sill
Silv
er R
ay in
trus
ions
I3R
D fr
act.
(I a
tm)
++
8 7 6
34
56
78
910
II12
recent version of TRACE.FOR described by Nielsen, 1988a,b). One atmosphere modelling
(Fig. 1) shows that the majority of the hypabyssal rocks formed at moderate pressures wherethe onset of pyroxene crystallization is earlier due to greater pyroxene stability at depth. This
earlier ciysta]iization results in the earlier depletion of Ca, leading to the offset of the data fromthe model trend. A number of hypabyssal rocks show evidence of fractionation at nearsurface levels (Fig. 2), falling on the model trend.
It is possible through polybaric fractionation to relate all of the varied hypabyssal rocks to
a single primary parent, the high-Al "primitive olivine tholeiites," which are a commoncomposition among the volcanic rocks associated with the Midcontinent Rift. In such a
scenario, a primitive olivine tholeiite magma crystallizes ol+plag+pyx at depth (—10 kb?),driving the composition to lower Ca and Mg (BRD in Fig. 3). Continued fractionation at
depth could lead to compositions similar to the Sonju Lake parent proposed by Miller et al.(1990). If a the magma of BRD composition is injected to an intermediate depth (—3-5 kb?),the crystallizing assemblage would lose pyroxene due to shrinkage of the pyroxene stabilityfield, and the ol+plag crystallization would carry the liquids to lower Mg (point B in Fig. 3),
where pyroxene would again crystallize, and the liquid compositions then migrate down inCa, along the trend seen by many of the hypabyssal rocks (toward C in Fig. 3). If someliquids of BRD composition were actually injected to near surface regions, pyroxenecrystallization could be delayed even further, and the liquids would follow the trend shown in
the modelling for 1-atm pressure (toward D in Fig. 3). Sills such as the Lester River and47th
Avenue are in this group, as are the Silver Bay intrusions. In reality, there can of course be
many cross trends, but the overall picture of multi-level fractionation is apparent.The resorption features seen in the plagioclase; evidence for evolution at various levels in
the crust, are consistent with the chemistry, which suggests the same process. The chemicalsimilarity among samples of the entire suite is also evidence for lateral communication among
chambers for great distances along the rift. These two processes have been inferred for theMid-Atlantic rift in Iceland, which indicates that some modem rift processes were occurring at
1.1 Ga.
References:Doe, B.R., Lipman, P.W., Hedge, C.E., and Kurasawa, II. (1969). Contributions to
Mineralogy and Petrology, 2 1, 142-156.Green, J.C., Bornhorst, Ti., Chandler, V.W., Mudrey, M.G.J., Meyers, P.E., Pesonen,
L.J. and Wilband, J.T. (1987) In: Mafic dyke swarms (Halls, R.B. and Fahrig, W.F.,ed.), Geological Association of Canada Special Paper 34, 289-302.
Lipman, P.W. (1969). Geological Society of America Bulletin, 80, 1343-1354.Miller, J.D., Jr., Schaap, B.D. and Chandler, V.W. (1990). 36th Annual Institute on Lake
Superior Geology, 66-68.Nielsen, R.L. (l988a) Geochimica a Cosmochimica Acta, 52, 27-38.Nielsen, R.L. (1988b) Computers and Geosciences, 14, 15-35.Paces, J.B. (1988) Ph.D. dissertation, Michigan Technological University, Houghton, 4l3p.Shank, S.G. (1990) M.S. thesis, University of Minnesota, Minneapolis, l3Op.
61
recent version of TRACEFOR described by Nielsen, 1988a,b). One atmosphere modelling (Fig. 1) shows that the majority of the hypabyssal rocks formed at moderate pressures where the onset of pyroxene crystallization is earlier due to greater pyroxene stability at depth. This earlier crystallization results in the earlier depletion of Ca, leading to the offset of the data from the model trend. A number of hypabyssal rocks show evidence of fractionation at near surface levels (Fig. 2), falling on the model trend.
It is possible through polybaric fractionation to relate all of the varied hypabyssal rocks to a single primary parent, the high-A1 "primitive divine tholeiites," which are a common composition among the volcanic rocks associated with the Midcontinent Rift. In such a sw&ho, a primitive olivine tholeiite magma crystallizes ol+plag+pyx at depth (-10 kb?), driving the composition to lower Ca and Mg (BRD in Fig. 3). Continued fractionation at depth could lead to compositions similar to the Sonju Lake parent proposed by Miller et al. (1990). If a the magma of BRD composition is injected to an intermediate depth (-3-5 kb?), the crystallizing assemblage would lose pyroxene due to shrinkage of the pyroxene stability field, and the ol+plag crystallization would cany the liquids to lower Mg (point B in Fig. 3), where pyroxene would again crystallize, and the liquid compositions then migrate down in Ca, along the trend seen by many of the hypabyssal rocks (toward C in Fig. 3). If some liquids of BRD composition were actually injected to near surface regions, pyroxene crystallization could be delayed even further, and the liquids would follow the trend shown in
the modelling for 1-atm pressure (toward D in Fig. 3). Sills such as the Lester River and 47th
Avenue are in this group, as are the Silver Bay intrusions. In reality, there can of course be many cross trends, but the overall picture of multi-level fractionation is apparent.
The resorption features seen in the plagioclase; evidence for evolution at various levels in the crust, are consistent with the chemistry, which suggests the same process. The chemical similarity among samples of the entire suite is also evidence for lateral communication among chambers for great distances along the rift. These two processes have been inferred for the Mid-Atlantic rift in Iceland, which indicates that some modem rift processes were occurring at 1.1 Ga.
References: Doe, B.R., Lipman, P.W., Hedge, C.E., and Kurasawa, H. (1969). Contributions to
Mineralogy and Petrology, 2 1, 142-156. Green, J.C., Bornhorst, T.J., Chandler, V.W., Mudrey, M.G.J., Meyers, P.E., Pesonen,
L.J. and Wilband, J.T. (1987) In: Mafic dyke swarms (Halls, R.B. and Fahrig, W.F., cd.). Geological Association of Canada Special Paper 34,289-302.
Lipman, P.W. (1969). Geological Society ofArnericaBulletin, 80, 1343-1354. Miller, J.D., Jr., Schaap, B.D. and Chandler, V.W. (1990). 36th Annual Institute on Lake
Superior Geology, 66-68. Nielsen, R.L. (1988a) Geochimica et Cosmochimica Ada, 5 2, 27-38. Nielsen, R.L. (1988b) Computers and Geosciences, 1 4, 15-35. Paces, J.B. (1988) Ph.D. dissertation, Michigan Technological University, Houghton, 413p. Shank, S.G. (1990) M.S. thesis. University of Minnesota, Minneapolis, 130p.
JSURFACE EXPRESSION OF MAJOR BEDROCK STRUCTURAL FEATURES
James S. Johnson, Denise A. Stavish, Mary K. Tozer, and George W. ShurrDepartment of Earth Sciences, St. Cloud State University, St. Cloud, MN 56301
Continental lithosphere in Minnesota is subdivided into a series of struc-tural blocks bounded by fault zones that are components of Precambrian platemargins. Major fault zones in the crystalline basement have expression inlandfonns and surface sediments. For example, in western Minnesota, the GreatLakes Tectonic Zone is located at the southern limit of the Lake Agassiz plain;and in southwestern Minnesota, faults bounding the Fulda and Pipestone structuralbasins (Southwick and Mossler, 1984) correspond with the end moraine complex onthe southwestern margin of the Des Moines lobe. Equivalence of basement structureand surface features will be described in three other areas: 1) Agassiz beacharea in northwestern Minnesota, 2) Swanville spillway area in central Minnesota,and 3) north of Lake Mille Lacs.
In northwestern Minnesota, the beach complex at the margins of the LakeAgassiz plain forms a broad arc opening to the south and east. Northwest—trending faults in the Archean basement generally converge toward the curve ofthe arc and parallel the sides of the arc of beach ridges. Near Greenbush, MN,sharp variations in the elevation of the beach-ridge crest, mark the positionof large areomagnetic gradients associated with the Vermillion Fault Zone. Incentral Minnesota, the St. Croix end moraine complex has a broad arc to thewest between the Serpent Lake and Malmo structural discontinuities mapped bySouthwick, Morey, and McSwiggen (1988). On the inside of the arc of the endmoraine, several eskers trend perpendicular to and terminate at the positionof the Malmo discontinuity. North of Lake Mille Lacs patterns of lowland peatand upland till are similar to the geometry of Proterozoic thrust duplexes.Eskers in this vicinity are generally located along areas of steep areomagneticgradient and trend parallel to the contours.
Correspondence of major structural features in the Precambrian basementwith geomorphic features and surface sediments helps to explain why linearfeatures mapped on Landsat images appear to mark basement structures. Beachridges, morainic margins, and some eskers are all visible as Landsat linearfeatures; eskers trending perpendicular to structural features are not easilymapped as obvious linear features. Paleotectonic and recent earth movementsalong the lithosphere block boundaries influenced patterns of erosion anddeposition. As a consequence, patterns of erosion and deposition visible aslinear features on Landsat images show fidelity with structures in the Pre-cambrian basement.
REFERENCES CITED
Southwick, D.L., and Mossler, J.H., 1984, The Sioux Quartzite and subjacentregolith in the Cottonwood County Basin, Minnesota, in Southwick, D.L.,ed., Shorter contributions to the geology of the Sioux Quartzite (EarlyProterozoic), Southwestern Minnesota: Minnesota Geological Survey Reportof Investigations 32, p. 17—44.
Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map of thePenokean Orogen, central and eastern Minnesota, and accompanying text:Minnesota Geological Survey Report of Investigations 37, 25 p.
62
SURFACE EXPRESSION OF MAJOR BEDROCK STRUCTURAL FEATURES I
James S. Johnson, Denise A. Slavish, Mary K. Tozer, and George W. Shurr Department of Earth Sciences, St. Cloud State University, St. Cloud, MN 56301
Continental lithosphere in Minnesota is subdivided into a series o f struc- tural blocks bounded by fault zones that are components of Precambrian plate margins. Major fault zones in the crystalline basement have expression in landforms and surface sediments. For example, in western Minnesota, the Great Lakes Tectonic Zone is located at the southern limit of the Lake Agassiz plain; and in southwestern Minnesota, faults bounding the Fulda and Pipestone structural basins (Southwick and Mossier, 1984) correspond with the end moraine complex on the southwestern margin of the Des Moines lobe. Equivalence of basement structure and surface features will be described in three other areas: 1) Agassiz beach area in northwestern Minnesota, 2) Swanville spillway area in central Minnesota, and 3) north of Lake Mille Lacs.
In northwestern Minnesota, the beach complex at the margins of the Lake Agassiz plain forms a broad arc opening to the south and east. Northwest- trending faults in the Archean basement generally converge toward the curve of the arc and parallel the sides of the arc of beach ridges. Near Greenbush, MN, sharp variations in the elevation of the beach-ridge crest, mark the position of large areomagnetic gradients associated with the Vermillion Fault Zone. In central Minnesota, the St. Croix end moraine complex has a broad arc to the west between the Serpent Lake and Malmo structural discontinuities mapped by Southwick, Morey,and McSwiggen (1988). On the inside of the arc of the end moraine, several eskers trend perpendicular to and terminate at the position of the Malmo discontinuity. North of Lake Mille Lacs patterns o f lowland peat and upland till are similar to the geometry of Proterozoic thrust duplexes. Eskers in this vicinity are generally located along areas of steep areomagnetic gradient and trend parallel to the contours.
Correspondence of major structural features in the Precambrian basement with geomorphic features and surface sediments helps to explain why linear features mapped on Landsat images appear to mark basement structures. Beach ridges, morainic margins, and some eskers are all visible as Landsat linear features; eskers trending perpendicular to structural features are not easily mapped as obvious linear features. Paleotectonic and recent earth movements along the 1 ithosphere block boundaries influenced patterns of erosion and deposition. As a consequence, patterns of erosion and deposition visible as linear features on Landsat images show fidelity with structures in the Pre- cambri an basement.
REFERENCES CITED
Southwick, D.L., and Mossier, J.H., 1984, The Sioux Quartzite and subjacent regolith in the Cottonwood County Basin, Minnesota, in Southwick, D.L., ed., Shorter contributions to the geology of the Sioux Quartzite (Early Proterozoic), Southwestern Minnesota: Minnesota Geological Survey Report of Investigations 32, p. 17-44.
Southwick, D.L., Morey, G.B., and McSwiggen, P.L., 1988, Geologic map of the Penokean Orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Investigations 37, 25 p.
THE LYNNE MASSIVE SULFIDE DEPOSIT, ONEIDA COUNTY, WISCONSIN
Lawrence P. Kennedy, Teresa A. Harding, John N. Schaff,& Anne N. Zielinski, Noranda Exploration, Inc.,
Rhinelander, Wisconsin
The Lynne deposit is a volcanic- to volcanicl•ast ic-hosted, Zn-Pb-Cu—Ag orebody located in the early Proterozoic magmatic terraneof northern Wisconsin. A preliminary ore reserve, based on thefirst 39 holes in an open-pit mine design, is calculated at 6.1million tons grading 7.14% Zn, 0.34% Cu, 1.89% Pb, 3.09 opt Ag, and0.013 opt Au. The mineralization is hosted by a gently-dippingsequence of fragmental rhyolites, chiefly lapilli to crystal tuffs,and less abundant dacite to andesite flows, volcaniclasticsandstones and siltstones. Facies relationships in the rhyoliticpyroclastic rocks to the north (down-dip) suggests proximity to avolcanic constructional feature. A footwall tonalite intrudes andpartly encloses the lowermost part of the deposit, and a number ofsubvertical, east— to southeast-trending faults disrupt the down-dip extension of mineralization. Many of these faults are occupiedby feldspar-phyric, rhyolitic to basaltic dikes.
The bulk of the Zn-Pb-Cu-Ag mineralization occurs within asequence of chemical sedimentary rocks which thicken and coalescein the central part of the deposit. Lenses, lobes and stratiformbodies of massive to semi-massive sulfide are interbedded withconformable talc units, chert, laminated to disrupted carbonaterocks, and narrow beds of volcaniclastic and sedimentary rocks.The talcose rocks underlie and partly enclose lenses of laminated,pyrrhotite—rich chert and sphalerite-rich masEive sulfide, and alsohost disseminated sphalerite, pyrrhotite, and galena. The talcoseunit is overlain by thick beds of massive sulfide and lenticularmasses of carbonate and chert containing disseminated to beddedsulfides. The aggregate thickness of the sulfide-rich sedimentaryrocks exceeds 325 feet in the center of the deposit; its lenticularnature and symmetry suggests that it may have formed as a
hydrothermal mound, with an undetermined amount of replacementmineralization.
Sphalerite is the most abundant sulfide mineral, followed inabundance by pyrrhotite, galena, pyrite, and chalcopyrite.Tetrahedrite (var. freibergite) or polybasite, native silver,pyrargyrite, electrum, and native gold are also economicallyimportant minerals. The sulfide distribution and metal zonation issimilar to the zoning documented in other volcanogenic massivesulfide deposits. Chalcopyrite and pyrrhotite are most abundant inthe lowermost talcose unit, whereas silver minerals and galena areconcentrated in diopside-rich cherts and cherty beds in the upperpart of the deposit. Calc-silicate minerals, including diopside,ferrosalite, garnet, tremolite, and epidote, are widespread; mostof these minerals formed as a result of decarbonation reactionsduring the emplacement of the footwall tonalite, or perhaps duringthe late stages of hydrothermal activity.
63
THE LYNNE MASSIVE SULFIDE DEPOSIT, ONEIDA COUNTY, WISCONSIN
Lawrence P. Kennedy, Teresa A. Harding, John N. Schaff, & Anne M. Zielinski, Noranda Ex~loration. Inc..
The Lynne deposit is a volcanic- to volcaniclastic-hosted, Zn- Pb-Cu-Ag orebody located in the early Proterozoic magmatic terrane .of northern Wisconsin. A preliminary ore reserve, based on the *Â¥firs 39 holes in an open-pit mine design, is calculated at 6.1 illion tons grading 7.14% Zn, 0.34% Cu, 1.89% Pb, 3.09 opt Ag, and .013 opt Au. The mineralization is hosted by a gently-dipping
sequence of fragmental rhyolites, chiefly lapilli to crystal tuffs, and less abundant dacite to andesite flows, volcaniclastic sandstones and siltstones. Facies relationships in the rhyolitic yroclastic rocks to the north (down-dip) suggests proximity to a olcanic constructional feature. A footwall tonalite intrudes and partly encloses the lowermost part of the deposit, and a number of subvertical, east- to southeast-trending faults disrupt the down- dip extension of mineralization. Many of these faults are occupied b y feldspar-phyric, rhyolitic to basaltic dikes.
The bulk of the Zn-Pb-Cu-Ag mineralization occurs within a ence of chemical sedimentary rocks which thicken and coalesce
in the central part of the deposit. Lenses, lobes and stratiform odies of massive to semi-massive sulfide are interbedded with onformable talc units, chert, laminated to disrupted carbonate rocks, and narrow beds of volcaniclastic and sedimentary rocks. The talcoserocks underlie and partly enclose lenses of laminated,
pyrrhotite-rich chert and sphalerite-rich massive sulfide, and also host disseminated sphalerite, pyrrhotite, and galena. The talcose unit is overlain by thick beds of massive sulfide and lenticular masses of carbonate and chert containing disseminated to bedded sulfides. The aggregate thickness of the sulfide-rich sedimentary rocks exceeds 325 feet in the center of the deposit; its lenticular nature and symmetry suggests that it may have formed as a hydrothermal mound, with an undetermined amount of replacement mineralization.
Sphalerite is the most abundant sulfide mineral, followed in abundance by pyrrhotite, galena, pyrite, and chalcopyrite. Tetrahedrite (var. freibergite) or polybasite, native silver, pyrargyrite, electrum, and native gold are also economically important minerals. The sulfide distribution and metal zonation is similar to the zoning documented in other volcanogenic massive sulfide deposits. Chalcopyrite and pyrrhotite are most abundant in the lowermost talcose unit, whereas silver minerals and galena are concentrated in diopside-rich cherts and cherty beds in the upper part of the deposit. Calc-silicate minerals, including diopside, ferrosalite, garnet, tremolite, and epidote, are widespread; most of these minerals formed as a result of decarbonation reactions during the emplacement of the footwall tonalite, or perhaps during the late stages of hydrothermal activity.
jTHICK-SKINNED BACKTHRUSTING IN THE JFELCH-CALUMET TROUGHS REGION, NORTHERN MICHIGAN--
A COMPARISON WITH THE SOUTHERN ALPS
JOHN S. KLASNER, Department of Geology, Western Illinois University, and U.S.Geological Survey, Macomb, Illinois 61455 and P. K. SIMS, U.S. Geological Survey, MS 905,Box 25046, Federal Center, Denver, Colorado 80225-0046 j
The Felch and Calumet troughs region is on the south edge of the Superior craton.Rocks of the Wisconsin magmatic terranes were accreted to the craton approximately 1850Ma along the north-verging Niagara fault, causing complex deformation and metamorphismof Archean basement and Early Proterozoic strata on the foreland. In the study area,deformation is characterized by thick-skinned, south-verging backthrusting and backfoldingopposite the overall sense of north-verging deformation on the continental foreland. Thatnear-recumbent minor folds with accompanying axial-planar subhorizontal foliation areevident in both Archean and Early Proterozoic rocks suggests the presence of crystalline-cored nappes, such as that of the Carney Lake block.
Deep tectonic levels are exposed in the Felch and Calumet troughs region, indicatingmajor involvement of Archean rocks in the deformation. Both involvement of basement andthe existence of backthrusts are common in younger orogens, such as in the southern Alps.Comparing a section of the 1850 Ma Penokean orogen in northern Michigan with a sectionin the 30 Ma southern Alpine orogen reveals several common features. Proceeding inwardfrom the continental margin, both orogens have 1) accreted oceanic crust as shown by thepresence of ophiolite; 2) a zone of thick-skinned, complex deformation with associatedbackthrusting and backfolding; 3) a master thrust along which strata were tectonicallytransported over a basement arch; and 4) inboard of the basement arch, deformation thatis mainly thin skinned, but also involves some thrusting of basement rocks. The Alpineorogen consists of multiply deformed nappes, whereas deep erosion of the Penokean orogenmakes recognition of nappes difficult. Nevertheless, the minor recumbent folds and a Jsubhorizontal foliation in the Felch and Calumet troughs area are structures similar to thosein the Alps, and their presence strongly suggests that nappes, now mostly eroded, may alsoexist in the Penokean orogen of northern Michigan. j
jJ
jj
K:\PKS\ABS\NorMich (3-22-91)
-j64
-J
- THICK-SKINNED BACKTHRUSTING IN THE
FELCH-CALUMET TROUGHS REGION, NORTHERN MICHIGAN-- A COMPARISON WITH THE SOUTHERN ALPS
JOHN S. KLASNER, Department of Geology, Western Illinois University, and U.S. Geological Survey, Macomb, Illinois 61455 and P. K. SIMS, U.S. Geological Survey, MS 90q Box 25046, Federal Center, Denver, Colorado 80225-0046
The Felch and Calumet troughs region is on the south edge of the Superior craton. Rocks of the Wisconsin magmatic terranes were accreted to the craton approximately 1850 Ma along the north-verging Niagara fault, causing complex deformation and metamorphism of Archean basement and Early Proterozoic strata on the foreland. In the study area, deformation is characterized by thick-skinned, south-verging backthrusting and backfolding opposite the overall sense of north-verging deformation on the continental foreland. That near-recumbent minor folds with accompanying axial-planar subhorizontal foliation are evident in both Archean and Early Proterozoic rocks suggests the presence of crystallin'-- cored nappes, such as that of the Carney Lake block.
Deep tectonic levels are exposed in the Felch and Calumet troughs region, indicating major involvement of Archean rocks in the deformation. Both involvement of basement and the existence of backthrusts are common in younger orogens, such as in the southern Alps. Comparing a section of the 1850 Ma Penokean orogen in northern Michigan with a section in the 30 Ma southern Alpine orogen reveals several common features. Proceeding inward from the continental margin, both orogens have 1) accreted oceanic crust as shown by the presence of ophiolite; 2) a zone of thick-skinned, complex deformation with associated backthrusting and backfolding; 3) a master thrust along which strata were tectonically transported over a basement arch; and 4) inboard of the basement arch, deformation that is mainly thin skinned, but also involves some thrusting of basement rocks. The Alpine orogen consists of multiply deformed nappes, whereas deep erosion of the Penokean orogen makes recognition of nappes difficult. Nevertheless, the minor recumbent folds and a subhorizontal foliation in the Felch and Calumet troughs area are structures similar to those in the Alps, and their presence strongly suggests that nappes, now mostly eroded, may a'=" exist in the Penokean orogen of northern Michigan.
EARLY PROTEROZOIC ROCKS IN THE MONICO, WISCONSIN AREA:IMPLICATIONS ON THE WISCONSIN MAGMATIC TERRANE
GENE L. LABERGE, Geology Dept., UW—Oshkosh, Oshkosh, WI 54901
The Wisconsin magmatic terrane is a major east—trending beltof Early Proterozoic volcanic and plutonic rocks in the LakeSuperior region. The Monico area is a "window" into the EarlyProterozoic rocks through the Pleistocene deposits that blanketmost of northern Wisconsin, and, therefore, may afford anopportunity to determine the relationships between major unitswithin the volcanic belt. This paper presents preliminary datafrom a project funded by a Faculty Development Grant from UW-Oshkosh, with support from the Wisconsin Geological Survey, andthe U.S. Geological Survey.
Similar to other parts of the Wisconsin magmatic terrane, atleast two distinctly different sequences of rocks are present inthe Monico area. One sequence, exposed mainly north of Highway8, consists mainly of amphibolite—facies gneissic rocks withlittle preservation of primary features. The dominant lithologyis a foliated granitoid gneiss with abundant mafic blocks.Numerous segmented mafic dikes are also present.
The other sequence, exposed mainly south of Highway 8,consists of several mafic to felsic successions of greenschist—facies volcanic rocks with subordinate volcanogenic sedimentaryrocks. The repetitions in successions may be due to acombination of tight folding and faulting. Several largemetadiabase dikes and the "Jennings granite" intrude the volcanicsuccessions. Although pillow basalts, debris flows, and flow—banded rhyolite and felsic breccias are dominant in the Monicaarea, drill cores to the east and west of the area of outcropcontain higher proportions of sedimentary rocks. This suggeststhat rocks in the Monica area may represent a volcanic center (avolcanic island?) within the magmatic terrane. By contrast, theareas with abundant sedimentary rocks may represent intra—arc orback—arc basins between volcanic centers.
The two major sequences of rocks in the map area arepresumably in fault contact, and it is probable that the gneissicrocks represent uplifted, deeper levels of volcanogenic rocksthat were formed during earlier stages of subduction within amature island arc. By contrast, the greenschist—facies rocks mayrepresent later products of the general subduction event. Thefault separating the two rock sequences may be the result ofdeformation during docking of the magmatic terrane with rocks ofthe Superior Craton to the north. Thus, the rocks exposed in theWisconsin inagmatic terrane may represent various tectonic slicesof a mature island arc and associated intra—arc and back—arcbasins.
65
EARLY PROTEROZOIC ROCKS IN THE MONICO, WISCONSIN AREA: IMPLICATIONS ON THE WISCONSIN MAGMATIC TERRANE
GENE L. LABERGE, Geology Dept., UW-Oshkosh, Oshkosh, WI 54901
The Wisconsin magmatic terrane is a major east-trending belt of Early Proterozoic volcanic and plutonic rocks in the Lake Superior region. The Monico area is a "window" into the Early Proterozoic rocks through the Pleistocene deposits that blanket most of northern Wisconsin, and, therefore, may afford an opportunity to determine the relationships between major units within the volcanic belt. This paper presents preliminary data from a project funded by a Faculty Development Grant from UW- Oshkosh, with support from the Wisconsin Geological Survey, and the U.S. Geological Survey.
Similar to other parts of the Wisconsin magmatic terrane, at least two distinctly different sequences of rocks are present in the Monico area. One sequence, exposed mainly north of Highway
consists mainly of amphibolite-facies gneissic rocks with :ittle preservation of primary features. The dominant lithology is a foliated granitoid gneiss with abundant mafic blocks. Numerous segmented mafic dikes are also present.
The other sequence, exposed mainly south of Highway 8, consists of several mafic to felsic successions of greenschist- fades volcanic rocks with subordinate volcanogenic sedimentary rocks. The repetitions in successions may be due to a combination of tight folding and faulting. Several large metadiabase dikes and the "Jennings granite" intrude the volcanic successions. Although pillow basalts, debris flows, and flow- banded rhyolite and felsic breccias are dominant in the Monico area, drill cores to the east and west of the area of outcrop contain higher proportions of sedimentary rocks. This Suggests that rocks in the Monico area may represent a volcanic center (a volcanic island?) within the magmatic terrane. By contrast, the areas with abundant sedimentary rocks may represent intra-arc or back-arc basins between volcanic centers.
The two major sequences of rocks in the map area are presumably in fault contact, and it is probable that the gneissic rocks represent uplifted, deeper levels of volcanogenic rocks that were formed during earlier stages of subduction within a mature island arc: By contrast, the greenschist-facies rocks may represent later products of the general subduction event. The fault separating the two rock sequences may be the result of deformation during docking of the magmatic terrane with rocks of the Superior Craton to the north. Thus, the rocks exposed in the Wisconsin magmatic terrane may represent various tectonic slices of a mature island arc and associated intra-arc and back-arc basins.
Ske
tch
of h
ypot
hetic
al e
ast-
wes
t (lo
ngitu
dina
l) se
ctio
n al
ong
the
Wis
cons
in m
agm
atic
terr
ane
show
ing
vario
us s
tage
s of
dev
elop
men
tof
vol
cani
c ce
nter
s al
ong
the
isla
nd a
rc.
L-
LJ L
L L
LL
LL L
L
The Geology and Geophysics of major post-Keweenawanfaults in the eastern Lake Superior region
Manson, M.L. and Halls, H.C.Department of Geology, University of Toronto, Erindale campus,
3359 Mississauga Rd., Mississauga, Ontario L5L 1C6
The eastern Lake Superior shoreline has a characteristic saw-tooth outlinein which promontories are primarily occupied by Middle Keweenawan volcanicsof the l.lGa Midcontinent Rift of North America (MCR), and embayments byUpper Keweenawan clastics. This pattern appears to be generated by major ENEto WSW faults, some of which are related to Archean structures within thesouthern Superior Province. At Mamainse Point, more than 5km of volcanics arebound against sandstones to the south by a fault that appears, in new OntarioGeological Survey aeromagnetic data, to extend inland along the southern marginof the adjacent greenstone belt. The Montreal River fault, possibly related to theKapuskasing structure, defines the northern margin of these greenstones andextends into rift straw underlying eastern Lake Superior in GSC-GLIMPCEaeromagnetic data. At Havilland Bay. a 500m thick succession of Jacobsvillesandstone has been upturned and folded against a little-recognised fault exposeddirectly alongside Highway 17. Further east-west faults have been observedbounding Keweenawan strata in the Goulais Bay area and at Grindstone Point.
Whilst the exact nature of these faults is unclear, significant verticaldisplacements across the Mamainse Point and Havilland Bay faults is suggestedfrom detailed gravity profiles collected during 1990. Regional potential field dataindicate the lateral extent of many of the faults and suggest broad structural unitsnormal to the rift axis. This contrasts with post-Keweenawan faults in the westernlake region which are generally parallel to the axis of the rift. The change instructural style from west to east is highlighted in the centre of the lake, where aseries of arcuate fault segments appear to link the Isle Royale and MichipicotenIsland faults via Superior Shoal (Teskey et at, 1991). Sampling with a
submersible has revealed a complex structure for Keweenawan volcanics exposedat the Shoal, possibly involving drag folding and block rotations (Manson andHalls, 1991).
Constraints on the sense and timing of the eastern faults will haverelevance to models describing the closure of the MCR, possibly requiring aGrenville influence, and offer a method for quantifying post-Keweenawandeformation within the southern Superior Province.
Manson, M.L., and Halls, H.C., 1991, An investigation of Superior Shoal, centralLake Superior, with a manned submersible, in press, CJ.E.S.
Teskey, D.J., Thomas, M.D., Gibb, R.A., Dods, S.D., Kucks, R.P., Chandler,V.W., Fadaie, K., Phillips, J.D., 1991, High resolution aeromagnetic surveyof Lake Superior, EOS, V.72, No.8
67
The Geology and Geophysics of major post-Keweenawan faults in the eastern Lake Superior region
Manson, M.L. and Halls, H.C. Department of Geology, University of Toronto, Erindale campus,
3359 Mississauga Rd., Mississauga, Ontario L5L 1C6
The eastern Lake Superior shoreline has a characteristic saw-tooth outline in which promontories are primarily occupied by Middle Keweenawan volcanics of the l.lGa Midcontinent Rift of North America (MCR), and embayments by Upper Keweenawan elastics. This pattern appears to be generated by major ENE to WSW faults, some of which are related to Archean structures within the southern Superior Province. At Mamainse Point, more than 5km of volcanics are bound against sandstones to the south by a fault that appears, in new Ontario Geological Survey aeromagnetic data, to extend inland along the southern margin of the adjacent greenstone belt. The Montreal River fault, possibly related to the Kapuskasing structure, defines the northern margin of these greenstones and extends into rift strata underlying eastern Lake Superior in GSC-GLIMPCE aeromagnetic data. At Havilland Bay, a 500m thick succession of Jacobsville sandstone has been upturned and folded against a little-recognised fault exposed directly alongside Highway 17. Further east-west faults have been observed bounding Keweenawan strata in the Goulais Bay area and at Grindstone Point.
Whilst the exact nature of these faults is unclear, significant vertical displacements across the Mamainse Point and Havilland Bay faults is suggested from detailed gravity profiles collected during 1990. Regional potential field data indicate the lateral extent of many of the faults and suggest broad structural units normal to the rift axis. This contrasts with post-Keweenawan faults in the western lake region which are generally parallel to the axis of the rift. The change in structural style from west to east is highlighted in the centre of the lake, where a series of arcuate fault segments appear to link the Isle Royale and Michipicoten Island faults via Superior Shoal (Teskey et al., 1991). Sampling with a submersible has revealed a complex structure for Keweenawan volcanics exposed at the Shoal, possibly involving drag folding and block rotations (Manson and Halls, 1991).
Constraints on the sense and timing of the eastern faults will have relevance to models describing the closure of the MCR, possibly requiring a Grenville influence, and offer a method for quantifying post-Keweenawan deformation within the southern Superior Province.
Manson, M.L., and Halls, H.C., 1991, An investigation of Superior Shoal, central Lake Superior, with a manned submersible, in press, C.J.E.S.
Teskey, D.J., Thomas, M.D., Gibb, R.A., Dods, S.D., Kucks, R.P., Chandler, V.W., Fadaie, K., Phillips, J.D., 1991, High resolution aeromagnetic survey of Lake Superior, EOS, V.72, No.8
-j
GEOPHYSICAL INVESTIGATIONS OF THE MWCONTINENT RIFT IN EASTERN JLAKE SUPERIOR USING VARIABLE MAGNETIZATION MODELINGMARIANO, J. AND HINZE, W.J. (Dept. of Earth and Atmospheric Sciences,Purdue Univ.,West Lafayette, IN. 47907 JMagnetic anomalies over eastern Lake Superior are very useful in tracing the subsurface extentand geometry of the Keweenawan volcanic rocks of the Midcontinent Rift. In 1987, a highresolution aeromagnetic survey was flown over Lake Superior as part of the Great LakesMultidisciplinary Program on Crustal Evolution. In order to take full advantage of these newdata, improved quantitative magnetic modeling techniques have been developed and applied tothe volcanic rocks. The Keweenawan basalts have retained a strong remanent magnetizationderived when they cooled through the Curie point isotherm of magnetite. Post-depostionalstructural deformation of these rocks has rotated the remanent magnetization vector resulting ina spatially varying direction and intensity of magnetization when vectorially added to the Jinduced magnetization vector. An algorithm to accomodate these variations in magnetizationhas been formulated and tested and has been applied to the volcanic rocks of eastern LakeSuperior.
The magnetic modeling procedure, which is based on the equivalent point sourceconcept, is termed the equivalent point dipole (EPD) method, and calculates the magneticresponse of a given volume of anomalous magnetization by summing the magnetic fields due toa series of point dipoles of specified spacing contained within the volume. This approach allowsthe direction and magnitude of magnetization to vary freely over an anomalous unit. Theaccuracy and speed of this calculation are directly dependent on the point dipole spacing. Theoptimum spacing minimizes calculation time, but still results in acceptable calculation error.Given the simplicity of the dipole calculation and the speed of modern computing facilities, thisis fortunately not a problem. In general, a dipole spacing equal to one tenth the distance betweenthe observation plane and the point dipoles yields errors less than one percent. Figure 1 showscomparisons of the EPD calculation with the conventional method of anomaly calculation(Tal\vani and Heirtzler,1964).
The EPD method has been applied on a two dimensional basis to profiles that have beenstudied with the gravity and seismic reflection methods in eastern Lake Superior. These profilesinclude recently released seismic reflection sections with up to 8 s of TWYI' as well aspreviously available GLIMPCE data with up to 16 s of T\V1T. On these sections, theKeweenawan basalts are clearly imaged as a series of strong, laterally continuous reflectionswith dips generally ranging from -30 to ±30 degrees. Extensive paleomagnetic studies in theLake Superior region indicate that the basalts possess both a normal and reversed remanentmagnetization component in addittion to the induced component. Assuming original Jhorizontality of the basalt flows, the apparent dips derived from the seismic reflection sectionsare used to rotate the remanent magnetization vector back to the horizontal in the plane of theprofile. The rotated remanent vector is then added to an induced magnetization vector, resulting Jin a spatially varying direction and magnitude of total magnetization.
Modeling results indicate that the consideration of variable magnetization will providemuch greater confidence in the structural interpretations of the Keweenawan basalts.Preliminary models in eastern Lake Superior suggest that a large volume of reversely polarizedrocks is necessary to account for the observed aeromagnetic anomalies.
68
GEOPHYSICAL INVESTIGATIONS OF THE MIDCONTINENT RIFT IN EASTERN LAKE SUPERIOR USING VARIABLE MAGNETIZATION MODELING MARIANO, J. AND HINZE, W.J. (Dept. of Earth and Atmospheric Sciences.Purdue Univ., West Lafayette, IN. 47907 Magnetic anomalies over eastern Lake Superior are very useful in tracing the subsurface extent and geometry of the Keweenawan volcanic rocks of the Midcontinent Rift. In 1987, a high resolution aeromagnetic survey was flown over Lake Superior as 'part of the Great Lakes Multidisciplinary Program on Crustal Evolution. In order to take full advantage of these new data, improved quantitative magnetic modeling techniques have been developed and applied to the volcanic rocks. The Keweenawan basalts have retained a strong remanent magnetization derived when they cooled through the Curie point isotherm of magnetite. Post-depostional structural deformation of these rocks has rotated the remanent magnetization vector resulting in a spatially varying direction and intensity of magnetization, when vectorially added to the induced magnetization vector. An algorithm to accomodate these variations in magnetization has been formulated and tested and has been applied to the volcanic rocks of eastern Lake Superior.
The magnetic modeling procedure, which is based on the equivalent point source concept, is termed the equivalent point &pole (EPD) method, and calculates the magnetic response of a given volume of anomalous magnetization by summing the magnetic fields due to a series of point dipoles of specified spacing contained within the volume. This approach allows the direction and magnitude of magnetization to vary freely over an anomalous unit. The accuracy and speed of this calculation are directly dependent on the point dipole spacing. The optimum spacing minimizes calculation time, but still results in acceptable calculation error. Given the simplicity of the dipole calculation and the speed of modern computing facilities, this is fortunately not a problem. In general, a dipole spacing equal to one tenth the distance between the observation plane and the point dipoles yields errors less than one percent. Figure 1 shows comparisons of the EPD calculation with the conventional method of anomaly calculation (Talwani and Heirtzler.1964).
The EPD method has been applied on a two dimensional basis to profiles that have been studied with the gravity and seismic reflection methods in eastern Lake Superior. These profiles include recently released seismic reflection sections with up to 8 s of TWTT as well as previously available GLIMPCE data with up to 16 s of TWTT. On these sections, the Keweenawan basalts are clearly imaged as a series of strong, laterally continuous reflections with dips generally ranging from -30 to +30 degrees. Extensive paleomagnetic studies in the Lake Superior region indicate that the basalts possess both a normal and reversed remanenl magnetization component in addittion to the induced component. Assuming original horizontality of the basalt flows, the apparent dips derived from the seismic reflection sections are used to rotate the remanent magnetization vector back to the horizontal in the plane of the profile. The rotated remanent vector is then added to an induced magnetization vector, resulting in a spatially varying direction and magnitude of total magnetization.
Modeling results indicate that the consideration of variable magnetization will provide much greater confidence in the structural interpretations of the Keweenawan basalts. Preliminary models in eastern Lake Superior suggest that a large volume of reversely polarized rocks is necessary to account for the observed aeromagnetic anomalies.
s (a) N
020 .0 4!o 50 0 iS 50 50 10
DISTANCE (1(M)
a
2 C
= S—S SC
o 10 20 50 40 50 50 70 50 50 '00DISTANCE (1(M)
s (b) Nz
—
0° ¶0 20 50 40 50 60 70 60 90 '00— DISTANCE (1(M)
0•
I'S 0 t'o.'oiot'o7'0 5'O 100DISTANCE (1(M)
Figure 1. (a) The magnetic anomaly due to combined induced and remanent magnetizationcalculated by the EPD method (dashed) and the Talwani and Heirtzler method (solid). Note thatthese are essentially coincident over the entire length of the profile. The total magnetizationvector in the plane of the profile is shown for every fifth dipole. (b) The magnetic anomaly dueto combined induced and remanent magnetization with a gradational rotation of the remanentvector using the EPD method (dashed) and with homogeneous magnetization using the Talwaniand Heirtzler method (solid). A single horizontal plane of total magnetization vectors is shownThe induced and remanent magnetization vectors and the disturbance of the remanent vector arecompatible with conditions in the Lake Superior region and the volcanic rocks of theMideontinent Rift.
Reference
Talwani, M. and Heinzler, J;, 1964, Computation of magnetic anomalies caused by two-dimensional structures of arbitrary shape. IN: Computers in the Mineral Industries, 1.Stanford Univ. Publ. Geol. Sci., 90): 69
- " I 0 10 2 0 4 0 70 .o *o 100
Figure 1. (a) The calculated by the EPD method (dashed) and the Talwani and Heirtzler method (solid). Note that these are essentially coincident over the entire length of the profile. The total magnetization vector in the plane of the profile is shown for every fifth dipole. (b) The magnetic anomaly due to combined induced and remanent magnetization with a gradational rotation of the remaneni vector using the EPD method (dashed) and with homogeneous magnetization using the Talwani and Heirtzler method (solid). A single horizontal plane of total magnetization vectors is shown. The induced and remanent magnetization vectors and the disturbance of the remanent vector are compatible with conditions in the Lake Superior region and the volcanic rocks of the Midcontinent Rift.
Reference . , .
Talwani, M. and Heirtzler. 1.. 1964, Computation of magnetic anomalies caused by two- dimensional structures of arbitrary shape. IN: Computers in the Mineral Industries, 1. Stanford Univ. Publ. Geol. Sci.. 9(1): 464-480 69
70
A PRELIMINARY STUDY OF REJUVENATED MOVEMENT ALONGA PRECAMBRIAN FAULT, St CROIX COUNTY, WISCONSIN
Michael I). Middleton, Department of Plant and Earth Science,University of Wisconsin-River Falls, River Falls, WI 54022
The eastern margin of the Midcontinent Rift system is marked by theHastings-Lake Owen fault in western Wisconsin. Precambrian displacement alongthis high angle fault has been estimated at approximately 3,000 m. (Craddock, 1972)based upon geophysical evidence. A number of workers have suggested that theHastings-Lake Owen fault was reactivated in the late Cambrian, causing thinning ofthe Cambrian section and an unconformity between the Cambrian and Ordoviciansequences in Minnesota (Morey and Rensink, 1969; Cavaleri, et. al., 1987). Evidencefrom a number of exposures in St. Croix County, Wisconsin supports these views,and further refines the timing and amount of post-Precambrian movement alongthe fault.
The surface expression of the Hastings-Lake Owen fault in St. Croix County isa series of low escarpments with a northeast trend formed by differential erosion ofthe softer Cambrian sandstones to the west which have been displaced upwardagainst more resistant Ordovician dolomites to the east. A quarry along this trendcut into dolomites of the Lower Ordovician Prairie du Chien Group shows evidenceof this displacement. In sharp contrast to the relatively flat-lying bedrock units inthis region, the sttata in the quarry dip approximately 20° to the east. These stratadip away from the fault on its downthrown side, and these dips were most probablycaused by drag. The displacement of the Cambrian sandstones against LowerOrdovician dolomites and the drag folding within the dolomites both demonstratepost-Early Ordovician movement along the fault. In addition, features within thedolomite itself provide evidence for movement of the fault in the Early Ordovicianduring deposition of the sediments. The beds in the quarry thicken away from thefault, with lower strata showing higher dips than those above. Sharply truncatedstromatolite beds, thick dolomitic conglomerates, erosional relief along contacts, andmudcracks all attest to periodic erosion and exposure of the sea floor duringdeposition of the Prairie du Chien Group at this site, perhaps due to recurrent smallmovements along the fault.
A PRELIMINARY STUDY OF REJUVENATED MOVEMENT ALONG A PRECAMBRIAN FAULT, ST. CROIX COUNTY, WISCONSIN
Michael D. Middleton, Department of Plant and Earth Science,
University of Wisconsin-River Falls, River Falls, WI 54022
The eastern margin of the Midcontinent Rift system is marked by the Hastings-Lake Owen fault in western Wisconsin. Precambrian displacement along this high angle fault has been estimated at approximately 3,000 m. (Craddock, 1972)
based upon geophysical evidence. A number of workers have suggested that the
Hastings-Lake Owen fault was reactivated in the late Cambrian, causing thinning of the Cambrian section and an unconforrnity between the Cambrian and Ordovician sequences in Minnesota (Morey and Rensink, 1969; Cavaleri, et. al., 1987). Evidence from a number of exposures in St. Croix County, Wisconsin supports these views, and further refines the timing and amount of post-Precambrian movement along the fault.
The surface expression of the Hastings-Lake Owen fault in St. Croix County is
a series of low escarpments with a northeast trend formed by differential erosion of the softer Cambrian sandstones to the west which have been displaced upward
against more resistant Ordoviaan dolomites to the east. A quarry along this trend cut into dolomites of the Lower Ordoviaan Prairie du Chien Group shows evidence of this displacement. In sharp contrast to the relatively flat-lying bedrock units in this region, the strata in the quarry dip approximately 20' to the east. These strata
dip away from the fault on its downthrown side, and these dips were most probably
caused by drag. The displacement of the Cambrian sandstones against Lower
Ordovician dolomites and the drag folding within the dolomites both demonstrate
post-Early Ordovician movement along the fault. In addition, features within the
dolomite itself provide evidence for movement of the fault in the Early Ordovician during deposition of the sediments. The beds in the quarry thicken away from the fault, with lower strata showing higher dips than those above. Sharply truncated
stromatolite beds, thick dolomitic conglomerates, erosional relief along contacts, and mudaacks all attest to periodic erosion and exposure of the sea floor during
deposition of the Prairie du Chien Group at this site, perhaps due to recurrent small
movements along the fault.
The amount of Post-Precambrian displacement along the Hastings-LakeOwen fault in this region can be determined at exposures in the vicinity of Hudson,Wisconsin. Cambrian sandstones have been displaced upward approximately 85-100m. against Ordovician strata along the fault. Along the Cottage Grove fault whichparallels the Hastings-Lake Owen fault to the northwest, displacement is evengreater, approximately 115-125 m. The evidence gathered in St. Croix Countydemonstrates later recurrent movements and greater Post-Precambriandisplacement than has previously been reported for the Precambrian faults in thisregion.
References Cited
Cavaleri, M., Mossier, J.H. and Webers, G.F., 1987, The Geology of the St. Croix RiverValley, in Balaban, N.H., Ed., Field Trip Guidebook for the Upper MississippiValley, Minnesota, Iowa and Wisconsin. Minnesota Geological Survey,Guidebook Series #15, p. 23-43.
Craddock, C., 1972. Keweenawan Geology of East-central and SoutheasternMinnesota, in Sims, P.K. and Morey, G.B., Eds, Geology of Minnesota: ACentennial Volume. Minnesota Geological Survey, p. 416-424.
Morey, C.B. and Rensink, D.C., 1969. Rejuvenated Precambrian faults as a cause ofPaleozoic structures in southeastern Minnesota. Annals of the Institute ofLake Superior Geology. 15th, Wisconsin State University, Dept. Ceol.,Oshkosh, WI, May 8-9.
71
The amount of Post-Precambrian displacement along the Hastings-Lake Owen fault in this region can be determined at exposures in the vicinity of Hudson, Wisconsin. Cambrian sandstones have been displaced upward approximately 85-100 m. against Ordoviaan strata along the fault. Along the Cottage Grove fault which parallels the Hastings-Lake Owen fault to the northwest, displacement is even greater, approximately 115-125 m. The evidence gathered in St. Croix County demonstrates later recurrent movements and greater Post-Precambrian displacement than has previously been reported for the Precambrian faults in this region.
References Cited
Cavaleri, M., Mossier, J.H. and Webers, G.F., 1987, The Geology of the St. Croix River Valley, in Balaban, N.H., Ed., Field Trip Guidebook for the Upper Mississippi Valley, Minnesota, Iowa and Wisconsin. Minnesota Geological Survey, Guidebook Series #15, p. 23-43.
Craddock, C., 1972. Keweenawan Geology of East-central and Southeastern Minnesota, in Sims, P.K. and Morey, G.B., Eds, Geology of Minnesota: A Centennial Volume. Minnesota Geological Survey, p. 416-424.
Morey, G.B. and Rensink, D.G., 1969. Rejuvenated Precambrian faults as a cause of Paleozoic structures in southeastern Minnesota. Annals of the Institute of Lake Superior Geology. 15th, Wisconsin State University, Dept. Geol., Oshkosh, WI, May 8-9.
GEOCHRONOLOGY OF THE DULUTH COMPLEX: A PROGRESS REPORT
MILLER, .ID., JR., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114 and
LB. Paces and R.E. Zartman, U.S. Geological Survey, Box 25046, MS 963, Denver Federal Center, Denver,CO 80225
Recent high-precision dating of volcanic rocks associated with the Midcontinent Rift have contributedmuch toward understanding the timing, duration, and tectonomagmatic evolution of the Li-Ga rifting event.However, similar geochronological control for associated plutonic rocks is not yet available. The primaryobjective of the present study is to date individual gabbroic intrusions within the Duluth Complex in northeast-ern Minnesota using high-precision U-Pb techniques (1) to determine the temporal range of intrusive activitywithin several well-studied areas of the complex, as well as between these intrusions and the overlying volcanicrocks, and (2) to test models, which have previously been based on geologic mapping and geophysical interpre-tation, for the emplacement of the Duluth Complex.
The Duluth Complex is large, composite mafic intrusive supersuite, which was emplaced at shallowdepths (<7 km) beneath a comagmatic edifice of plateau basalt during the development of the Midcontinent Rift(Fig. I). Multiple intrusions were also emplaced higher into the volcanic pile, with the most voluminous ofthese being the Beaver Bay Complex. The Duluth Complex is composed of several lithologieally distinctiverock series, previously interpreted to be temporally distinct (Weiblen & Morey, 1980). The oldest intrusiverocks are a layered suite of gabbroic cumulates in the northern area, termed Nathan's layered series (Fig. I).Their evolved compositions and reversed magnetic polarity suggest that these rocks may be similar in age tothe nearby Logan sills (1109 ± 2 Ma; Davis & Sutcliffe, 1985). These gabbros are intruded by a structurallycomplicated suite of dominantly plagioclase cumulates, termed the anorthositic series (Fig. I), which occurthroughout the complex and are thought to have intruded as plagioclase crystal mushes (Miller & Weiblen,1990). Anorthositic series rocks are intruded throughout the complex by layered intrusions of iroctolitic togabbroic cumulates, collectively termed the troctolitic series. The apparent truncation of aeromagnetic patternsrelated to SE.dipping layering in intrusions along the unexposed western margin of the Duluth Complex (Fig.1) led Miller et al. (1990) to speculate that troctolitic series intrusions young to the south as a result of succes-sive intrusive overplating. A felsic series also comprises a significant portion of the Duluth Complex (Fig. I)but was not included in this geochronologic study.
Differentiated samples from seven mafic intrusive bodies were chosen that, based on geologic and geo-physical evidence, span the temporal and spatial (areal and stratigraphic) limits of intrusive activity in theDuluth Complex (Table 1; Fig. 1). Between 30 and 40kg of rock was processed for each sample yielding clearzircon (± baddeleyite) grains lacking obvious cores or rims. Air-abrasion and hand-picking yielded crack- andinclusion-free grains, which were analyzed individually or as 2-5 grain aggregates weighing 30 to 150 jig. Ucontents range from 200 to 1000 ppm, and most fractions are less than 1.5% discordant. Preliminary ages forfive of these samples are reported in Table 1. The two anorthositie samples (FCI, AS3) plus one troctoliticseries ferrogabbro (Dl) yield identical ages at the 95% confidence level. Fractions from all three rocks re-gressed together result in an age of 1099.1 ± 0.6 Ma with a lower intercept age of-I ± 40 Ma (MSWD = 1.3),
The troetolitic series olivine gabbro (P02), as well as the hypabyssal ferrodiorite from the Beaver Bay Com-plex (SBG2), yield younger ages of 1096.1 ± 0.8 and 1095S ± 0.5 Ma, respectively, which are clearly resolvedfrom the older ferrogabbro and anorthosite dates. Although additional analyses are required before final agedeterminations are made, the following have been established:— Although the onset of igneous activity as represented by Nathan's layered series has not yet been deter-
mined, the bulk of the Duluth Complex and associated hypabyssal intrusions were emplaced between1099 and 1096 Ma. This further supports previous observations that volumetrically most Keweenawanigneous activity occurred over a time span between 1099 and 1094 Ma (Davis & Sutcliffe, 1985; Davis &Paces, 1990; Van Schmus et a!., 1990). Since parts of the Duluth Complex are older than the normal-polarity lavas exposed along rift margins, these intrusive rocks may represent a means of samplingmagmas associated with lavas that are more deeply buried within the central rift graben.
— The relative ages of intrusive rocks based on their magnetic polarity must be viewed with considerablecaution. Although all of the analyzed rocks possess normal magnetic polarity, three of the intrusive
•bodies have emplacement ages of 1099 Ma, older than the major R-N magnetic reversal placed at aboutI09L6 ± 3.7 Ma (Fig. 1; Davis & Sutcliffe, 1985). This disparity is likely related to the slow cooling of
72
GEOCHRONOLOGY OF THE DULUTH COMPLEX: A PROGRESS REPORT
MILLER, J.D., JR., Minnesota Geological Survey, 2642 University Ave., St. Paul, MN 55114 and
J.B. Paces and R.E. Zarunan, U.S. Geological Survey. Box 25046, MS 963, Denver Federal Center, Denver, CO 80225
Recent high-precision dating of volcanic rocks associated with the Midcontinent Rift have contributed much toward understanding the timing, duration, and tectonomagmatic evolution of the 1.1-Ga rifting event. However, similar geochmnological control for associated plutonic rocks is not yet available. The primary objective of the present study is to date individual gabbroic intrusions within the Duluth Complex in nonheast- em Minnesota using high-precision U-Pb techniques (1) to determine the temporal range of intrusive activity within several well-studied areas of the complex, as well as between these intrusions and the overlying volcanic rocks, and (2) to test models, which have previously been based on geologic mapping and geophysical interpre- tation, for the emplacement of the Duluth Complex.
The Duluth Complex is large, composite mafic intrusive supersuite, which was emplaced at shallow depths (<7 km) beneath a comagmatic edifice of plateau basalt during the development of the Midcontinent Rift (Fig. 1). Multiple intrusions were also emplaced higher into the volcanic pile, with the most voluminous of these being the Beaver Bay Complex. The Duluth Complex is composed of several lithologically distinctive rock series, previously interpreted to be temporally distinct (Weiblen & Morey, 1980). The oldest intrusive rocks are a layered suite of gabbmic cumulates in the northern area, termed Nathan's layered series (Fig. 1). Their evolved compositions and reversed magnetic polarity suggest that these rocks may be similar in age to the nearby Logan sills (1 109 Â 2 Ma, Davis & Sutcliffe, 1985). These gabbros are intruded by a structurally complicated suite of dominantly plagioclase cumulates, termed the anorthositic series (Fig. 1). which occur throughout the complex and are thought to have intruded as plagioclase crystal mushes (Miller & Weiblen, 1990). Anorthositic series rocks are intruded throughout the complex by layered inbusions of troctolitic to gabbroic cumulates, collectively termed the troctolitic series. The apparent truncation of aeromagnetic patterns related to SE-dipping layering in intrusions along the unexposed western margin of the Duluth Complex (Fig. 1) led Miller et al. (1990) to speculate that troctolitic series intrusions young to the south as a result of succes- sive intrusive overplating. A felsic series also comprises a significant portion of the Duluth Complex (Fig. 1) but was not included in this geochronologic study.
Differentiated samples from seven mafic intrusive bodies were chosen that, based on geologic and geo- physical evidence, span the temporal and spatial (areal and stratigraphic) limits of intrusive activity in the Duluth Complex (Table 1; Fig. 1). Between 30 and 40 kg of rock was processed for each sample yielding clear zircon (Â baddeleyite) grains lacking obvious cores or rims. Air-abrasion and hand-picking yielded crack- and inclusion-free grains, which were analyzed individually or as 2-5 grain aggregates weighing 30 to 150 ug. U contents range from 200 to 1000 ppm, and most fractions are less than 1.5% discordant. Preliminary ages for five of these samples are reported in Table 1. The two anorthositic samples (FC1, AS3) plus one troctolitic series ferrogabbro (Dl) yield identical ages at the 95% confidencelevel. Fractions from all three rocks re- gressed together result in an age of 1099.1 Â 0.6 Ma with a lower intercept age of -1 Â 40 Ma (MSWD = 1.3). The troctolitic series olivine gabbm (PG2). as well as the hypabyssal ferrodiorite from the Beaver Bay Com- plex (SBG2). yield younger ages of 1096.1 Â 0.8 and 1095.9 Â 0.5 Ma, respectively, which are clearly resolved from the older ferrogabbm and anorthosite dates. Although additional analyses are required before final age determinations are made, the following have been established: - Although the onset of igneous activity as represented by Nathan's layered series has not yet been deter-
mined, the bulk of the Duluth Complex and associated hypabyssal intrusions were emplaced between 1099 and 1096 Ma. This further supports previous observations that volmetrically most Keweenawan igneous activity occurred over a time span between 1099 and 1094 Ma (Davis & Sutcliffe, 1985; Davis & Paces, 1990; Van Schmus et a]., 1990). Since pans of the Duluth Complex are older than the normal- polarity lavas exposed along rift margins, these intrusive rocks may represent a means of sampling magmas associated with lavas that are more deeply buried within the central rift graben.
- The relative ages of intrusive rocks based on their magnetic polarity must be viewed with considerable caution. Although all of the analyzed rocks possess normal magnetic polarity, three of the intrusive bodies have emplacement ages of 1099 Ma, older than the major R-N magnetic reversal placed at about 1097.6 Â 3.7 Ma (Fig. 1; Davis & Sutcliffe, 1985). This disparity is likely related to the slow cooling of
.;.WISCONSIN .•Figure 1. Generalized geology of northeastern Minnesota show
Table 1. Preliminary U-Pb ages for zircons from 0
ing samples collected for U-Pb isotope analyses.
uluth Complex and related intrusions
Sample Rock Series T-R-Sec. Rock Type Upper Intercept No. of Lower InterceptAge (Ma) ±2a analyses Age (Ma) ±2a
error error
DlPG2A53FCI
NLS5LLG2
SBG2
Troctolitic 49-15-1 fen-ogabbroTroctolitic 59-12-8 olivine gabbro
Anorthositic 49-14-6 gabbroic anorthositeAnorthositic 61-8-10 gabbroic anorthositeNathan's L.S. 64-2-2 olivine gabbroBeaver Bay 56-7-6 fenogabbroComplex
Beaver Bay 56-7-29 ferrodioriteComplex
1099.0 ± 0.8 3 -2 ± 411096.1 ± 0.8 3 0"
1098.9*1099.2 ± 1.7 4
in progressin progress
1095.9 ± 0.5 4 5 ± 7
.2rnpb,206pb age from single analysis**Regression forced through 0 Ma lower intercept due to clustering of data near concordia
73
1I CANADA 90
r___aiScRIPTloNor MAP UNITS
Middle ProlerpzO,c (Keoae enasyan SuperoroupiSedsmentary Rocks (Baylield & Oronto Groups)
• shale and Ieldspalhic ID quartz Ose sandslone
nlrusive Rocacs l,ncludes Duluth COmoleal
R - granod.oriI.c IC graniI.c bc I5
domnaruly subvolcanc malic rocks
dominanbly Irootolilic rocks
!-dom, nanlly gabbro.c rocksXc -dorn.nanlty anorlh ostIbc rocks
• need lroclob,t,c so goat crc rocksabundant
Volcan.c Rocks North Shore Volcan,c Gtou:-SoleyI,c basall beat caIn some anoes!teshyol,be. and nt ertlom seo,men lacy C;.
cl0°
p
Early P.ocarn,n!r lAnjrnrk.p O'puolarolillIe and greyvacke Rove S V!n.r.a nrc
ran lornlat!crr re a;-..
&ctlarn lyerm,ll,cn dIstrictntaas.ae to OnelSs.c. yr anItIc 504; IIC 1 ;kSlocally micat,tscmetacoboan,; and metased.me-:a -'
'sty banded ron bornOatcI] sno.r.n
SYMBOLS-— . Aopron,malo geolog.c corrtaCI. dariec .srre,r
,nberred Irom geopnysscal data— . Large-scale laull
40 20 30 4CM.20 —u
Figure 1. Generalized geology of northeastern Minnesota showing samples collected for U-Pb isotope analyses.
Table 1. Preliminary U-Pb ages for zircons from Duluth Complex and related intrusions
Sample Rock Series T-R-Sec. Rock Type Upper Intercept No. of Lower Intercept Age (Ma) i2a analyses Age (Ma) i 2 a
error error
Dl Troctolitic 49-15-1 ferrogabbm 1099.0 Â 0.8 3 - 2 i 4 1 PG2 Troctolitic 59-12-8 olivine gabbro 1096.1 i 0.8 3 O* * AS3 Anorthositic 49-14-6 gabbmic anorthosite 1098.9. 1 FC1 Anorthositic 61-8-10 gabbroic anorthosite 1099.2 Â 1.7 4 O*
NLS5 Nathan's L.S. 64-2-2 olivine gabbro in progress LLG2 Beaver Bay 56-7-6 ferrogabbro in progress
Complex SBG2 Beaver Bay 56-7-29 ferrodiorite 1095.9 Â 0.5 4 5 Â 7
Complex
*2CTPb/2çà age from single analysis **Regression forced through 0 Ma lower intercept due to clustering of data near concordia
74
these plutonic rocks between zircon crystallization temperatures (—900°C) and the Curie point of the Fe-Tioxides (c590°-350°C).
— The similar ages of anorthositic series rocks (AS3 & PC!) taken from widely separate areas of thecomplex suggest that the emplacement of these rocks occurred as a restricted temporal event throughoutthe Duluth Complex around 1099 Ma. This is consistent with their unique petrogenesis (Miller &Weiblen, 1990).
— The essentially identical 1099-Ma age of anorthositic and troctolitic rocks at Duluth (AS3 & Dl), whilenot contradicting field relations indicating that the anorthositic rocks are older, does lead to the specula-tion that the petrogenesis of these two series may be more closely linked than previously thought. At thevery least, it implies that the types of parent magmas that gave rise to these two rock series were eithervery closely related or were generated by mechanisms that were very nearly synchronous.
— Speculation (Miller et al., 1990) that the troctolitic series intrusions along the western margin young tothe south is strongly contradicted by the distinctly younger age of the troctolitic series sample from thenorthwest (Kawishiwi) part of the complex (PG2; 1096 Ma) compared to the sample at Duluth (Dl; 1099Ma). Given the complexity of intrusive relationships in the Kawishiwi area (Severson & Hauck, 1990), itseems likely that troctolitic series rocks of various ages may exist in this region.
— The similarity of 1096-Ma ages between the Kawishiwi-area olivine gabbro (PG2) and the ferrogabbro(SBG2) representing the youngest intrusive unit of the hypabyssal Beaver Bay Complex (Miller, 1988)implies that magmas were simultaneously emplaced at several stratigraphic levels within the volcanic pile,at least late in the intrusive history. Dating of the Lax Lake gabbro, which mapping shows to be amongthe earliest intrusions in the Beaver Bay Complex (Miller, 1988), will determine whether intrusive activityin this hypabyssal environment began as early as it did in the deeper Duluth Complex.
In summary, high-precision dates generated by this study place very important constraints on the timingand emplacement mechanisms for the Duluth Complex and related intrusions, as well as providing additionaldata bearing on the broader evolution of the Midcontinent Rift. These preliminary results give cause to rethinkpreviously proposed concepts hypothesized from field, geochemical, and geophysical data, and offer incentivesfor further integration of these data toward better understanding of rift processes.
REFERENCES CITED
Davis, D. W., & Paces, 1. B., 1990, Time resolution of geologic events on the Keweenawan Peninsula andimplications for development of the Midcontinent Rift system: Earth Plan. Sci. Letters, v. 97, p. 54-64.
Davis, D.W., & Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon plate and northem Lake Superior: Geol.Soc. Am. Bull., v.96, p. 1572-1579.
Miller, J.D., Jr., 1988, Geologic map of the Silver Bay and Split Rock Point NE quadrangles, Lake County,Minnesota: Minn. Geol. Surv. Misc. Map M-65, scale 1:24,000.
Miller, J.D., Jr, Chandler, V.W., Southwick, D.L., & Cambray, F.W., 1990, Style of emplacement of the DuluthComplex: Geol. Soc. Am. Abstr. with Programs, v. 22, p. A369.
Miller, J.D., Jr., & Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: examples of rocks formedfrom plagioclase crystal mush: J. Petrol., v. 31, p. 295-339.
Palmer, H.C., & Davis, D.W., 1987, Paleomagnetism and U-Pb geochronology of volcanic rocks fromMichipicoten Island, Lake Superior, Canada: precise calibration of the Keweenawan polar wander track:Precambrian Res., v.37, p. 151-171.
Severson, M.J., & Hauck, S.A., 1990, Geology, geochemistry, and siratigraphy of a portion of the PartridgeRiver intrusion: Duluth, Univ. of Minn., Nat. Resources Research Inst. Tech. Report NRRJ/GMIN-TR-89-11,236 p.
Van Schmus, W.R., Martin, M.W., Sprowl, D.R., Geissman, J., & Berendsen, P., 1990, Age, Nd and Pbisotopic composition, and magnetic polarity for subsurface samples of the 1100 Ma Midcontinent Rift:Geol. Soc. Am. Abstr. with Programs, v. 22, p. A174.
Weiblen, P.W. and Morey, 0.8., 1980, A summary of the stratigraphy, petrology, and structure of the DuluthComplex: Am J. Sci., v. 220-A, p. 88-133.
these plutonic racks between zircon crystallization temperatures (-90O0C) and the Curie point of the Fe-Ti oxides (c59O0-350T).
- The similar ages of anonhositic series rocks (AS3 & FC1) taken from widely separate areas of the complex suggest that the emplacement of these rocks occurred as a restricted temporal event throughout the Duluth Complex around 1099 Ma. This is consistent with their unique petrogenesis (Miller & Weiblen, 1990).
- The essentially identical 1099-Ma age of anorthositic and troctolitic rocks at Duluth (AS3 & Dl), while not contradicting field relations indicating that the anorthositic rocks are older, does lead to the specula- tion that the petrogenesis of these two series may be more closely linked than previously thought At the very least, it implies that the types of parent magmas that gave rise to these two rock series were either very closely related or were generated by mechanisms that were very nearly synchronous.
- Speculation (Miller et al., 1990) that the troctolitic series intrusions along the western margin young to the south is strongly contradicted by the distinctly younger age of the troctolitic series sample from the northwest (Kawishiwi) part of the complex (PG2; 1096 Ma) compared to the sample at Duluth (Dl; 1099 Ma). Given the complexity of intrusive relationships in the Kawishiwi area (Severson & Hauck, 1990). it seems likely that troctolitic series rocks of various ages may exist in this region.
- The similarity of 1096-Ma ages between the Kawishiwi-area olivine gabbm (PG2) and the ferrogabbro (SBG2) representing the youngest intrusive unit of the hypabyssal Beaver Bay Complex (Miller, 1988) implies that magmas were simultaneously emplaced at several stratigraphic levels within the volcanic pile, at least late in the intrusive history. Dating of the Lax Lake gabbro, which mapping shows to be among the earliest intrusions in the Beaver Bay Complex (Miller, 1988). will determine whether intrusive activity in this hypabyssal environment began as early as it did in the deeper Duluth Complex.
In summary, high-precision dates generated by this study place very important constraints on the timing and emplacement mechanisms for the Duluth Complex and related intrusions. as well as providing additional data bearing on the broader evolution of the Midcontinent Rift. These preliminary results give cause to rethink previously proposed concepts hypothesized from field, geochemical, and geophysical data, and offer incentives for further integration of these data toward better understanding of rift processes.
REFERENCES CITED
Davis, D. W., & Paces, J. B.. 1990, Time resolution of geologic events on the Keweenawan Peninsula and implications for development of the Midcontinent Rift system: Earth Plan. Sci. Letters, v. 97, p. 54-64.
Davis, D.W., & Sutcliffe, R.H., 1985, U-Pb ages from the Nipigon plate and northern Lake Superior: Geol. Soc. Am. Bull., v. 96, p. 1572-1579.
Miller, J.D., Jr., 1988, Geologic map of the Silver Bay and Split Rock Point NE quadrangles. Lake County, Minnesota: Minn. Geol. Surv. Misc. Map M-65, scale 1:24,000.
Miller, J.D., Jr, Chandler, V.W., Southwick, D.L., & Cambray, F.W., 1990, Style of emplacement of the Duluth Complex: Geol. Soc. Am. Abstr. with Programs, v. 22, p. A369.
Miller, J.D., Jr., & Weiblen, P.W., 1990, Anorthositic rocks of the Duluth Complex: examples of rocks formed from plagioclase crystal mush: J. Peml., v. 31, p. 295-339.
Palmer, H.C., & Davis, D.W., 1987, Palmmagnetism and U-Pb geochronology of volcanic rocks from Michipicoten Island, Lake Superior, Canada: precise calibration of the Keweenawan polar wander track: Precambrian Res., v. 37, p. 151-171.
Severson, M.J., & Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion: Duluth, Univ. of Minn., Nat. Resources Research Inst. Tech. Report NRRVGMlN-TR-89- 11,236 p.
Van Schmus, W.R., Martin, M.W., Sprowl, D.R.. Geissman, J., & Berendsen, P., 1990. Age, Nd and Pb isotopic composition, and magnetic polarity for subsurface samples of the 1100 Ma Midcontinent Rift: Geol. Soc. Am. Abstr. with Programs, v. 22, p. A174.
Weiblen, P.W. and Morey, G.B., 1980, A summary of the stratigraphy, petrology, and structure of the Duluth Complex: Am J. Sci., v. 280-A, p. 88-133.
MANGANESE MINERALIZATION IN EARLY PROTEROZOIC IRON-FORMATION OFTHE EMILY DISTRICT, CUYUNA RANGE, EAST-CENTRAL MINNESOTA
MOREY, G.E., and 0. L. Southwick, Minnesota Geological Survey, 2642 University Avenue, St. Paul,Minnesota 55114
Early Proterozoic strata of the Emily district at the far northern end of the Cuyuna Iron Range definethe southwestern closure of the Animikie basin in east-central Minnesota (Southwick and others, 1988). Assuch, the rocks of the Emily district are correlative with strata of the well known Anitnikie Group of theMesabi range. However, unlike the monodlinal nature of the Mesabi range, strata in the Emily district arethrown into a series of broad, open, eastward-plunging folds with near-vertical axial planes. Geometricrelationships imply that the basal contact of the Animikie Group overlies an unconformity cut onto olderfolded rocks of the North range (Chandler and Malek, 1991). That unconformity marks the boundarybetween twice-deformed rocks of the Penokean fold-and-thrust belt and the once-deformed rocks of theAnimilde basin
On the Mesabi range, the Animikie Group consists of a lower quartz arenitic sequence (PokegamaQuartzite). an intermediate iron-rich sequence (Biwabik Iron Formation), and an upper black-shale—graywacke sequence (Virginia Formation). In the Emily district, however, the stratigraphic position of theBiwabik is occupied by several lenticular units of iron-formation intercalated within both Pokegarna- andVirginia-like materials.
The main iron-formation of the Emily district—termed Unit A of the Ruth Lake area—can be dividedinto seven lithotopes (Fig. 1). They are (1) an epiclastic lithotope of quartz-rich siltstone and shale; (2) amixed epiclastic—jaspery-chert lithotope; (3) an oolitic and pisolitic lithotope; (4) a thick-bedded lithotope ofcherry or granular iron-formation; (5) a mixed thick- and thin-bedded lithotope characterized by thickintervals of slaty or nongranular iron-formation; (6) a thin-bedded lithotope consisting of laminae to verythin beds of nongranular iron-formation; and (7) a ferruginous chert lithotope. In general. lithotopes 1, 2,and 3 contain primary hematite and have textural attributes indicative of the oxide (hematite) facies of iron-formation. In contrast. lithotope 6 contains appreciable carbonates or silicates and has textural attributesindicative of the carbonate facies of iron-formation. Lithotopes 4 and 5 are interlayered and containmixtures of oxides (hematite-magnetite) and silicates, and thus were deposited in generally similarsedimentological regimes in waters variably affected by currents and at depths intermediate between oxidesand carbonate facies. The sedimentological significance of lithotope 7 is uncertain. Iris a thin-beddedlaminated unit deposited in a deeper water regime as evidenced by a stratigraphic position transitionalbetween thin-bedded iron-formation and overlying black shale; yet it contains primary hematite.
Sedimentological parameters imply that Unit A was deposited during two transgressive-regressivecycles in a basin with a strandline to the west and deeper water to the north and east. Well-rounded grainsof terrigenous quartz, which persist throughout lithotopes 1-5. imply that much of the sedimentationoccurred relatively close to the strandline.
Although Unit A in the Emily district has many mineralogical, textural, and chemical attributesindicative of "ordinary" iron-formation, it locally contains manganese oxides in amounts 10 to 100 timesgreater than the norm of 0.6 to 0.8 percent in the Biwabik Iron Formation (Morey and Morey, 1990).Manganese oxides occur principally in lithotopes 1-5 as disseminated grains, as thin pods or lenses, and aslayers as thick as 1.5 meters that typically contain about 10 percent Mn; some contain as much as 20-30percent Mn. Manganese oxides are particularly abundant near Ruth Lake where two laterally persistentzones about 15 to 18 meters thick have manganese tenors enriched to the 10-50 percent range. Both zonesmore or less coincide with stratigraphic positions occupied by the oolitic-pisolitic lithotope. They containvarious proportions of psiomelane and cryptomelane. as well as hematite and quartz. Goethite andmanganite may be locally abundant where they occur, they are secondary phases that formed during aperiod of intense chemical weathering that modified these rocks in Late Jurassic or Early Cretaceous time.
75
MANGANESE MINERALIZATION IN EARLY PROTEROZOIC IRON-FORMATION O F THE EMILY DISTRICT, CUYUNA RANGE, EAST-CENTRAL MINNESOTA
MOREY, G.B., and D. L. Southwick, Minnesota Geological Survey, 2642 University Avenue, St. Paul, Minnesota 55 114
Eaily Pmtemwic strata of the Emily district at the far northern end of the Cuyuna Iron Range define [he southwestern closure of the Aniiikie basin in east-central Minnesota (Southwick and others, 1988). A such, the rocks of the Emily district are correlative with strata of the well known Animikie Gmup of the Mesabi range. However. unlike the monoclinal nature of the Mesabi range, strata in the Emily district are thrown inw a series of broad, open. eastward-plunging folds with near-vertical axial planes. Geometric relationships imply that the basal contact of the ~ ~ m i k i e Groupoverlies an unconfo&ty cut onto older folded rocks of the North ranee (Chandler and Malek. 1991 ). That unconformitv marks the boundary between twice-deformed rocks of the Penokean fold-and-thrust belt and the onc&efonned rocks of the Animikie basin-
On the Mesabi range, the Animikie Gmup consists of a lower quartz arenitic sequence (Pokegama Quartzite), an intermediate iron-rich sequence (Biwabik Iron Formation), and an upper black-shale- graywacke sequence (Virginia Formation). In the Emily district, however. the stratigraphic position of the Biwabik is occupied by several lenticular units of iron-formation intercalated within both Pokegama- and Virginia-like materials.
The main iron-formation of the Emily districtÑterme Unit A of the Ruth Lake areaÑca be dividei into seven lithotopes (Fig. 1). They are (1) anepiclastic lithotope of quartz-rich siltstone and shale; (2) a mixed epiclastic-jaspery-chert lithowpe; (3) an oolitic and pisolitic lithotope; (4) a ihick-bedded lithotope o cherty or granular iron-fonnation; (5) a mixed thick- and thin-bedded lithowpe characterized by thick intervals of slaty or nongranular iron-formation; (6) a thin-bedded lithowpe consisting of laminae w very thin beds of nongranulai iron-formation; and (7) a ferruginous chert lithowpe. In general, lithotopes I. 2. and 3 contain ~rimarv hematite and have textural attributes indicative of lhe oxide (hematite) facies of iron- formation. Incontra& lithowpe 6 contains appreciable carbonates or silicates and has textural attributes indicative of the carbonate facies of iron-formation. Lithotopes 4 and 5 are interlayered and contain mixtures of oxides (hematite-magnetite) and silicates, and thus were deposited in generally similar sedimenwlogical regimes in waters variably affected by currents and at depths intermediate between oxide: and carbonate facies. The sediienwlogical significance of lithotope 7 is uncertain. It is a thin-bedded laminated unit deposited in a deeper water regime as evidenced by a stratigraphic position transitional between thin-bedded iron-formation and overlying black shale; yet it contains p r imw hematite. - -
Sedimentological parameters imply that Unit A was deposited during two uan&ressive-regressive cycles in a basin with a strandline w the west and deeper wakr 10 the north and east. Well-rounded grains of temeenous auartz. which nersist ihroughout liihownes 1-5. imply that much of the sedimentation - &curred relatively close w the strandline.
Although unit A in the Emily district has many mineralogical. textural. and chemical attributes indicative of "ordinary" iron-formation. it locallv remains manganese oxides in amounts 10 to 100 times greater than the normof 0.6 w 0.8 percent in the Biwabik ~ronkormation (Morey and Money, 1990).
-
Manganese oxides occur principally in lithotopes 1-5 as disseminated grains, as ihin pods or lenses, and a layers as thick as 1.5 meters that typically wntain about 10 percent Mn; some wntain as much as 20-30 percent Mn. Manganese oxides are particularly abundant near Ruth Lake where two laterally persistent zones about 15 w 18 meters thick have manganese tenors enriched to the 10-50 percent range. Both zone more or less coincide with stratieraphic positions occupied by the oolitic-pisolitic lithotope. They Contain various proponions of psilomelane and cryptomelane, as well as hematite and quanz. Goethite and maneanite mav be locallv abundant- where they occur. they are secondary phases that formed during a " period of intense chemical weathering that modified these rocks in Late ~ u k s i c or Early Cretaceous time.
75
The manganese oxides are most likely epigenetic in origin. They are confined to the porous andpermeable pails of Unit A; rocks with the greatest primary porosity—such as quartz arenitic rocks in theepiclastic lithotope and the oolitic-pisolitic lithotope-.-.-have the most manganese. Morey and others (1991)have suggested that the manganese was deposited by a refluxing process involving reducing solutions thatleached manganese from older rocks of the North range. The manganese was subsequently precipitatedwhere the reducing solutions came into contact with oxidizing conditions in the depositional basin justbelow the sediment-water interface. Much of the manganese must have been precipitated early in thediagenetic history of the iron-formation, because texturally it occupies porn spaces normally filled with silicacement.
Mesozoic weathering of the rocks caused some redistribution of manganese. The abundance ofmanganese makes the Ruth Lake area in the Emily district a potential target for in-situ mining techniquescurrently being developed by the U.S. Bureau of Mines and the Mineral Resources Research Center of theUniversity of Minnesota.
l'his project was supported in part by the basic research component of the Minerals DiversificationProgram, administered by the Minerals Cooniinating Committee for the Minnesota State Legislature.
REFERENCES CITED
Chandler, V.W., and Malek, K.C., 1991, Moving-window Poisson analysis of gravity and magnetic datafrom the Penokean orogen, east-central Minnesota: Geophysics, v. 56, p. 123—132.
Morey, G.B., and Morey, P.R., 1990, Major and minor element chemistry of the Biwabik Iron Formationand associated rocks, Minnesota, in AIME, Minnesota Section, 163rd Annual Meeting, and MiningSymposium, 51st, 1990. Proceedings: University of Minnesota, Duluth, Continuing Education andExtension, Center for Professional Development, p. 259—287.
Morey, G.B., Southwick, D.L., and Schonler, S.P., 1991, Manganiferous zones in the Early Proterozoiciron-formation in the Emily district of the Cuyuna Iron Range, east-central Minnesota: MinnesotaGeological Survey Report of Investigations 39, 42 p.
Southwick, D.L., Morey, G.B., and McSwiggen, P.L. 1988, Geologic map (scale 1:250,000) of thePenokean orogen, central and eastern Minnesota, and accompanying text: Minnesota Geological SurveyReport of Investigations 37, 25 p.. 1 p1.
E whMrashett basin Ramp Back ramp platform litholopes
Solo.. fa,.weaiher wave base wave ag.Iated Low energy
Figure 1. Schematic profile of lithotopes in the Emily district. No vertical or horizontal scale intended.Environmental nomenclature is typical of that used in Phanerozoic limestone-shale sequences.
76
,a1wy flyer
e,alor. 050ood ,.se 00a1s
The manganese oxides are most likely eqigenetic in origin. They are confined to the porous and permeable pans of Unit A, rocks with the greatest primary porosity~such as q u m arenitic rocks in the epiclastic lithotope and the oolitic-pisolitic lithotopeÑhav the most manganese. Morey and others (1991) have suggested that the manganese was deposited by a refluxing process involving reducing solutions that leached manganese from older mcks of the Nonh range. The manganese was subsequently precipitated where the reducing solutions came into contact with oxidizing conditions in the depositional basin just below the sediment-water interface. Much of the manganese must have been precipitated early in the diagenetic history of the iron-formation, because texturally it occupies pore spaces normally filled with silica cement.
Mesozoic weathering of the mcks caused some redistribution of manganese. The abundance of manganese makes the Ruth Lake area in the Emily district a potential target for in-situ mining techniques currently being developed by the U.S. Bureau of Mines and the Mineral Resources Research Center of the University of Minnesota.
This project was supported in pan by the basic research component of the Minerals Diversification Program, administered by the Minerals Coordinating Committee for the Minnesota State Legislature.
REFERENCES CITED
Chandler, V.W., and Malek, K.C., 1991, Moving-window Poisson analysis of gravity and magnetic data from the Penokean omgen, east-central Minnesota: Geophysics, v. 56, p. 123-132.
Morey, G.B., and Morey, P.R., 1990, Major and minor element chemistry of the Biwabik Iron Formation and associated rocks, Minnesota, in AIME, Minnesota Section, 163rd Annual Meeting, and Mining Symposium, 5lst. 1990, Proceedings: University of Minnesota, Duluth, Continuing Education and Extension, Center for Professional Development, p. 259-287.
Morey, G.B., Southwick, D.L., and Schottler, S.P., 1991, Manganiferous zones in the Early Proterozoic iron-formation in the Emily district of the Cuyuna Iron Range, east-central Minnesota: Minnesota Geological Survey Report of Investigations 39.42 p.
Southwick, D.L., Morey. G.B., and McSwiggen, P.L. 1988, Geologic map (scale 1:250,000) of the Penokean omgen, central and eastern Minnesota, and accompanying text: Minnesota Geological Survey Report of Invesligations 37.25 p., 1 pi.
Figure 1. Schematic profile of lithotopes in the Emily district. No vertical or horizontal scale intended. Environmental nomenclature is typical of that used in Phanemzoic limestone-shale sequences.
PICK LAKE ZINC—COPPER—SILVER DEPOSIT
IAN P. MORRISON, MINNOVA INC.
The Pick Lake massive sulphide deposit is situated 20
kin north of Schreiber, 145 kin east of Thunder Bay, Ontarioand 1.5 km from Ninnova's Winston Lake mine.
The deposit occurs 1000 meters stratigraphically belowthe Winston Lake deposit at the base of a sequence of
calc—alkalic volcanics and sediments. First indications ofthe deposit came in 1984 when Minnova drill tested a weaklymineralized horizon within a zone of strong hydrothermalalteration. Drill hole WL—9 intersected 0.3 meters of massivepyrrhotite within a thick sequence of altered clasticsediments and intentiediate to felsic ash which was weaklymineralized with zinc. Subsequent drilling down dip led tothe discovery of thin but high grade massive suiphides.
Exploration drilling has continued to systematicallytest the Pick Lake deposit at depth and has defined a steeplyplunging sheet with a strike length of 400 meters, dip lengthof 1400 meters but an average thickness of less than 2
meters. In 1990, drill hole WL—67 intersected 13.4 meters ofmassive sulphides grading 2.6% Cu, 26.0% Zn and 106 g/t Ag ata vertical depth of 1050 meters. Drilling to date at PickLake totals 27,000 meters over 36 holes.
Core and poster display:— regional geological map of N. W. Ontario— geological map of the Winston Lake property- 1:2000 composite section (Pick Lake stratigraphy)- 1:2000 longitudinal section (Pick Lake deposit)— surface geology map of the Anderson showing (Pick Lake)— core representative of the fringe mineralization— core representative of thin massive suiphides- massive sulphide intersection WL—67- sample suite of Pick Lake stratigraphy
77
PICK LAKE ZINC-COPPER-SILVER DEPOSIT
IAN R . MORRISON, MINNOVA INC.
The Pick Lake massive sulphide deposit is situated 20
north of Schreiber, 145 Ion east of Thunder Bay, Ontario
nd 1.5 1cm from Minnova's Winston Lake mine.
The deposit occurs 1000 meters stratigraphically below
the Winston Lake deposit at the base of a sequence of
calc-alkalic volcanics and sediments. First indications of
the deposit came in 1984 when Minnova drill tested a weakly
mineralized horizon within a zone of strong hydrothermal
alteration. Drill hole WL-9 intersected 0.3 meters of massiv
pyrrhotite within a thick sequence of altered clasti
sediments and intermediate to felsic ash which was weak1
mineralized with zinc. Subsequent drilling down dip led t
the discovery of thin but high grade massive sulphides.
Exploration drilling has continued to systematically
test the Pick Lake deposit at depth and has defined a steeply
plunging sheet with a strike length of 400 meters, dip length
of 1400 meters but an average thickness of less than 2
meters. In 1990, drill hole WL-67 intersected 13.4 meters of
massive sulphides grading 2.6% Cu, 26.0% Zn and 106 g/t Ag at
a vertical depth of 1050 meters. Drilling to date at Pick
Lake totals 27,000 meters over 36 holes.
Core and poster display:
regional geological map of N. W. Ontario
geological map of the Winston Lake property
1:2000 composite section (Pick Lake stratigraph
1:2000 longitudinal section (Pick Lake deposit)
surface geology map of the Anderson showing (Pick Lake)
core representative of the fringe mineralization
core representative of thin massive sulphides
massive sulphide intersection WL-67
sample suite of Pick Lake stratigraphy
PLATINUM GROUP ELEMENT POTENTIAL OFKEWEENAWAN INTRUSIVE ROCK IN WISCONSIN
H.G. Itudrey. Jr., Wisconsin Geological and Natural History Survey, 3817Mineral Point Road, Madison, Wisconsin 53705, and
Bruce A. Brown, Wisconsin Geological and Natural History Survey, 3817Mineral Point Road, Madison, Wisconsin 53705
Whole rock analyses of platinum in the Round Lake Intrusion, SawyerCounty, Wisconsin discloses platinum enrichment with increasing iron andtitanium accompanied by decreasing paladium. Anomalous chromium and va-nadium also correlate with iron and titanium.
The Round Lake Intrusion is known only from boreholes and geophys-ical data. It is at least 8 km long and 22 km wide and is inferred to ex-tend to a depth of 1400 m (Roder, 19773; Stuhr, 1976). Diabasic olivinegabbro and troctolite occur at the margin of an oxide-rich core of thetrough-shaped intrusion. Dominant minerals are olivine, plagioclase andtitanomagnetite in various proportions. In the core of the intrusion,olivine and plagioclase crystallized early and iron-titanium oxides crys-tallized late. The core of the intrusion consists of a magnetitite unitof 12 to 26 percent clusters of oxide and silicates, 9-15 percentolivine, 2-10 percent plagioclase, 54 to 72 percent titanomagnetite andless than 3 percent ilmenite. Plagioclase is An50; titanomagnetiteaverages Mt Usp50 and contains 1.3 percent V 0. Commercial po-tential of Re vanadium-rich core is limited ecause ilmenite-ulvospinelintergrowths are narrow, and ultra-fine grinding wouldbeneficiation. However, added values from platinumalter the economics.
RL-Ol-O316 diabase <5 38 35 35 408RL-05-O557 dibase 7 103 28 94 478RL-Ol-0324 diabase 8
RL-09-1190 lower magnetite/troctolite 11RL-09-1179 lower rnagnetite/troctolite 30RL- 01-0184 Magnetite/troctolite 75RL-05-0320 magnetitite 106RL-02-0078 magnetitite 116RL-10-0386 magnetitite 120
Stuhr, S.W., 1976, Geology of the Round Lake Intrusion, Sawyer County,Wisconsin: unpublished M.S. thesis, University of Wisconsin-Madison, 148p.
Roder, D.L., 1973, The petrology and chemistry of the Round Lake Intru-sion, Northwest Wisconsin: unpublished H.S thesis, University of
78 Wisconsin-Madison, 115 p.
ABSTRACT
U
j
be required forgroup elements may
Pt PcI Coppb ppb ppm
Crppm
Vppm
41 29 501 54 3493 105 7903 155 5414 142 24201 189 25305 170 2820
391609
1050nanana
>2000
PLATINUM GROUP ELEMENT POTENTIAL OF KEWEENAWAN INTRUSIVE ROCK IN WISCONSIN
M.G. Hudrey, Jr., Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison, Wisconsin 53705, and
Bruce A. Brown, Wisconsin Geological and Natural History Survey, 3817 Mineral Point Road, Madison. Wisconsin 53705
ABSTRACT >
Whole rock analyses of platinum in the Round Lake Intrusion, Sawyer County, Wisconsin discloses platinum enrichment with increasing iron and titanium accompanied by decreasing paladium. Anomalous chromium and va- nadium also correlate with iron and titanium.
The Round Lake Intrusion is known only from boreholes and geophys- ical data. It is at least 8 km long and 2 2 km wide and is inferred to ex- tend to a depth of 1400 m (Roder, 19773; Stuhr, 1976). Diabasic olivine gabbro and troctolite occur at the margin of an oxide-rich core of the trough-shaped intrusion. Dominant minerals are olivine, plagioclase and titanomagnetite in various proportions. In the core of the intrusion, olivine and plagioclase crystallized early and iron-titanium oxides crys- tallized late. The core of the intrusion consists of a magnetitite unit of 12 to 26 percent clusters of oxide and silicates, 9-15 percent olivine, 2-10 percent plagioclase, 54 to 72 percent titanomagnetite and less than 3 percent ilmenite. Plagioclase is An ; titanomagnetite averages Mt U s p and contains 1.3 percent V on. 50~ommercial po- tential of the vanadium-rich core is limited because ilmenite-ulvospinel intergrowths are narrow, and ultra-fine grinding would be required for beneficiation. However, added values from platinum group elements may alter the economics.
RL-01-0316 diabase ,.. ,, , < . .. C5 RL-05-0557 dibase ~ * .,. - . 7 RL-01-0324 diabase 8 RL-09-1190 lower magnetite/troctolite 11 RL-09-1179 ' lower magnetite/troctolite 30 RL-01-0184 Magnetite/troctolite 7 5 RL-05-0320 magnetitite 106 RL-02-0078 magnetitite ,- 116 RL-10-0386 magnetitite 120
Stuhr, S.W., 1976, Geology of the Round Lake Intrusion, Sawyer County, Wisconsin: unpublished M.S. thesis, University of Wisconsin-Madison, 148 P.
Roder, D.L., 1973, The petrology and chemistry of the Round Lake Intru- sion. Northwest Wisconsin: unpublished M.S. thesis, University of
7Q Wisconsin-Madison, 115 p.
The Porcupine Mountains area, Michigan - a Keweenawan centralvolcano?
by
S.W. Nicholson. K.). Schulz, W.F. Cannon. and L.G. WoodruffU.S. Geological Survey, National Center - MS 954, Reston, VA. 22092
Although magmatism related to the Midcontinent rift system is mostly basalticcomposition, intermediate and felsic rocks comprise about 10 to 15% of thevolcanic section in the Lake Superior region. Locally, intermediate and felsicrocks dominate the volcanic suite, as they do in the Porcupine Mountains area ofnorthern Michigan. The Porcupine Mountains area is anomalous in theMidcontinent rift system not only because of its abundance of felsic rocks but alsobecause of the coincidence of two geophysical anomalies -- a gravity low and amagnetic low -- superimposed on a broader magnetic high (Fig. 1). In addition,the abundant rhyolite and intermediate rocks form a partial ring of topographichighlands making up the structurally complex and apparently folded felsicvolcanic pile. For clarity, the entire area underlain by the elliptical broadmagnetic high (and the corresponding topographic highlands) will be called thePorcupine Mountains volcanic center. Within this volcanic center, thePorcupine Mountains themselves make up the northern highlands, PorcupinePeak forms the western highlands, and the hills around the Bergland firetowermake up the southeastern highlands (Fig. 1). Earlier workers suggested that thisarea is a remanent of an Icelandic-style central volcanic complex (White, 1972;Green, 1977; Kopydlowski, 1983), but evidence supporting this suggestion hasbeen largely undocumented. To evaluate this hypothesis, mapping, geochemical,isotopic, and geophysical studies were recently undertaken in the PorcupineMountains area.
Preliminary results of mapping and geophysical studies indicate manysimilarities between Icelandic central volcanoes and the inferred PorcupineMountains volcanic center. For instance, the Porcupine Mountains center formedat the edge of an active rift zone and is about 30 to 40 km wide, comparable in sizeto Icelandic central volcanoes. Earlier workers recognized that an accumulationof felsic and intermediate rocks, called the 'unnamed" formation, formed a lens-shaped body underlain by the northwest-dipping basalts of the Portage LakeVolcanics (Fig. 1). The "unnamed' formation is being formally named thePorcupine Volcanics (Cannon and Nicholson, in review). This formation is morethan 2 km thick at its thickest but thins away from the Porcupine Mountains area.The thickness of the overlying Copper Harbor Conglomerate decreasessubstantially over the Porcupine Volcanics.
Recent mapping has shown that much rhyolite in the southern highlandsoccurs as subvolcanic bodies that intrude flows of intermediate composition.These rhyolites are commonly porphyritic, containing both quartz and feldsparphenocrysts. Near the top of the Porcupine Volcanics in the southern highlands.an extrusive rhyolite body contains mostly small feldspar phenocrysts. Incontrast, most rhyolite bodies in the northern highlands are extrusive flows.However, these rhyolites are typically massive aphyrie flows or they may containonly sparse feldspar ± quartz phenocrysts. Understanding the chemicalrelationships among the rhyolite bodies in the Porcupine Volcanics awaits theresults of detailed geochemical studies.
Gravity modeling across the Porcupine Mountains volcanic center requires thepresence of a felsic stock to account for the low gravity anomaly beneath the
79
T h e P o r c u p i n e M o u n t a i n s a r e a , Michigan - a Keweenawan c e n t r a l I v o l c a n o ? ."
b Y
S.W. Nicholson. K.J. Schulz. W.F. Cannon, and L.G. Woodruff U S . Geological Survey, National Center - MS 954, Reston, VA. 22092
Although magmatism related to the Midcontinent rift system is mostly basaltic composition, intermediate and felsic rocks comprise about 10 t o 15% of the volcanic section in the Lake Superior region. Locally, intermediate and felsic rocks dominate the volcanic suite. as they do in the Porcupine Mountains area of northern Michigan. The Porcupine Mountains area i s anomalous in the Midcontinent rift system not only because of its abundance of felsic rocks but also because of the coincidence of two geophysical anomalies - a gravity low and a magnetic low -- superimposed on a broader magnetic high (Fig. 1). In addition, the abundant rhyolite and intermediate rocks form a partial ring of topographic highlands making up the structurally complex and apparently folded felsic volcanic pile. For clarity, the entire area underlain by the elliptical broad magnetic high (and the corresponding topographic highlands) will be called the Porcupine Mountains volcanic center. Within this volcanic center , the Porcupine Mountains themselves make up the northern highlands. Porcupine Peak forms the western highlands, and the hills around the Bergland firctowcr make up the southeastern highlands (Fig. 1). Earlier workers suggested that this area is a remanent of an Icelandic-style central volcanic complex (White. 1972; Green, 1977; Kopydlowski. 1983). but evidence supporting this suggestion has been largely undocumented. T o evaluate this hypothesis, mapping. geochemical, isotopic, and geophysical studies were recently undertaken in the Porcupine Mountains area.
Preliminary results of mapping and geophysical studies indicate many similarities between Icelandic central volcanoes and the inferred Porcupinc Mountains volcanic center. For instance, the Porcupine Mountains center formed at the edge of an active rift zone and is about 30 to 40 km wide, comparable in size to Icelandic central volcanoes. Earlier workers recognized that an accumulation of felsic and intermediate rocks, called the "unnamed" formation, formed a lens- shaped body underlain by the northwest-dipping basalts of the Portage Lake Volcanics (Fig. 1). The "unnamed" formation is being formally named the Porcupine Volcanics (Cannon and Nicholson, in review). This formation is more than 2 km thick at its thickest but thins away from the Porcupine Mountains area. The thickness o f the over ly ing Copper Harbor Conglomerate decreases substantially over the Porcupine Volcanics.
Recent mapping has shown that much rhyolite in the southern highlands occurs as subvolcanic bodies that intrude flows of intermediate composition. These rhyolites are commonly porphyritic, containing both quartz and feldspar
hcnocrysts. Near the top of the Porcupine Volcanics in the southern highlands, n extrusive rhyolite body contains mostly small feldspar phenocrysts. In
ntrast, most rhyolite bodies in the northern highlands are extrusive flows. owever. these rhyolites are typically massive aphyric flows or they may contain nly sparse feldspar  quar tz phenocrysts. Understanding the chemical
relationships among the rhyolite bodies in the Porcupine Volcanics awaits the results of detailed geochemical studies.
Gravity modeling across the Porcupine Mountains volcanic center requires the presence of a felsic stock to account for the low gravity anomaly beneath the
0
EX
PLA
NA
TIO
N
GE
OLO
GIC
UN
ITS
Kew
eena
wan
____
sedi
men
tary
roc
ks
Lilli
ll P
orcu
pine
Voi
cani
cs
IP
orta
ge L
ake
1Vpl
1V
olca
nics
GE
OP
HY
SIC
AL
AN
OM
ALI
ES
Mag
netic
low
Mag
netic
hig
h
/0 G
ravi
ty (
in m
gals
)
j_.
Str
ike
and
dip
FIG
.1:
Map
sho
win
glo
catio
n of
peni
nsul
a of
Mic
higa
n.el
liptic
al m
agne
tical
lyar
e su
perim
pose
d on
the
Por
cupi
ne M
ount
ains
vol
cani
cce
nter
in th
e no
rthw
este
rnpa
rt o
f the
upp
erT
he to
pogr
aphi
c hi
ghla
nds
of th
e P
orcu
pine
Vol
cani
cs c
oinc
ide
with
an
high
ano
mal
y.S
mal
ler
mag
netic
low
and
grav
ity lo
w a
nom
alie
sco
inci
de a
nd
mid
dle
of th
e m
agne
tichi
gh.
L C
[C
LL.
..I
L t
L. L
I
_
L_(L
-c±
i L_
C
[Vs
1 II
510
-S.-
'km
w m - x - (ft
a
\
-a la C w m n n w 3 C V
m .- 0) 0 Z - C : C ¥*- 4-1 ¥!
.P- 0 * < - x u 0
w ' n ¥*->- 0) s- .- .- m 0- Q C m - E c 0 0 I-<->-= w m ¥I-' - 0 3 a%- 0 s C - .£ m ¥*- 0 =, %-¥I- 0 0-7 C > >
m W W L -c C cr ¥*- .-
c-0 C -3 C. .- 0 (0
s- s - 0 % w Q - 0 Â¥I- IÃ
C w w-c: u u ¥*- .- -
I
.
volcanic center (Kiasner and Jones. 1989). A felsic stock also is consistent withthe coincident low magnetic anomaly (King, 1987). The geophysical inference ofa felsic stock lying beneath the volcanic center supports the comparison with theIcelandic model of a central volcano overlying a shallow magma chamber.
Numerous faults and apparent folding complicate structural interpretation inthis area. For example, in the northern highlands (in the Porcupine MountainsState Wilderness Area). a change in the dip of volcanic units from south to northled Hubbard (1975) to propose an anticline. However, detailed mapping indicatesthat some individual stratigraphic units cannot be traced across the proposed fold.An alternative model for the structure of the northern highlands is that differentrhyolite bodies may represent individual small (1-3 km wide) flow-domecomplexes, some with carapace breccias still intact, and possibly localized along amajor caldera-bounding fault. Similar flow-dome complexes have beendocumented in the Torfajokull volcanic complex in Iceland (Macdonald et al..1990). Thus, we suggest that the elliptical area of highlands and the coincidentgeophysical anomalies reflect the original shape of the volcanic shield, althoughit has been somewhat modified by post-rift compressional deformation.
The Porcupine Mountains volcanic center appears to share many similaritieswith Icelandic-style central volcanoes, but a more complete understanding of thearea will require careful interpretation of structures along nearby seismicreflection profiles, detailed correlation of stratigraphic units and structuralanalysis within the center, and careful analysis of chemical, isotopic andvolcanological data.
ReferencesCannon, W.F., and Nicholson, S.W., in review, Revisions of stratigraphic
nomenclature within the Keweenawan Supergroup of northern Michigan.U.S. Geological Survey, Bulletin 1970-A.
Green, J.C., 1977. Keweenawan plateau volcanism in the Lake Superior region.Geological Association of Canada, Special Paper 16, p. 407-422.
Hubbard, 11.A., 1975, Geology of Porcupine Mountains in Carp River and WhitePine quadrangles, Michigan. U.S. Geological Survey, Journal of Research, v. 3,p. 519-528.
King, E.R., 1987, Aeromagnetic map of the Iron River 1° x 2° quadrangle.Michigan and Wisconsin. U.S. Geological Survey, MiscellaneousInverstigations Series, 1-1360-F. 1:250,000.
Klasner, J.S., and Jones, W.J., 1989, Bouger gravity anomaly map and geologicinterpretation of the Iron River 10 x 20 quadrangle, Michigan and Wisconsin.U.S. Geological Survey, Miscellaneous lnverstigations Series. J-1360-E.1:250,000.
Kopydlowski, P.J., 1983, The Oak Bluff Volcanics. a Middle Keweenawan centralvolcano; Porcupine Mountains region. Michigan. unpublished M.S. thesis,Michigan Technological University. Houghton. 88 pp.
Macdonald, R., McGarvie, D.W., Pinkerton, H., Smith, R.L., and Palacz, Z.A., 1990.Petrogenetic evolution of the Torfajokull volcanic complex. Iceland I.Relationship between the magma types. Journal of Petrology. v. 31, p. 429-459.
Walker, G.P.L., 1963, The Breiddalur central volcano, eastern Iceland. QuarterlyJournal of the Geological Society of London. v. 119. p. 29-63.
White, W.S., 1972, The base of the Upper Keweenawan, Michigan and Wisconsin.U.S .Geological Survey Bulletin 1354-F, 23 pp.
81
volcanic center (Kiasner and Jones. 1989). A felsic stock also is consistent with the coincident low magnetic anomaly (King, 1987). The geophysical inference of a felsic stock lying beneath the volcanic center supports the comparison with the Icelandic model of a central volcano overlying a shallow magma chamber.
Numerous faults and apparent folding complicate structural interpretation in area. For example. in the northern highlands (in the Porcupine Mountains
e Wilderness Area). a change in the dip of volcanic units from south to north 5) to propose an anticline. However. detailed mapping indicates ual stratigraphic units cannot be traced across the proposed fold.
n alternative model for the structure of the northern highlands is that different may represent individual small (1-3 km wide) flow-dome
exes. some with carapace breccias still intact, and possibly localized along a caldera-bounding fault. Similar f low-dome complexes have been
ented in the Torfajokull volcanic complex in Iceland (Macdonald e t al.. Thus, we suggest that the elliptical area of highlands and the coincident
sical anomalies reflect the original shape of the volcanic shield, although been somewhat modified by post-rift compressional deformation. The Porcupine Mountains volcanic center appears to share many similarities
ith Icelandic-style central volcanoes, but a more complete understanding of the ea will require careful interpretation of structures along nearby seismic flection profiles, detailed correlation of stratigraphic units and structural
nalysis within the center. and careful analysis of chemical , isotopic and volcanological data. ., : 7
R e f e r e n c e s
Cannon, W.F.. and Nicholson, S.W.. in review, Revisions of stratigraphic nomenclature within the Keweenawan Supergroup of northern Michigan. U.S. Geological Survey, Bulletin 1970-A.
reen. J.C., 1977, Keweenawan plateau volcanism in the Lake Superior region. Geological Association of Canada. Special Paper 16, p. 407-422.
ubbard, H.A., 1975, Geology of Porcupine Mountains in Carp River and White Pine quadrangles, Michigan. U.S. Geological Survey, Journal of Research, v. 3.
King, E.R., 1987. Aeromagnetic map of the Iron River 10 x 20 quadrangle. Michigan and Wisconsin. U.S. Geological S u r v e y , Misce l l aneous Inverstigalions Series, I-1360-F, 1:250,000.
Klasner. J.S., and Jones. W.J., 1989, Bouger gravity anomaly map and geologic interpretation of the Iron River 10 x 20 quadrangle, Michigan and Wisconsin. U.S. Geological Survey, Miscellaneous Inverst igations Series. I-1360-E.
. 1:250,000. opydlowski. P.J.. 1983. The Oak Bluff Volcanics. a Middle Keweenawan central
volcano; Porcupine Mountains region, Michigan. unpublished M.S. thesis. Michigan Technological University. Houghton. 88 pp.
Macdonald, R., McGarvie, D.W.. Pinkerton, H., Smith. R.L., and Palacz. Z.A.. 1990. Petrogenetic evolution of t h e Torfajokull volcanic complex, Iceland I. Relationship between the magma types. Journal of Petrology, v. 31, p. 429459.
alker, G.P.L.. 1963. The Breiddalur central volcano. eastern Iceland. Quarterly Journal of the Geological Society of London, v. 119, p. 29-63.
hite. W.S.. 1972, The base of the Upper Keweenawan, Michigan and Wisconsin. U S .Geological Survey Bulletin 1354-F. 23 pp.
82
PHYSICAL VOLCANOLOGY OF THE FOOTWALLROCKS AT THE WINSTON LAKE MASSIVE SULFIDE DEPOSIT
Steven A. OsterbergGeology Dept., Economic Volcanological Research Lab.University of Minnesota—Duluth, Duluth, MN 55812
Ian R. Morrison,Minnova, Inc., Thunder Bay, Ontario P7C lE7
Footwall racks at the Winston Lake massive sulfide depositconsist of an amphibolite grade, hydrothermal].y—altered sequenceof north—south trending volcanic and volcaniclastic. rocks. Geo-logical mapping, drill core, and petrographic studies allow de-tailed subdivision and volcanological analysis of the strati-graphy.
Thick (100—325m), massive to poorly—bedded tuafic to felsicvolcaniclastic rocks form the base of the stratigraphy and arebounded to the west by granitic intrusive rocks. Intercalatedfelsic pyroclastic rocks form marker horizons and suggest thedeposits thicken southward.
The clastic rocks are overlain to the east by l00—850m ofinterlayered felsic and maf Ic lava flows which, in general,thicken southward. Felsic flows, which constitute 80% of thesuccession, are quartz—feldspar—phyric, massive, and rarelyflow—banded and brecciated. Mafic lava flows vary from massiveto well-pillowed, and aphyric to feldspar-phyric. Local maficand felsic dikes are present near the south end of the sequenceand are thought to represent feeders to the lava flows.
Immediately above the lava flows is a 75-200m thicksequence of pyroclastic and volcaniclastic rocks. Pumice—bear-ing felsic pyroclastic rocks predominate to the north withpumice size and content increasing northward. The pyroclasticrocks interfinger to the south with mafic-felsic volcaniclasticrocks. The volcaniclastic rocks are locally transected by alaterally limited (100—125n), but thick (150m), succession ofaphyric mafic lava flows, sills, and dikes. The mafic rocks arethought to fill an axial trough, and to represent a feeder com-plex to lavas higher in the section. The flows vary from mas-sive to pillowed; peperites and cm—scale sheet flows are locallywell-developed.
The Winston Lake Horizon caps the footwall sequence andconsists of 3—4 interlayered mafic lava flows and mafic—felsicvolcaniclastic rocks and cherty tuffs. The lavas are aphyric,laterally extensive (>2km), and thicken (2-BOrn) southward. Vo].-caniclastic rocks and tuffs vary from l-20m in thickness and aremassive, to well—laminated.
The stratigraphy at Winston Lake developed with cyclicaccumulation of volcaniclastic and volcanic rocks; lava flowswere derived from a rift—related southward source. In contrast,felsic pyroclastic rocks appear to have been derived from an un-related northward source. The lack of hydrovolcanic or vesi-cular deposits suggests deep water volcanism prevailed atWinston Lake.
PHYSICAL VOLCANOLOGY OF THE FOOTWALL ROCKS AT THE WINSTON LAKE MASSIVE SULFIDE DEPOSIT
Steven A. Osterberg Geology Dept., Economic Volcanological Research Lab. University of Minnesota-Duluth, Duluth, MN 55812
Ian R. Morrison, Minnova, Inc., Thunder Bay, Ontario P7C 1E7
Footwall rocks at the Winston Lake massive sulfide deposit consist of an amphibolite grade, hydrothermally-altered sequence of north-south trending volcanic and volcaniclastic rocks. Geo- logical mapping, drill core, and petrographic studies allow de- tailed subdivision and volcanological analysis of the strati- graphy.
Thick (100-325m), massive to poorly-bedded mafic to felsic volcaniclastic rocks form the base of the stratigraphy and are bounded to the west by granitic intrusive rocks. Intercalated felsic pyroclastic rocks form marker horizons and suggest the deposits thicken southward.
The clastic rocks are overlain to the east by 100-850m of interlayered felsic and mafic lava flows which, in general, thicken southward. Felsic flows, which constitute 80% of the succession, are quartz-feldspar-phyric, massive, and rarely flow-banded and brecciated. Mafic lava flows vary from massive to well-pillowed, and aphyric to feldspar-phyric. Local mafic and felsic dikes are present near the south end of the sequence and are thought to represent feeders to the lava flows.
Immediately above the lava flows is a 75-200m thick sequence of pyroclastic and volcaniclastic rocks. Pumice-bear- ing felsic pyroclastic rocks predominate to the north with pumice size and content increasing northward. The pyroclastic rocks interfinger to the south with mafic-felsic volcaniclastic rocks. The volcaniclastic rocks are locally transected by a laterally limited (100-125m), but thick (150m), succession of aphyric maf ic lava flows, sills, and dikes. The maf ic rocks are thought to fill an axial trough, and to represent a feeder com- plex to lavas higher in the section. The flows vary from mas- sive to pillowed; peperites and cm-scale sheet flows are locally well-developed.
The Winston Lake Horizon caps the footwall sequence and consists of 3-4 interlayered mafic lava flows and mafic-felsic volcaniclastic rocks and cherty tuffs. The lavas are aphyric, laterally extensive (>2km), and thicken (2-80m) southward. Vol- caniclastic rocks and tuffs vary from 1-2Om in thickness and are massive to well-laminated.
The stratigraphy at Winston Lake developed with cyclic accumulation of volcaniclastic and volcanic rocks; lava flows were derived from a rift-related southward source. In contrast, felsic pyroclastic rocks appear to have been derived from an un- related northward source. The lack of hydrovolcanic or vesi- cular deposits suggests deep water volcanism prevailed at Winston Lake.
THE NEDA IRON FORMATION —— A PRODUCT OF VOLCANISM?
William F. Read, 1905 14. Alexander St., Appleton, WI 54911
The Neda iron formation crops out along the east, south, and southwest flanks of theWisconsin Arch (Fig. 1). It lies at the top of the upper Ordovician Maguoketa shale and isoverlain by Silurian Mayville dolomite. The formation is discontinuous. It reaches a maximumthickness of about 12 meters. Downdip. sway from the arch, its extent is not well known.
The Neda is characterized by an abundance of oblate spheroidal structures not more thanabout 2 mm in diameter. These have generally been described as ooids, implying that theyoriginated by wave and current action on the aea floor.
Near the village of Neda in eastern Wisconsin. the type locality, the formation wasfonnerly mined in small open cuts and shallow underground workings. Two types of ore wererecognized: "soft ore.' in which the "ooids" are weakly cemented together, and "hard ore, inwhich they are well cemented. Hard Ore Occurs as a capping on the soft ore. It is generally notmore than a meter thick.
My ideas concerning the origin of the Neda formation are based mainly on petrographicexamination of a few thin sections of hard ore from the type locality. Lakeside 70 was used toattach the rock to the slide. This cement, unlike epoxies, does not gelatinize in acids. 5ysoaking the section for several days in strong (6N) nitric acid I got rid of most of the hematiteand linonite which makes an untreated section largely opaque. Removal of these ingredients didnot leave holes in the section. It appears that the iron oxide minerals are finely disseminatedthrough the host materi•ala in which they occur.
-
0'0-,
Fig. 1 outcrop of Ordovician—Silurian boundary in parts of Minmesota,Iowa, Illinois, and Wisconsin. Dots are on the down—dip aide of the line.Areas where Neda formation is exposed are stippled. The Ordovician—Silurianboundary is overlain in places by post—Siluriem sedisenta: 0 — Devonien; C —carboniferous. Geology, except Weds areas, from U.S. Geological Survey,Geologic Map of the United States, 1/250,000, 1932.
THE NEDA IRON FORMATTON -- A PRODUCT OF VULCANISM? W i l l F. Read, 1905 N. Alexander St., Appleton, WI 54911
&&gnized: "soft ore," in which the "ooids" are weakly cemented together, a which they are well cemented. ~ a r d ore occurs as a capping on the soft ore. xt is generally not more than a meter thick.
My i d i n c o m i n g the origin of the Neda formation are based mainly on petrographic 0 0 a r e h " i o n Of hard ore from the localit". Lakeside 7 0 was
Fig. 1 Outcrop of ordovician-silurian boundary in parts of HinnÇÈot Iowa, Illinois, and wimconsin. ~ o t s are on the &--dip side of the line. Ar-as wh-r- Nçd formation is exposed are stippled. The Ordovician-Silurian boundary is overlain in plat-s by post-Silurian sediments: D - mvonian; C - Carbonirxrous. Geology, except ~ e d a areas. from U.S. Geological Survey, Geologic Map of the United States, 1~250,000, 1932.
so far as I an aware, very little work has been done On thin sections of the Hedaformation from which most of the iron oxides have been removed. Most workers have contentedthemselves with the examination of polished surfaces or acetate peels. A rather detailed studyof the mineralogy of the Neda formation at Neda was published in 1934 by Hawley and Beavan (1).They worked mainly with crushed material obtained from a mining company —— presumably a mixtureof hard and soft ore. Thin fragments were cleared of hematite and lisonite by soaking them in acombination of nitric and hydrochloric acid, then examined by transmitted light under apetrographic microscope. If the identification of a mineral by petrographic methods wasuncertain, and the mineral seemed important, they checked it by powder X—ray diffraction.
In the hard ore from Neda, the "ooids" alone are so varied and complex that it is not
easy to draw conclusions about their origin. I am convinced, however, that they were not formed
by sedimentary processes and are therefore not ooids in the usual sense of the term. It seems to
me more likely that they are miniature accretionary lapilli of volcanic origin. A volcanic
origin is suggested by Hawley and Beavan's observation that 'the most abundant transportedfragments in the ore are rough, angular grains of scoriaceous lava." They also obser-ved that thelava, when cleared of iron oxides, im a "dirty, green to brown volcanic glass. I found
fragments of what I take to be volcanic glass mainly as nuclei in ooids." They are not
acoriaceous and it is open to question whether they are really glass Or some amorphous orsubmicroscopically crystalline material which may have been derived from glass.
Where did this glass come from and how old is it? Hawley and meavan reject the idea that
it came from nearby volcanoes which were active when the Neda formation was deposited. Theyprefer a precambrian source to the "west or northwest" from which the glassy fragments Weretransported by streams or marine currents. Their objection to a nearby, contemporary source is
that "as yet no igneous activity of such age is recognized in this part of North America." I
think there is a much sore serious objection to their proposed alternative, The youngest
Precambrian volcanics to the west (along with axis of the Wisconsin Arch) and northwest (thepresent take Superior region) are Keweenawan. only a very small percentage of whatever glsaa mayoriginally have been present in the Keweenawan volcanics is likely to have escapeddevitrification as early as OrdovicisnSilurian time. Even if we assume that there was more
fleas in the geweenawan velsasics then than there is now, how could transported fragments of it remain
apparently unaltered in the rieds formation while, in the source area, alteration was actively
progressing? one may also ask why, in the Neda formation, glassy fragments should be abundant
while fragments of crystalline lava, which certainly would have been more abundsnt in the source
area, are miasinq. Igneous activity of Pa,leozoic or later age may not have been recognized in
the area where the Neds formation was deposited in Hawley and meavan's day but now we have thetake Ellen kimberlite to remind us that there has, in fact, been post—Keweenawan volcanicactivity south of Lake Superior. Whether it extended down into the area where the Made was
deposited remains to be seen.Most of the •'ooids" in my thin sections show a thick costing of slightly anisotropic
material surrounding any nucleus that may be visible. This, together with the nucleus, say be
regarded as the "core" of the "ooid." In "ooids" which lack a visible nucleus, the core consists
entirely of the nearly isotropic material. Most of it is pale green or brown. There is evidence
that the brown variety was formed by oxidation of the green. Both varieties show imperfectconcentric lamination around the nucleus, if one is visible —— otherwise around what may be taken
roughly as the center of the core. The laminae are most clearly ylsible XN. Light passing
through them is polarized parallel to the long dimensions of the laminae (and also, of course, at
right angles to the long dimensions) . My guess is that this is a result of stra,n in otherwise
isotropic material. There are abundant small inclusions which look as if they may be particles
of volcanic ash, also platy or lath—shaped crystals which are elongatad parallel to the long
dimensions of the laminae. The material which forms cores also occurs in alternate shells around
the cores,The same question may be asked concerning this slightly anisotropic core material as was
asked about certain nuclei in the "ooids': is it glass or some amorphous or sumicroscopically
crystalline material? Hawley and seavan identified the material of which many shell fragments
they observed consisted as halloysite. According to Ross and Kerr (2) , halloysite is a
submicroscopically crystalline clay mineral related to kaolinite. If the green or brown core -
material which I observed is halloysite, it has presumably been weathered. The material
weathered may very well have been volcanic glass. I suggest that the material forming cores in
the "ooids" I observed, and shells separated from the cores but composed of the same material,were formed by the accretion of tiny droplets of liquid lava onto nuclei which, in some cases,
are visible; in others, not. The droplets. I think, spread out over the rounded surface of
previously accreted material which had had time to harden or become extremely viscous. Thus the
imperfect lamellae were formed.Alternating with the shells of green or brown core material are shells composed of
material which is transparent and colorless. It is generally completely isotropic but maycontain very small, nearly equidimenaional crystals showing low birefrimgence. The refractive
index I found to be close to 1.54. a little lower than the mean refractive index of core
material, opaque patches of iron oxide minerals are commonly present in it. Scattered reflected
light from these patches makes a good deal of the adjoining clear material look tomato—red XN.Hawley and Beavan identified the material which they found in transparent, colorless shells as
opal. They too found that it has a refractive index of about 1.54-The mode of deposition, or emplacement, of the colorless transparent shells seems to have
been rather extraordinary. Some of these shells were originally composed Of relatively large
crystals, mow paeudomorphs. Euhedral outlines of these crystals may be seen projecting from theinner and/or outer surfaces of shells composed of thews In one case which I observed a shell ofthis type pinches out and a single, isolated. euhedral crystal of the same material is present in
core material a short distance beyond its tip. I see this as evidence that the shell was growing
toward the isolated crystal, which is in line with it. What controlled the direction of growth
is uncertain. Transparent, colorless sheila roughly perallel the lameliae in adjacent core
material, This means that they also parallel the outer surfaces of the core and of the "ooid' as
a whole. Possibly more or less evenly spaced zones of tension developed in core material of
84
whic4 the "oojd" sees originally to have been entirely Composed. If these zones of tension were
due to shrinksge as a result of etier cooling or dehydration it is to be expected that they
would parallel the outer surface of the "ooid."Many of the colorless, transparent shells show no pseudOmorphs of antecedent crystals and
were preeusably amorphous or sumnicroscopically crystalline from the start. some ehells of thistype are clearly composites madeup of many very thin shells in contact with each other. Thismay be the result of deposition in a crack, which opened, was filled, and then reopened severaltines.
I am doubtful about the idea, embraced by other workers, that the oblateness of the
"ooids" in the Neda formation is due to compreçsion under the load of overlying sediments. The
soft ore contains cylindricalstructures a few cm in diameter which appear to be masses of
material, mud—like in consistency, which rolled down an inclined depositioflal surface, picking up
additional material as they progressed. "Ooida" in these structures are flattened tangentially.
indicating that the flattening took place before overlying sediments were deposited. If so. the
most likely cause of the flattening would be rapid rotation while the "ooids" were still soft and
L. airborne.Kimberlites and kimberlitic ejects contain spheroidal bodies 1-10 mm in diameter known as
"pelletal lspilli." In these, some kind of a nucleus—Usually a sineral grain——is surrounded by
very fine—grained. optically unresolvable, material. The latter often exhibits a poorly defined
concentric structure and contains platy or lath—shaped sinerals with their long dimensions
parallel to the concentric layering, if this is present; otherwise parallel to the surface of the
lapillus. clement (3) originally explained these bodies as quenched lava droplets. This does
not eccouflt for their concentric structure, however. i attribute the concentric structure in
core material of Neda "ooids' to accretion, clement (4) later suggested that accretion may also
be responsible for the concentric etructure in pelletsi lapilli of kisberliteS. it is not clear
whether the rapid rotation postulated to account for the oblateness of Neda ooids and also
numerous pellets1 lepilli was in progress during accretion or was somehow initieted after
accretion had ended, -
In comparing certain features of Neda "ooids" with those of pelletsl lapllli I do not
mean to imply that the volcanic activity which, in my opinion, produced the weda formation was
L - the same as, or similar to, that which produces kimberlites. This is not an impossibility.
however. Many "ooids" in the Neda hard ore show evidence of deformation after the rlgld clear
shells had been emplaced but before core material had completely solidified. This caused
fragmentation of the clear layers and displacement of the fragments. accompanied by flowage of
core material, Possible causes of deformation are (1) shock of landing; (2) impact by a later
L. arrival after landing; (3) viscous flow of the accumulated "ooids" and matrix; (4) compression
under the load of overlying sediments, Where deformation decreased the volume of an 'ooid" core
materialwas squeezed out into the surrounding matrix,
Many workers have observed fragments of fossils, sand grains. Wd ieces of pre—existing"ooids" serving, as nuclei in Neda ooids - — these occurrences sppear to support the concept of asedimentary origin, rather than a volcanic one. I suggest, however, that a volcanic crater nearsea level may be filled, during intervals of quiescence, by sea water and that normal marinesediments containing the remains of organisms may be deposited in it. AccretionarY lapilli willfall inside, as well as outside of, the crater when en eruption occurs. Fragments of these
lapilli, the sediments, and hard parts of organisms all may be blown aloft by a subsequent
L. eruption.It does not see likely, to me, that the Neda formation was deposited under water. Ifthe fluidity of core material in 'ooids" and the matrix was due to high temperature as Ipropose, an aquatic environment of deposition is ruled out because it would have caused rapidcoaling. I think that deposition took place on land -- land which, to allow for occasionalflooding of the crster of the source volcano, was not far above sea level.
I have looked et a good deal of the matrix surrounding "ooida" in my thin sections. Itcontains many angular fragments, mostly opaque, which may be bits of volcanic glass replaced byiron oxides. Hawley and Beavan saw a lot of these, too. However, the fragments they saw were
scoriaceous; the ones I saw are not. There are other inclusions of many kinds. They are
imbedded in what appears to have been fluid or plastic material, kept so by high temperature in
my opinion. After hardening, a good deal of deuteric (?) replacement seems to have occurred,
shall not attempt further description and interpretation of the matrix, My main purpose here isto promote the theory of volcanic origin, and I think it is sufficiently promoted byconsideration of the ooids" alone.
Rr nmxN cr5
(1) Hawley. .7. E., and A. P. Besvan. 1934. "Mineralogy and Genesis of the Mayville Iron Ore of
Wisconsin. American Mineralogist. vol. 19. p. 493—514.(2) Ross. c. S., and P. F. Ierr. 1934. "Halloyaite arid Allophane." U, s. Geological Survey,
Profeapional Paper la5—G. p. 135—148,
(3) Clement. C. R. • 1973. "Icisberlites fros the Mao Pipe, L.esOtho." Lesotho Kimberlites.
p. 110—121.(4) Clement, c, IL, 1982. "A conparative Geological Study of Some Major Icilaberlite Pipes in the
Northern cape and orange Free State." Ph.D. thesis. univ. of cspe Town.
85
LITHOGEOCHEMISTRY AND GEOLOGICAL MAPPING IN THE VERMILION GREENSTONE BELT,MINNESOTA, AS AN AID TO MINERAL EXPLORATION
Reichhoff, J.A., Hauck, S.A.Natural Resources Research Institute
University of Minnesota-DuluthDuluth, MN 55811
Southwick, D.L.Minnesota Geological Survey
2642 University AvenueSt. Paul, MN 55114
The Vermilion greenstone belt in northern Minnesota is a complex tectonicassemblage of late Archean volcanic and sedimentary rocks that consists of theclassic Vermilion district, where iron ore has been mined for more than a century,and its subsurface extensions to the west beneath the glacial cover. Several cyclesof mineral exploration for commodities other than iron have occurred in the beltover the years, generally without success; perhaps the most intensive of these hasbeen in the last decade, following the general recognition that the Vermiliongreenstone belt is tectonically a part of the very productive Shebandowan-Wawa-Abitibi subprovince of the Superior province of Canada. To assist industry with thedifficult task of conducting exploration in this poorly exposed region of Minnesota,the State has conducted regional mapping projects aimed at providing the geological,geophysical, and geochemical framework required for mineral potential assessment andstrategic planning. The lithogeochemistry and mapping project described here isdesigned to provide regional data on rock compositions within the Vermiliongreenstone belt.
Recent geological mapping, supported by detailed aeromagnetic mapping andshallow drilling in poorly exposed areas, has led to the recognition of at leastthree volcanic-sedimentary cycles in the Vermilion greenstone belt. These are: (1)a dominantly calc-alkaline sequence that includes rocks south of the Mud Creek and UWolf Lake faults in the Vermilion district and southeast of the Bear River fault innortheast Itasca County; (2) a dominantly tholeiitic sequence that includes rocksnorth and northwest of these faults; and (3) a mixed volcanic sequence (calc- jalkaline and tholeiitic) of unknown stratotectonic affinity (in the "Virginia horn")that lies south of sequence (1) and is separated from it by intrusive granitic rocksof the Giants Range batholith. JApproximately 280 rock samples have been collected from these three volcanicsequences and analyzed for major elements, minor elements, and trace elements (70elements total). These data form an analytically consistent set that characterizesthe lithogeochemistry of the volcanic sequences and facilitates comparisons,correlations and petrochemical interpretations of them. The data also provide thestatistical background on the distribution of metal background concentrations, andpathfinder elemental associations in the rocks that is necessary for meaningfulinterpretation of the more than 6,200 partial rock analyses that have accumulatedover the past century from academic and exploration activities in the Vermiliongreenstone belt.
Products of the lithogeochemistry project will be: (1) a generalized regionalgeologic map of the central part of the Vermilion greenstone belt that shows samplelocations for the 280 new 70-element rock analyses; (2) a computer file (electronic)of the new analytical data; (3) a computer file (electronic) of the archivalanalytical data; and (4) a report that describes methodologies and providesinterpretations.
86
.
LITHOGEOCHEMISTRY AND GEOLOGICAL MAPPING IN THE VERMILION GREENSTONE BELT, MINNESOTA, AS AN AID TO MINERAL EXPLORATION
Reichhoff, J.A., Hauck, S.A. Natural Resources Research Institute
University of Minnesota-Duluth Duluth, MN 55811
Southwick, D.L. Minnesota Geological Survey
2642 University Avenue St. Paul, MN 55114
The Vermilion greenstone belt in northern Minnesota is a complex tectonic assemblage of late Archean volcanic and sedimentary rocks that consists of the classic Vermilion district, where iron ore has been mined for more than a century, and its subsurface extensions to the west beneath the glacial cover. Several cycles of mineral exploration for commodities other than iron have occurred in the belt over the years, generally without success; perhaps the most intensive of these has been in the last decade, following the general recognition that the Vermilion greenstone belt is tectonically a part of the very productive Shebandowan-Wawa- Abitibi subprovince o f t h e Superior province of Canada. To assist industry with the difficult task of conducting exploration in this poorly exposed region of Minnesota, the State has conducted regional mapping projects aimed at providing the geological, geophysical, and geochemical framework required for mineral potential assessment and strategic planning. The lithogeochemistry and mapping project described here is designed to provide regional data on rock compositions within the Vermilion greenstone be1 t.
Recent geological mapping, supported by detailed aeromagnetic mapping and shallow drilling in poorly exposed areas, has led to the recognition of at least three volcanic-sedimentary cycles in the Vermilion greenstone belt. These are: (1) a dominantly calc-alkaline sequence that includes rocks south of the Mud Creek and Wolf Lake faults in the Vermilion district and southeast of the Bear River fault in northeast Itasca County; (2) a dominantly tholeiitic sequence that includes rocks north and northwest of these faults; and (3) a mixed volcanic sequence (calc- alkaline and tholeiitic) of unknown stratotectonic affinity (in the "Virginia horn") that lies south of sequence (1) and is separated from it by intrusive granitic rocks of the Giants Range batholith.
Approximately 280 rock samples have been collected from these three volcanic sequences and analyzed for major elements, minor elements, and trace elements (70 elements total). These data form an analytically consistent set that characterizes the lithogeochemistry of the volcanic sequences and facilitates comparisons, correlations and petrochemical interpretations of them. The data also provide the statistical background on the distribution of metal background concentrations, and pathfinder elemental associations in the rocks that is necessary for meaningful interpretation of the more than 6,200 partial rock analyses that have accumulatec over the past century from academic and exploration activities in the Vermilior greenstone be1 t.
Products of the lithogeochemistry project will be: (1) a generalized regional geologic map of the central part of the Vermilion greenstone belt that shows sample locations for the 280 new 70-element rock analyses; (2) a computer file (electronic) of the new analytical data; (3) a computer file (electronic) of the archival analytical data; and (4) a report that describes methodologies and provide: interpretations.
THE NATURE AND SOURCE OF MIDCONTINENT RIFTIGNEOUS ROCKS
!1(ailT. Seçfrt1, ZeOT!E. fPetennan2 ant Scott tE. 'Thieben3
(Department of geological a' i4tmospfieric Sciences, Lowa State 'University, Thnes IA50011
2 fBrancfi of Isotope geology, V. S. geolbgica4TSurvey, (Denver yesleral Center, (Denver, Co80225
(Department of geologicalsciences, 'University of Texps, J.ustin, 'IX 78712
Trace element and Nd-Sr isotopic data indicate that the Keweenawan MidcontinentRift igneous rocks originated from both mantle and crustal sources with mafic rocks beingderived from the mantle and more felsic rocks being derived from the crust. The MineralLake intrusions near Mellen, Wisconsin, illustrate the occurrence of interlayed mafic andfelsic intrusions with mixed mantle and crustal origins. The bimodal source ofMidcontinent Rift igneous rocks explains the bimodal compositional distribution of theseand other flood basalt province rocks.
Nd-Sr isotopic data for Midcontinent Rift igneous rocks (Brannon, 1984; Dosso,1984; Paces and Bell, 1989) plot in three largely separate fields on an Nd-Sr(T) diagramwith T = 1100 Ma, suggesting limited mixing and contamination. The three fieldsrepresent a mafic rock mantle field surrounding Nd-Sr values of zero, and lower andupper crustal fields for more felsic rocks. The Mineral Lake mafic rocks plot in the fielddefined by other Midcontinent Rift mafic rocks indicating an origin from slightly depletedor near chondritic mantle. The Mineral Lake felsic rocks fall close to the crustal fieldsdefined by other Midcontinent Rift felsic rocks. Some mixing is indicated by the narrowisthmus connecting the mafic mantle field with the more felsic lower crustal field.
Spider diagrams of trace element data for the Mineral Lake mafic rocks relative tothe calculated field for flood basalts (Thompson et al., 1984) indicates that the maficrocks, and perhaps other Keweenawan mafic rocks, are contaminated. Consequently,the Mineral Lake mafic rocks are derived from slightly depleted mantle contaminated withcrustal material. Similar plots for the Mineral Lake felsic rocks indicates they are similarto felsic rocks with a mixed mantle and crustal origin on the basis of isotopic evidence(Thompson et al., 1984). The felsic melts were probably generated by heat fromupwelling mantle melts of the type which produced the mafic intrusion and followedchannels opened by earlier fast moving mafic magmas.
REFERENCES
Brannon, J. C. (1984) Geochemistry of successive lava flows of the Keweenawan NorthShore Volcanic Group. Ph.D. dissertation, Washington University, St. Louis.
87
THE NATURE AND SOURCE OF MIDCONTINENT RIFT IGNEOUS ROCKS
' 'Department of ~ e o l o w tV A ~ f k T V Sciences, lown State University, S^mes, Ifi 5001 1
'Branch of Isotope, (geology, 11.5. QeohgicdSurvey, 'Denw fahraf~enter, 'Denver, CO 8022.5
Trace element and Nd-Sr isotopic data indicate that the Keweenawan Midcontinent Rift igneous rocks originated from both mantle and crustal sources with mafic rocks being derived from the mantle and more felsic rocks being derived from the crust. The Mineral Lake intrusions near Mellen, Wisconsin, illustrate the occurrence of interlayed mafic and felsic intrusions with mixed mantle and crustal origins. The bimodal source of Midcontinent Rift igneous rocks explains the bimodal compositional distribution of these and other flood basalt province rocks.
Nd-Sr isotopic data for Midcontinent Rift igneous rocks (Brannon, 1984; Dosso, 1984; Paces and Ben, 1989) plot in three largely separate fields on an Nd-Sr(T) diagram with T = 1100 Ma, suggesting limited mixing and contamination. The three fields represent a mafic rock mantle field surrounding Nd-Sr values of zero, and lower and upper crustal fields for more felsic rocks. The Mineral Lake mafic rocks plot in the field defined by other Midcontinent Rift mafic rocks indicating an origin from slightly depleted or near chondritic mantle. The Mineral Lake felsic rocks fall close to the crustal fields defined by other Midcontinent Rift felsic rocks. Some mixing is indicated by the narrow isthmus connecting the mafic mantle field with the more felsic lower crustal field.
Spider diagrams of trace element data for the Mineral Lake mafic rocks relative to the calculated field for flood basalts (Thompson et at., 1984) indicates that the mafic rocks, and perhaps other Keweenawan mafic rocks, are contaminated. Consequently, the Mineral Lake mafic rocks are derived from slightly depleted mantle contaminated with crustal material. Similar plots for the Mineral Lake felsic rocks indicates they are similar to felsic rocks with a mixed mantle and crustal origin on the basis of isotopic evidence (Thompson et al., 1984). The felsic melts were probably generated by heat from upwelling mantle melts of the type which produced the mafic intrusion and followed channels opened by earlier fast moving mafic magmas.
REFERENCES
Brannon, J. C. (1 984) Geochemistry of successive lava flows of the Keweenawan North Shore Volcanic Group. Ph.D. dissertation, Washington University, St. Louis.
Dosso, L. (1984) The nature of the Precambrian subcontinental mantle. Isotopic study(strontium, lead, neodymium) of the Keweenawan volcanism of the north shore ofLake Superior. Ph.D. dissertation, University of Minnesota, Minneapolis.
Paces, J. B. and Bell, K. (1989) Non-depleted sub-continental mantle beneath theSuperior Province of the Canadian Shield: Nd-Sr isotopic and trace elementevidence from Midcontinent Rift basalts. Geoch. Cosmoch. Acta, 53, 2023-2035.
Thompson, R. N., Morrison, M. A., Hendry, 0. L and Parry, S. J. (1984) An assessmentof the relative roles of crust and mantle in magma genesis: an elemental approach.Phil. Trans. Roy. Soc. London A, 310, 549-590.
J
J
jIj
-A
-A
-A
88
-j
Dosso, L. (1 984) The nature of the Precambrian subcontinental mantle. Isotopic study (strontium, lead, neodymium) of the Keweenawan volcanism of the north shore of Lake Superior. Ph.D. dissertation, University of Minnesota, Minneapolis.
Paces, J. B. and Bell, K. (1989) Non-depleted sub-continental mantle beneath the Superior Province of the Canadian Shield: Nd-Sr isotopic and trace element evidence from Midcontinent Rift basalts. Geoch. Cosmoch. Acta, 53, 2023-2035.
Thompson, R. N., Morrison, M. A,, Hendry, G. L. and Parry, S. J. (1984) An assessment of the relative roles of crust and mantle in magma genesis: an elemental approach. Phil. Trans. Roy. Soc. London A, 310, 549-590.
CORRELATION OF IGNEOUS UNITSAT THE MINNAMAX DEPOSIT, NE MINNESOTA
Mark J. SeversonSteven A. Hauck
Natural Resources Research Inst.University of Minnesota, Duluth
The Minnamax Cu-Ni Deposit (Babbitt Deposit) is situated within what hasbeen informally referred to as the Partridge River intrusion of the DuluthComplex (1.1 Ga), northeastern Minnesota (Figure 1). Within the deposit are awide variety of troctolitic, ultramafic, and footwall rock types, and hornfelsedinclusions (both sedimentary and igneous(fl). Many specific rock types arecorrelative between drill holes and can grossly be categorized into sevensubhorizontal troctolitic units, three types of hornfelsed inclusions, and a latecross-cutting pegmatitic phase. Also present are correlative units within thefootwall rocks. These correlative rock units were identified by detailedrelogging of 61 drill holes (117,605' of core) and are portrayed on nine cross-sections through various portions of the deposit.
Most of the rock units defined at the Dunka Road Cu-Ni Deposit (located tothe immediate SW of Minnamax) by Severson and Hauck (1990) are present atMinnamax. However, the overall picture at Minnamax is somewhat more complicatedthan Dunka Road due to rock type changes that are manifest by: 1) pinch-out andreappearance of specific marker-bed units, 2) down-strike gradational changes ofultramafic horizons, 3) extremely limited areal extent of some ultramafichorizons, and 4) gradational changes in the troctolitic rock types between anytwo drill holes. In some areas a particular marker horizon may "disappear" andbe replaced by another marker horizon, which in turn, may "disappear" laterally.In spite of these local difficulties, a gross stratigraphy of seven horizontaligneous units (Figure 2) are present at Minnamax and consist of (from bottom totop): Unit I - heterogeneous, sulfide-bearing augite troctolite and troctolitewith abundant sedimentary inclusions; Unit II - homogeneous troctolites with abasal picrite horizon (II is present only in SW Minnamax); Unit III - mottledanorthositic troctolite to augite troctolite with characteristic olivineoikocrysts (III is present mainly in SW Minnamax and is enveloped by Unit I tothe NE); Unit IV - mixed troctolite and augite troctolite (at the base or top ofUnit IV in two different areas) with a basal ultramafic horizon ("± picrite");Unit V - homogeneous anorthositic troctolite (gradational contact with Unit IV);and Units VI and VII - homogeneous troctolites with basal ultramafic horizons(more abundant and thicker ultramafic horizons in the Bathtub cross-section).Specific marker horizons utilized in drill hole correlations include (Figure 2):Unit III, "± picrite", "pocket picrite", top of Unit IV (augite troctolite), andthe base of Units VI and VII. The troctolitic rock units are commonly cut bythin granophyric veins, hybrid hornblendite bodies, and pegmatitic pyroxenitebodies (OUI -oxide ultramafic intrusions). Chlorine-rich drops which coat thecore surface are common in the OUI and ultramafic horizons.
Several enigmatic hornfelsed inclusions are present in Units VI and VII atMinnamax. These are grouped in two categories which include: I) granular gabbroto olivine gabbro which contain crude ovoid-shaped plagioclase-filled zones(vesicles?), and 2) granular oxide-rich olivine gabbro which also containsplagioclase-filled ovoids and modally bedded magnetite (± hercynite). The laterinclusion type megascopically resembles the material present within the ColvinCreek "Hornfels" area (Smiles SW of Minnamax). While these two inclusion typesare readily correlative between drill holes, their exact nature remains unknown.
89
CORRELATION OF IGNEOUS UNITS AT THE MINNAMAX DEPOSIT, NE MINNESOTA
Mark J. Severson Steven A. Hauck
Natura l Resources Research I n s t . U n i v e r s i t y o f Minnesota, Duluth
The Minnamax Cu-Ni Deposit (Babb i t t Deposit) i s s i t u a t e d w i t h i n what has been i n f o r m a l l y re fe r red t o as t h e Par t r i dge R iver i n t r u s i o n o f t h e Du lu th Complex (1.1 Ga), nor theas tern Minnesota (F igure 1) . W i th in t h e depos i t are a wide v a r i e t y o f t r o c t o l i t i c , u l t ramaf i c , and foo twa l l rock types, and hornfe l sed i nc lus ions (both sedimentary and igneous(?)) . Many s p e c i f i c rock types are c o r r e l a t i v e between d r i l l ho les and can gross ly be categor ized i n t o seven subhor izonta l t r o c t o l i t i c u n i t s , t h ree types o f hornfe lsed i nc lus ions , and a l a t e c r o s s - c u t t i n g pegmat i t i c phase. Also present are c o r r e l a t i v e u n i t s w i t h i n the foo twa l l rocks. These c o r r e l a t i v e rock u n i t s were i d e n t i f i e d by d e t a i l e d re logg ing o f 61 d r i l l ho les (117,605' o f core) and are por t rayed on n ine cross- sect ions through var ious p o r t i o n s o f the deposi t .
Most o f the rock u n i t s def ined a t the Dunka Road Cu-Ni Deposit ( loca ted t o the immediate SW o f Minnamax) by Severson and Hauck (1990) are present a t Minnamax. However, t h e o v e r a l l p i c t u r e a t Minnamax i s somewhat more complicated than Dunka Road due t o r o c k type changes t h a t are manifest by: 1) p inch-out and reappearance o f s p e c i f i c marker-bed u n i t s , 2 ) down-st r ike gradat iona l changes o f u l t r a m a f i c hor izons, 3 ) extremely l i m i t e d areal ex ten t of some u l t r a m a f i c horizons, and 4) g rada t i ona l changes i n t h e t r o c t o l i t i c rock types between any two d r i l l ho les. I n some areas a p a r t i c u l a r marker hor izon may "disappear" and be replaced by another marker hor izon, which i n tu rn , may "disappear" l a t e r a l l y . I n s p i t e of these l o c a l d i f f i c u l t i e s , a gross s t ra t i g raphy of seven h o r i z o n t a l igneous u n i t s (F igure 2 ) a re present a t Minnamax and c o n s i s t o f ( f rom bottom t o top ) : U n i t I - heterogeneous, su l f i de -bea r ing aug i te t r o c t o l i t e and t r o c t o l i t e w i t h abundant sedimentary i nc lus ions ; U n i t I 1 - homogeneous t r o c t o l i t e s w i t h a basal p i c r i t e ho r i zon ( I 1 i s present on l y i n SW Minnamax); U n i t 111 - mo t t l ed a n o r t h o s i t i c t r o c t o l i t e t o aug i te t r o c t o l i t e w i t h c h a r a c t e r i s t i c o l i v i n e o i koc rys ts (111 i s present main ly i n SW Minnamax and i s enveloped by U n i t I t o the NE); U n i t I V - mixed t r o c t o l i t e and aug i te t r o c t o l i t e ( a t t h e base o r top of U n i t I V i n two d i f f e r e n t areas) w i t h a basal u l t r a m a f i c hor izon ( "Â p i c r i t e " ) ; U n i t V - homogeneous a n o r t h o s i t i c t r o c t o l i t e (gradat ional contac t w i t h U n i t IV) ; and U n i t s V I and V I I - homogeneous t r o c t o l i t e s w i t h basal u l t ramaf i c hor izons (more abundant and t h i c k e r u l t r a m a f i c horizons i n the Bathtub c ross-sec t ion) . S p e c i f i c marker hor izons u t i l i z e d i n d r i l l ho le c o r r e l a t i o n s i nc lude (F igure 2) : U n i t 111, "Â p i c r i t e " , "pocket p i c r i t e " , t o p o f U n i t I V ( aug i te t r o c t o l i t e ) , and the base o f U n i t s V I and V I I . The t r o c t o l i t i c rock u n i t s a re commonly c u t by t h i n granophyr ic veins, h y b r i d hornb lend i te bodies, and pegmat i t i c pyroxen i te bodies (OUI -ox ide u l t r a m a f i c i n t r u s i o n s ) . C h l o r i n e - r i c h drops which coat the core sur face are common i n t h e OUI and u l t r a m a f i c hor izons.
Several enigmat ic horn fe lsed i nc lus ions are present i n U n i t s V I and V I I a t Minnamax. These a r e grouped i n two ca tegor ies which inc lude: 1) g ranu la r gabbro t o o l i v i n e gabbro which con ta in crude ovoid-shaped p l a g i o c l a s e - f i l l e d zones (ves ic les?) , and 2) g ranu la r o x i d e - r i c h o l i v i n e gabbro which a l s o conta ins p l a g i o c l a s e - f i l l e d ovoids and modal ly bedded magnetite (Â he rcyn i te ) . The l a t e r i n c l u s i o n type megascopical ly resembles t h e ma te r ia l present w i t h i n t h e Co lv in Creek "Horn fe ls " area (5 m i l e s SW o f Minnamax). While these two i n c l u s i o n types are r e a d i l y c o r r e l a t i v e between d r i l l holes, t h e i r exact na ture remains unknown.
jInterestingly, modally bedded magnetite is also present in picritic horizonswhich occur at the same stratigraphic levels as the second inclusion type.
Another enigmatic rock type is present within the lower 50-100' of theVirginia Formation footwall rocks. This rock is generally concordant with theoverall bedding trend, exhibits a gradational(?) contact with the surroundingsedimentary rocks, and generally exhibits a granoblastic texture; the massivesulfide ore is restricted to the rock types above this horizon. Preliminarypetrography and geochemistry indicate that this horizon contains hornblende andolivine (serpentinized), high Cl contents (up to 1300 ppm) and high MG Numbers(10-20) which are similar to values within ultramafic horizons, and high Crcontents (800-2300 ppm) which are much higher than anything sampled within theoverlying troctolitic rocks. These date infer that this horizon may representeither a metamorphosed early Keweenawan sill (chilled material?) or a
metamorphosed mafic flow within the Virginia Formation -- further study on this jhorizon is pending.
jSeverson, M.J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy
of a portion of the Partridge River intrusion, northeastern Minnesota:Natural Resources Research Institute, Technical Report, NRRI/GMIN-TR-89-11,Duluth, Minnesota, 240 p.
11
J
jjU
jJ
J90
rIGURE 1 GENERAL CEOLIC MAP or THE PARTRIDGE RIVER ft4TRUSIDN
Interestingly, modally bedded magnetite is also present in picritic horizons which occur at the same stratigraphic levels as the second inclusion type.
Another enigmatic rock type is present within the lower 50-100' of the Virginia Formation footwall rocks. This rock is generally concordant with the overall bedding trend, exhibits a gradational(?) contact with the surrounding sedimentary rocks, and generally exhibits a granoblastic texture; the massive sulfide ore is restricted to the rock types above this horizon. Preliminary petrography and geochemistry indicate that this horizon contains hornblende and olivine (serpentinized), high C1 contents (up to 1300 ppm) and high MG Numbers (10-20) which are similar to values within ultramafic horizons, and high Cr contents (800-2300 ppm) which are much higher than anything sampled within the overlying troctolitic rocks. These date infer that this horizon may represent either a metamorphosed early Keweenawan sill (chilled material?) or a metamorphosed mafic flow within the Virginia Formation - - further study on this horizon is pending.
Severson, M. J., and Hauck, S.A., 1990, Geology, geochemistry, and stratigraphy of a portion of the Partridge River intrusion, northeastern Minnesota: Natural Resources Research Institute, Technical Report, NRRWGMIN-TR-89-11, Duluth, Minnesota, 240 p.
LEGEND PRTS- PARTRIDGE RIVER TROCTOLITE SERIES
PRGC - PARTRIDGE RIVER GABBRO COMPLEX
Cu-Nl Dewails MINNAMAX - MMAX DUNKA ROAD - DR VETLEGS - WL VYMAN CREEK - VC
- WtS tOU1) SECTION 17 - 17 LONGEAR - LE LONGNOSE - LN SECTION ZS - 22 SKU0 - SK
<? ¥
-Ã
OUTLINES AREA OF
FIGURE I- GENERAL GEOLOGIC MAP OF THE PARTRIDGE RIVER INTRUSION
in r
ri r
—r-
nr-
r IT
tr
rirt
PA
RT
RID
GE
RIV
ER
INT
RU
SIE
N
UN
ITS
'I
= S
utF
ide—
becr
ing
aug!
te -
troc
tolit
e,Ii
= tr
octo
lite
wth
p;c
rl-te
Loy
ers
etc.
,Ill
= 'M
ottle
d' o
nort
hosi
tic tr
octo
tite,
IV =
uug
rte
troc
toU
te, V
nort
host
ictr
octo
fltp,
VI 8
VII
= n
nort
hosi
tic tr
octo
Ute
to o
u9ite
troc
toU
te, O
Woi
de[3
El N
i El R
t L
I Z E
l U I
uttr
anaf
cnt
rL45
lon,
*-
n,.n
rpn-
ti,n,
Irr
'n+
n rt
H 'r
nl n
MnI
:r,4
Mn
rrk
1 Q
q
15 M
ILE
S
odd
bdd
CO
Ofl
'CF
IGU
RE
2:
Str
ike—
Leng
th c
orre
Latio
n of
naj
or Ig
neou
s un
its w
ithin
the
basa
L 30
00 F
t. of
the
Par
trid
ge R
iver
intr
usio
n.
PA
RT
RID
GE
R
IVE
R I
NT
RU
SIO
N
I 15
M
ILE
S
I
LON
GN
OS
E)
LON
GE
AR
S
EC
TIO
N V1
)
-
- FO
OTW
ALL
WYM
AN
CR
EE
K
DU
NK
A
RO
AD
M
INN
AM
AX
/BA
BB
IT
SOU
THER
N E
DG
E
LOC
AL
BOY
/ TI
GE
R
BOY
nodal
hdd
w
+ -0
"
-1-
v' v
v k
/-- -----
--
-
/
/
IV
IV
/
. - -
Ill
I I
. I
1
I I
^ - FO
OTW
ALL
FO
OTW
ALL
IÃ
IGU
RE
2:
Str
ike
-le
ng
th
co
rre
lati
on
of
ma
jor
Ign
eou
s u
nit
s
with
in
the
ba
sa
l 30
00 ft.
of
the
Pa
rtri
dg
e R
ive
r in
tru
sio
n.
'e
UN
ITSv
I
= S
ulf
lde-
beo
rln
g
au
@@
tro
cto
lite
, I1
- tro
cto
llte
wlt
h p
lcrl
te
lay
ers
etc
. Il
l =
'Mo
ttle
d'
an
ort
ho
slt
lc
tro
cto
llte
, IV
= a
uQ
lte
trO
cto
Ute
, V
=
on
ort
ho
slt
lc
tro
cto
lrte
, V
I &
V
II
= o
no
rth
os
itic
tr
oc
toll
te t
o a
uv
te t
roc
toll
te,
DUI
= o
xid
e u
~tr
nn
oti
e m
tru
sien
, G
EN
ER
ALI
ZE
D!
92
THREE-DIMENSIONAL MODELLING OF THE MAGNETIC ANOMALY -CENTRAL LAKE SUPERIOR
D. Teskey
ABSTRACT
Two and one-half dimensional modelling of the central portion of Lake Superior has been carried
out using the high resolution aeromagnetic data collected by the GSC's Oueenair aircraft in 1987
for the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLLMPCE).
Individual 2.5 dimensional models for profiles spaced approximately 10 km apart can be combined
using trigonal surfaces to produce 3 dimensional models. These models indicate up to 40 km of
volcanic flows, with the lower members having reversed magnetization, similar to the lower Osler
group. These models tend to agree with the interpretation of the seismic profiles shot in 1986 as
part of the GLIMPCE program.
j
J
J
jJ
THREE-DIMENSIONAL MODELLING OF THE MAGNETIC ANOMALY - CENTRAL LAKE SUPERIOR
D. Y
ABSTRACT
Two and one-half dimensional modelling of the central portion of Lake Superior has been carried
out using the high resolution aeromagnetic data collected by the GSC's Queenair aircraft in 1987
for the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE).
Individual 2.5 dimensional models for profiles spaced approximately 10 km apart can be combined
using trigonal surfaces to produce 3 dimensional models. These models indicate up to 40 km of
volcanic flows, with the lower members having reversed magnetization, similar to the lower Osier
group. These models tend to agree with the interpretation of the seismic profiles shot in 1986 as
part of the GLIMPCE program.
MIDCONTINENT RIFT STRUCTURE INTERPRETED FROMTHE GNI/ARCONNE SEISMIC DATA SET
M.D. Thompson & L.D. McGinnisArgonne National Laboratory
Argonne, IL 60439
M.G. Mudrey, Jr.Wis. Geol. & Nat. Hist. Sun., 3817 Mineral Point Rd.
Madison, WI 53706
C.?. ErvinDept. of Geology, Northern Illinois Univ.
DeKalb, IL 60115
Midcontinent rift structure is interpreted from 2500 km oforthogonally-oriented seismic profiles located in Lake Superiorto have evolved in the following sequence: A) Localizedvolcanic-filled basins first developed (Hinze et al., 1990) andlater coalesced to form the main axial rift basin. Volcanicflows extruded at this time are constant in thickness, suggestingthat the major component of rift subsidence began afterdevelopment of the primary axial basin. B) The main rift stageinvolved crustal extension, subsidence, and rapid lava extrusion.Lava flows interpreted as Portage Lake and Osler equivalentsexhibit a pronounced fanning geometry towards the rift's axis,indicating extrusion into a rapidly subsiding basin. C) Volcaniccessation is marked by a region-wide mixed sediment-volcanicfacies that separates a lower, dominantly volcanic sequence froman upper sedimentary series. D) Post-volcanic sedimentary basinsdeveloped, reaching thicknesses of 7 1cm, or more. E) Sedimentbasin development was interrupted by a compress ional event thatreactivated the normal boundary faults, producing high-anglereverse and thrust faults. Faulting and folding involved all ofthe volcanics and the older Oronto group sediments. F) Aregional unconformity developed and is overlain by the youngestKeweenaw-age sediments.
The geometry and location of several major faults is clearlydetailed by the seismic data set. A southeast extension of theKeweenaw Fault from the Keweenaw Peninsula is stronglycorroborated. The Douglas Fault must parallel the southernshoreline, juxtaposing its southern dip against the KeweenawFault's northward dip, and probably dies out near the PorcupineMountains. Ebinger (1989) described a similar fault geometry inthe Western Rift System of East Africa. A shear fault oraccommodation zone is imaged on the northeast side of WhiteRidge, which acts to separate a dominantly thrust fault regime tothe northeast from the high-angle reverse faults observed onWhite Ridge.
Synthesis of the structural features imaged by the seismicprofiles suggests that crustal shortening was directed NW-SE inwestern Lake Superior and NE-SW in the eastern lake. A net N-S
93
MIDCONTINENT RIFT STRUCTURE INTERPRETED FROM THE GNI/ARGONNE SEISMIC DATA SET
M.D. Thompson & L.D. McGinnis Argonne National Laboratory
Argonne, IL 60439
M.G. Mudrey, Jr. Wis. Geol. & Mat. Hist. Surv.. 3817 Mineral Point Rd.
Madison, WI 53706
C.P. Ervin Dept. of Geology, Northern Illinois Univ.
DeKalb, IL 60115
Midcontinent rift structure is interpreted from 2500 km of orthogonally-oriented seismic profiles located in Lake Superior to have evolved in the following sequence: A) Localized volcanic-filled basins first developed (Hinze et al., 1990) and later coalesced to form the main axial rift basin. Volcanic flows extruded at this time are constant in thickness, suggesting that the major component of rift subsidence began after development of the primary axial basin. B) The main rift stage involved crustal extension, subsidence, and rapid lava extrusion. Lava flows interpreted as Portage Lake and Osier equivalents exhibit a pronounced fanning geometry towards the rift's axis, indicating extrusion into a rapidly subsiding basin. C) Volcanic cessation is marked by a region-wide mixed sediment-volcanic fades that separates a lower, dominantly volcanic sequence from an upper sedimentary series. D) Post-volcanic sedimentary basins developed, reaching thicknesses of 7 km, or more. E) Sediment basin development was interrupted by a compressional event that reactivated the normal boundary faults, producing high-angle reverse and thrust faults. Faulting and folding involved all of the volcanics and the older Oronto group sediments. F) A regional unconformity developed and is overlain by the youngest Keweenaw-age sediments.
The geometry and location of several major faults is clearly detailed by the seismic data set. A southeast extension of the Keweenaw Fault from the Keweenaw Peninsula is strongly corroborated. The Douglas Fault must parallel the southern shoreline, juxtaposing its southern dip against the Keweenaw Fault's northward dip, and probably dies out near the Porcupine Mountains. Ebinger (1989) described a similar fault geometry in the Western Rift System of East Africa. A shear fault or accommodation zone is imaged on the northeast side of White Ridge, which acts to separate a dominantly thrust fault regime to the northeast from the high-angle reverse faults observed on White Ridge.
Synthesis of the structural features imaged by the seismic profiles suggests that crustal shortening was directed NW-SE in western Lake Superior and NE-SW in the eastern lake. A net N-S
-J
component of crustal shortening, as argued by Cambray (1988), Jcould explain the fault and fold geometries observed throughoutthe lake. The Keweenaw Volcanics behaved as a single "thrust"sheet in the western lake, overriding a basal decollement betweenAnimikie and pre-Portage Lake Volcanic units. Tilted, folded,and uplifted volcanic units imaged in the eastern lake alsosupport this "thrust" sheet interpretation.
REFERENCES CITED
Cambray, F.W., 1988, A Tectonic Model for the Mid-continent RiftSystem: Abstract Volume, 34th Institute on Lake Superior Geology,Marquette, MI, p. 17.
Ebinger, C.J., 1989, Geometric and Kinematic Development of JBorder Faults and Accommodation Zones, Kivu-Rusizi Rift, Africa,TECTONICS, v.8, no.1, pp. 117-134.
Hinze, W.J. , L.W. Braile, and V.W. Chandler, 1990, A Geophysical UProfile of the Southern Margin of the Midcontinent Rift System inWestern Lake Superior, TECTONICS, v.9, no.2, pp. 303-310.
J
j-3
jU
1jU
U
U
94
J
component of crustal shortening, as argued by Cambray (1988). could explain the fault and fold geometries observed throughout the lake. The Keweenaw Volcanics behaved as a single "thrust" sheet in the western lake, overriding a basal decollement between Animikie and pre-Portage Lake Volcanic units. Tilted, folded, and uplifted volcanic units imaged in the eastern lake also support this "thrust" sheet interpretation.
REFERENCES CITED
Caubray, F.W., 1988, A Tectonic Model for the Mid-continent Rift System: Abstract Volume, 34th Institute on Lake Superior Geology, Marquette, MI, p. 17.
Ebinger, C.J., 1989, Geometric and Kinematic Development of Border Faults and Accommodation Zones. Kivu-Rusizi Rift, Africa, TECTONICS, v.8, no.1, pp. 117-134.
Hinze, W.J., L.W. Braile, and V.W. Chandler, 1990, A Geophysical Profile of the Southern Margin of the Midcontinent Rift System in Western Lake Superior, TECTONICS, v.9, no.2, pp. 303-310.
REGIONAL STRATIGRAPHIC MODEL OF LATE CRETACEOUS SEDIMENTS AND THEIRRELATIONSHIP WITH THE UNDERLYING PRE-LATE CRETACEOUS WEATHERING PROFILE
ALONG THE MINNESOTA RIVER VALLEY, MINNESOTA.
Thomas A. Toth, John J. Home, and Steven A. Hauck
Natural Resources Research InstituteUniversity of Minnesota-Duluth Duluth, MN 55811
The presence of pre-Late Cretaceous residual kaolin deposits, andkaolinitic Cretaceous non-marine and marine sediments in southwesternMinnesota is well documented. Current work in the area from Redwood Falls,Mn. to Fairfax, Mn. is being conducted to understand the detailed regionalstratigraphic relationships of the pre-Late Cretaceous residuum and LateCretaceous sediments that are found on both sides of the Minnesota Rivervalley.
The residual deposits vary greatly in grade, both vertically andhorizontally, from kaolinite-rich to grus. The parent rocks for theseresidual deposits consists of granites and gneisses. A number of factorsinfluence the grade of this residual material. These factors can be groupedinto three groups: 1) composition of the parent rock, 2) chemical weatheringeffects, and 3) physical influences. Presently, some of these residualdeposits are used in the production of portland cement, and may soon be usedas filler in livestock feed.
Overlying the pre-Late Cretaceous residual deposits is a sequence ofsecondary kaolinites, composed of kaolmnitic sandstones, siltstones,mudstones and lignite. These sediments are discontinuous, commonly fillingtopographic low areas in the residual deposits. Secondary kaolinites arewhite to light brown in color, and commonly pisolitic. Near The top of somesections of secondary kaolinites, a pisolite bench of iron-cemented materialis present. In the Minnesota River valley, the secondary kaolinites appearto occur in Late Cretaceous stream channel and overbank deposits. Thesecondary kaolinitic material is presently being mined for use in themanufacture of face-brick.
Late Cretaceous sediments overlie residual and secondary deposits, andare composed of non-marine and marine shales, lignites, sandstones andsiltstones. Some of the shale units in this sequence are considered to beball clays, and are the most economically significant. The ball clays, whichare dominantly kaolinite in composition contain organic material, whichimpart a light gray to black color to the sediments. This package of LateCretaceous sediments is wide-spread in southwestern Minnesota. Currently,the ball clays are used in face-brick manufacture in Minnesota.
Pleistocene glaciation has subsequently scoured and eroded parts or allof the Late Cretaceous sequence, resulting in deposition of a thick blanketof till and outwash material of varying thickness. Glacial deposits covermuch of the study area, leaving the best exposures for study along the wallsof the Minnesota River valley and tributaries.
95
REGIONAL STRATIGRAPHIC MODEL OF LATE CRETACEOUS SEDIMENTS AND THEIR RELATIONSHIP WITH THE UNDERLYING PRE-LATE CRETACEOUS WEATHERING PROFILE
ALONG THE MINNESOTA RIVER VALLEY, MINNESOTA.
Thomas A. Toth, John J. Heine, and Steven A. Hauck
Natural Resources Research Institute University of Minnesota-Duluth Duluth, MN 55811
The presence of pre-Late Cretaceous residual kaolin deposits, and kaol ini tic Cretaceous non-marine and marine sediments in southwestern Minnesota is well documented. Current work in the area from Redwood Falls, Mn. to Fairfax, Mn. is being conducted to understand the detailed regional stratigraphic relationships of the pre-Late Cretaceous residuum and Late Cretaceous sediments that are found on both sides of the Minnesota River val 1 ey.
The residual deposits vary greatly in grade, both vertically and horizontally, from kaolinite-rich to grus. The parent rocks for these residual deposits consists of granites and gneisses. A number of factors influence the grade of this residual material. These factors can be grouped into three groups: 1) composition of the parent rock, 2) chemical weathering effects, and 3) physical influences. Presently, some of these residual deposits are used in the production of portland cement, and may soon be used as filler in livestock feed.
Overlying the pre-Late Cretaceous residual deposits is a sequence of secondary kaol inites, composed of kaol initic sandstones, siltstones, mudstones and lignite. These sediments are discontinuous, commonly filling topographic low areas in the residual deposits. Secondary kaolinites are white to light brown in color, and commonly pisolitic. Near The top of some sections of secondary kaolinites, a pisolite bench of iron-cemented material is present. In the Minnesota River valley, the secondary kaolinites appear to occur in Late Cretaceous stream channel and overbank deposits. The secondary kaolinitic material is presently being mined for use in the manufacture of face-brick.
Late Cretaceous sediments overlie residual and secondary deposits, and are composed of non-marine and marine shales, lignites, sandstones and siltstones. Some o f the shale units in this sequence are considered to be ball clays, and are the most economically significant. The ball clays, which are dominantly kaolinite in composition contain organic material, which impart a light gray to black color to the sediments. This package of Late Cretaceous sediments is wide-spread in southwestern Minnesota. Currently, the ball clays are used in face-brick manufacture in Minnesota.
Pleistocene glaciation has subsequently scoured and eroded parts or all of the Late Cretaceous sequence, resulting in deposition of a thick blanket of till and outwash material of varying thickness. Glacial deposits cover much of the study area, leaving the best exposures for study along the walls of the Minnesota River valley and tributaries.
Initial observations concerning the stratigraphic model for theMinnesota River valley between North Redwood and Fairfax include:
1. The grade of the primary kaolinite is dependent upon the typeparent material.
2. The degree of weathering of the bedrock to a primary kaolinite isinconsistent. This inconsistency is either the result of differentialremoval by glacial activity or reflects the Late Cretaceous topographyof the area.
3. The secondary kaolinite is discontinuous and variable in thicknessacross the study area.
4. There is a change in grain size and texture of the secondarykaolinite from east to west.
5. Evidence of fluvial deposition includes the quartz, kaolinitic-supported conglomerate found at the base of the secondary kaolinite.
6. Elevations of the pisolite bench are consistent along Crow Creekand can be used as a time line if the thick pisolitic bench is indeedthe result of the same laterization event.
7. The Late Cretaceous non-marine and marine shales are rarelypreserved.
8. Sedimentary structures and fossils that would provide a
stratigraphic marker are lacking.
9. Organic remains are found in the form of carbonized imprints in theshale, which form the lignitic material. These imprints are theremains of leafs and thin twigs. Tree branches and trunks formed thenucleus for siliceous concretions with the woody material sinceremoved.
These observations are being incorporated into a stratigraphic modelfor the Late Cretaceous sequence in the Minnesota River valley area. j
J
J
jJ
J96
-J
Initial observations concerning the stratigraphic model for the 1 Minnesota River valley between North Redwood and Fairfax include: .; , .>..*>., .:, .,.- : . , . I .
These observations are being incorporated into a stratigraphic model for the Late Cretaceous sequence in the Minnesota River valley area. I
1. The grade of the primary kaolinite is dependent upon the type parent material. I 2. The degree of weathering of the bedrock to a primary kaolinite is inconsistent. This inconsistency is either the result of differential removal by glacial activity or reflects the Late Cretaceous topography of the area.
I 3. The secondary kaol inite is discontinuous and variable in thickness across the study area.
I 4. There is a change in grain size and texture of the secondary kaolinite from east to west.
I 5. Evidence of fluvial deposition includes the quartz, kaol initic- supported conglomerate found at the base of the secondary kaolinite.
6. Elevations of the pisolite bench are consistent along Crow Creek and can be used as a time line if the thick pisolitic bench is indeed the result of the same laterization event. 1
7. The Late Cretaceous non-marine and marine shales are rarely preserved.
I 8. Sedimentary structures and fossils that would provide a stratigraphic marker are lacking.
9. Organic remains are found in the form of carbonized imprints in the shale, which form the lignitic material. These imprints are the remains of leafs and thin twigs. Tree branches and trunks formed the
I nucleus for siliceous concretions with the woody material since removed. I
CHEMISTRY AND METAMORPHISM OF AN ARCHEAN PILLOW BASALT
Turriock A.C.(2), Karnineni D.C. (l),MoGregor H. (1)(1) AECL Research, Whiteshell Laboratory, Pinawa, Manitoba,ROE lLO, Canada.(2) Geological Sciences, University of Manitoba, Winnipeg,R3T 2N2, Canada.
AbstractIn southeastern Manitoba, on the western edge of the Superior
Province of Archean age, the Bird River greenstone belt is asmall sub—province of supracrustal rocks. Pillow basalts are alower stratigraphic formation, approx. 2 km thick, in a sequenceof bimodal mafic and felsic volcanics and olastic meta-sedimentary rooks.
Two samples of basalt have been analysed (both core and rim),plus a sheared basalt, plus a cross—cutting dyke. Thecompositions of the pillow cores, and the sheared basalt, aresimilar to other Archean tholeiites of diverse volcanicformations, arid to modern pillow tholeiites. They have flat EElpatterns, and in Nb-Zr-V they plot as E—MORB. They are enrichedin C02 and Sr compared to rims.
The rims of the pillows are darker (abundance of hornblende)and contain corranon garnet porphyroblasts. They are enriched inFe+3, Fe, Mg, Ca, Mn compared to cores. They also have flat REEpatterns, but in Nb-Zr-V and Ti-Zr-V they plot outside the fieldof igneous rooks due to enrichment in Y.
The cross—cutting dyke is an andesite which has not beensubject to the alteration seen ira the rims of the pillow basalt.Its primary magma is distinct from that of the pillow basalt, asshown by enrichment in light REE, decreasing from La to Dy.
The formations are steeply dipping, and show post-foldingshear zones. The alteration of pillow rims is prercietarnorphic,arid did not affect the sheared basalt or the late dyke. Duringrnetamorph 1 sm, hornbl ende and cummington ite have overgrown the 52foliation (main folding event). This is interpreted asindicating that heat flow, probably from the adjacent bathoiit.hcintrusion, continued after deformation.
97
I CHEMISTRY AND METAMORPHISM OF AN ARCHEAN PILLOW BASALT
I Turriock A. C. ( 2 ) , Kamineni D. C. (1) ,McGregor R . ( I ) (1) AECL Research, Whiteshell Laboratory. Pinawa, Manitoba, ROE 1L0, Canada.
I (2 ) Geological Sciences, University of Manitoba, Winnipeg, R3T 2N2, Canada.
I Abstract In southeastern Manitoba, on t h e western edge of t h e Superior
rovince of Archean age, t h e Bird River greenstone b e l t is a
I mall sub-province of supracrus ta l rocks. Pillow b a s a l t s are a ower s t r a t i g r a p h i c formation, approx. 2 km th ick , i n a sequence
of bimodal mafic and f e l s i c volcanics and c l a s t i c meta- edimentary rocks.
I Two samples of b a s a l t have been analysed (both core and r im), l u s a sheared ba sa l t , p lus a cross-cut t ing dyke. The ompositions of t h e pi l low cores, and t h e sheared b a s a l t , are
I irnilar t o o t h e r Archean t h o l e i i t e s of d ive r se volcanic ormations, and t o modem pil low t h o l e i i t e s . They have f l a t REE a t t e r n s , and i n Nb-Zr-Y they p l o t as E-MORB. They are enriched n CO2 and S r compared t o r i m s .
I The r i m s of t h e pi l lows a r e darker (abundance of hornblende) and contain common garne t porphyroblasts. They are enriched i n
e+3, Fe, Mg, C a , Mn compared t o cores. They a l s o have f l a t REE
I a t t e r n s , but i n Nb-Zr-Y and Ti -Zr -Y they p l o t ou t s ide t h e f i e l d f igneous rocks due t o enrichment i n Y.
The cross-cut t ing dyke is an andes i te which has not been
I ubjec t t o t h e a l t e r a t i o n seen i n t h e r i m s of t h e p i l low basa l t .
I t s primary magma is d i s t i n c t from t h a t of t h e p i l low basa l t , a s shown by enrichment i n l i g h t REE, decreasing from La t o DY.
The formations a r e s t e ep ly dipping, and show post-folding
I shear zones. The a l t e r a t i o n of pi l low r i m s is premetamorphic, nd d id not a f f e c t t h e sheared ba sa l t o r t h e l a t e dyke. During ~et.amorphisrri, hornblende arid cummingtonite have overgrown t h e 52
I 01 i a t i on (main fo ld ing even t ) . This is in te rp re ted a s
indica t ing t h a t heat flow, probably from the ad ~ a c t i n t ba thc i i i t h i e in t rus ion , continued a f t e r deformatio I
Geochemistry of Archean Rocks from the Virginia Horn area:Preliminary Interpretations
James L. WelshDepartment of Geology
Gustavus Adoiphus CollegeSt. Peter, MN 56082
Jayne ReichhoffNatural Resource Research Institute
University of Minnesota, Duluth
Archean supracrustal rocks exposed in the anticlinal core of the Virginia Hornstructure of the Mesabi Iron Range of northeastern Minnesota comprise asequence of folded metavolcanic and metasedimentary units, some and perhapsall of which are tectonically stacked, and into which are intruded a series of smallgold-bearing felsic porphyry bodies. A major through-going structure, the PikeRiver fault/shear system cuts this sequence. Chemical analyses for major andminor elements have been obtained for a suite of 66 samples representing allprincipal lithologies.
Metavolcanic rocks of the area are principally flows, some pillowed, withsubordinate intercalated fragmental units. Major element analyses of these rocksshow them to range in composition from basalt to dacite, with basalt and andesitepredominating. The major and minor element data suggest that these rocks aregrouped into two distinct geochemical suites. The basaltic rocks show tholeiiticaffinities and have flat lIFE patterns; the andesitic and dacitic rocks have caic-alkaline affinities and are somewhat enriched in the light REF. Rocks in theGilbert and McKinley areas appear to be tholeiitic, while those in the Biwabik areaare calc-alkaline. The nature of the field expression of the transition betweenthese two suites is not yet clear, though the Pike River Fault may play a role.
The felsic porphyries plot in a very tight cluster around 70% Si02 and 8% Na20+1(20. These rocks would appear to be more silicic than other dacite porphyriesfrom the Vermilion District. These rocks have undergone sericite-carbonatealteration and the high 5i02 values might suggest secondary silicification. Withthe exception of occasional quartz veins, however, there appears to be littlepetrographic evidence for secondary silicification. Likewise, the consistent 5102values of the various samples would seem to rule out silica introduction. Perhapsthe higher Si02 values, as occur in these intrusives, might be a clue as to whythese bodies are auriferous, while similar-appearing dacitic bodies with lower5i02 contents elsewhere in the Vermilion District are not.
Greywackes range from 55-70% Si02, the majority plotting as "dacites",suggesting possible derivation from the calc-alkaline portions of the volcanic pile.The composition of these greywackes is consistent with greywackes throughoutthe Vermilion District.
98
Geochemistry of Arehean Rocks from the Virginia Horn area: Preliminary Interpretations
James L. Welsh Department of Geology
Gustavus Adolphus College St. Peter, MN 56082
Jape Reichhoff Natural Resource Research Institute
University of Minnesota, Duluth
Archean supracrustal rocks exposed in the anticlinal core of the Virginia Horn structure of the Mesabi Iron Range of northeastern Minnesota comprise a sequence of folded metavolcanic and metasedimentary units, some and perhaps all of which are tectonically stacked, and into which are intruded a series of small gold-bearing felsic porphyry bodies. A major through-going structure, the Pike River faultlshear system cuts this sequence. Chemical analyses for major and minor elements have been obtained for a suite of 66 samples representing all principal lithologies.
Metavolcanic rocks of the area are principally flows, some pillowed, with subordinate intercalated fragmental units. Major element analyses of these rocks show them to range in composition from basalt to dacite, with basalt and andesite predominating. The major and minor element data suggest that these rocks are grouped into two distinct geochemical suites. The basaltic rocks show tholeiitic affinities and have flat REE patterns; the andesitic and dacitic rocks have calc- alkaline affinities and are somewhat enriched in the light REE. Rocks in the Gilbert and McKinley areas appear to be tholeiitic, while those in the Biwabik area are calc-alkaline. The nature of the Held expression of the transition between these two suites is not yet clear, though the Pike River Fault may play a role.
The felsic porphyries plot in a very tight cluster around 70% Si02 and 8% Na20 + K20. These rocks would appear to be more silicic than other dacite porphyries from the Vermilion District. These rocks have undergone sericite-carbonate alteration and the high Si02 values might suggest secondary silicification. With the exception of occasional quartz veins, however, there appears to be little petrographic evidence for secondary silicification. Likewise, the consistent S102 values of the various samples would seem to rule out silica introduction. Perhaps the higher Si02 values, as occur in these intrusives, might be a clue as to why these bodies are auriferous, while similar-appearing dacitic bodies with lower Si02 contents elsewhere in the Vermilion District are not.
Greywackes range from 55-70% Si02, the majority plotting as "dacitesn, suggesting possible derivation from the calc-alkaline portions of the volcanic pile. The composition of these greywackes is consistent with greywackes throughout the Vermilion District.
ARCHEAN AND PROTEROZOIC TECTONO-MAGMATICACTIVITY ALONG ThE SOUTHERN MARGIN OF THE
SUPERIOR PROVINCE IN NORTHWESTERN IOWA, USA
Kenneth B. Windom', W. R. Van Schmus2, Karl B. SeiferO,B. T. Wallin2, and R. R. Anderson3
'Department of GeolQgical and Atmospheric Sciences, Iowa State University, Ames, IA 500102Department of Geology, University of Kansas, Lawrence, KS 660463Geological Survey Bureau, Iowa Department of Natural Resources, Iowa City, IA 52242
A Precambrian igneous complex of layered ultramafic and mafic rocks occurs within thebasement of northwestern Iowa beneath approximately 300 meters of Phanerozoic sediments.This layered series has been named the Otter Creek complex. It is marked by a circularmagnetic anomaly, one of several that lie north and west of an inferred suture that has beenpostulated as the boundary between the Archean Superior Province and an Early Proterozoicterrane. The layered series is tilted steeply to the northwest; rocks representing the upperportion of the original magma chamber have been removed by erosion or tectonic processes.A block of banded iron formation, itself intruded by lamprophyre dikes, is contained within thelayered sequence. The iron formation/lamprophyre block has undergone high-temperaturemetamorphism followed by a retrograde event.
The banded iron formation was deposited in a supracrustal environment, then intrudedby the lamprophyre. This assemblage was apparently incorporated into a mafic magma thatsubsequently crystallized to form the layered complex. Radiometric dating of the layered rocksand the lamprophyre, using Nd/Sm isotopes, yields an isochron of approximately 2.9 Ga and an
value of zero to slightly negative values. The lamprophyre and layered complex fall on thesame isochron and thus appear to be essentially coeval. Archean lamprophyre has been reportedfrom the Superior Province as having been formed during the closing stages of the varioussubprovinces. Trace element abundances in both the lamprophyre and layered complex areconsistent with derivation from a primitive to partially depleted mantle and thus support theisotopic evidence.
The entire layered complex has undergone low-temperature alteration, similar to that seenin other mafic rocks from Archean greenstone belts. We interpret the high-temperaturemetamorphism experienced by the iron formation and lamprophyre to have occurred duringincorporation into the mafic magma and the retrograde metamorphism of these lithologies asresulting from the same event that produced prograde paragenesis of low-temperature mineralsin the layered complex. Although most primary phases were replaced with secondary phasessuch as serpentine, chlorite, uralitic amphibole, epidote, and albite, major chemical redistributiondoes not appear to have affected these rocks
A younger Archean quartz monzodiorite gneiss occurs near the layered complex, but thecontact relations are not known. The magma from which the monzodiorite originally crystallized
99
ARCHEAN AND PROTEROZOIC TECTONO-MAGMATIC ACTIVITY ALONG THE SOUTHERN MARGIN OF THE
SUPERIOR PROVINCE IN NORTHWESTERN IOWA, USA
Kenneth E. Windom', W. R. Van Schmus2, Karl E. Seifert', E. T. Wallin2, and R. R. Anderson3
. .24 'Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50010 . 'Department of Geology, University of Kansas, Lawrence, KS 66046
'Geological Survey Bureau, Iowa Department of Natural Resources, Iowa City, IA 52242
A Precambrian igneous complex of layered ultramafic and mafic rocks occurs within the basement of northwestern Iowa beneath approximately 300 meters of Phanerowic sediments. This layered series has been named the Otter Creek complex. It is marked by a circular magnetic anomaly, one of several that lie north and west of an inferred suture that has been postulated as the boundary between the Archean Superior Province and an Early Proterozoic terrane. The layered series is tilted steeply to the northwest; rocks representing the upper portion of the original magma chamber have been removed by erosion or tectonic processes. A block of banded iron formation, itself intruded by lamprophyre dikes, is contained within the layered sequence. The iron fonnation/lamprophyre block has igh-temperature metamorphism followed by a retrograde event.
The banded iron formation was deposited in a supracrustal environment, then intruded by the lamprophyre. This assemblage was apparently incorporated into a mafic magma that subsequently crystallized to form the layered complex. Radiometric dating of the layered rocks and the lamprophyre, using NdISm isotopes, yields an isochron of approximately 2.9 Ga and an eNd value of zero to slightly negative values. The lamprophyre and layered complex fall on the same isochron and thus appear to be essentially coeval. Archean lamprophyre has been reported from the Superior Province as having been formed during the closing stages of the various subprovinces. Trace element abundances in both the lamprophyre and layered complex are consistent with derivation from a primitive to partially depleted mantle and thus support the isotopic evidence.
The entire layered complex has undergone low-temperature alteration, similar to that seen in other mafic rocks from Archean greenstone belts. We interpret the high-temperature metamorphism experienced by the iron formation and lamprophyre to have occurred during incorporation into the mafic magma and the retrograde metamorphism of these lithologies as resulting from the same event that produced prograde paragenesis of low-temperature minerals in the layered complex. Although most primary phases were replaced with secondary phases such as serpentine, chlorite, uralitic amphibole, epidote, and albite, major c istribution does not appear to have affected these rocks
A younger Archean quartz monzodiorite gneiss occurs near the layered complex, but the contact relations are not known. The magma from which the monzodiorite originally crystallized
formed at approximately 2523 Ma (Van Schmus et al., 1987). A mantle separation age of 2.9Ga has been determined for this rock, indicating that it could have formed by partial melting ofmaterial derived from the same mantle rocks that produced the Otter Creek layered complex.Tectonic uplift tilted the layered igneous body and the quartz monzodiorite and exposed themto erosion.
Felsic volcanism occurred at approximately 1775 Ma (Wallin and Van Schmus, 1988)and covered the exposed rocks. These volcanics yield a depleted mantle separation age of 2.7Ga. They are similar in age and stratigraphic position to rhyolites found in central Wisconsin(Smith, 1978; Van Schmus, 1978), but have been more strongly altered than the Wisconsinrhyolites, being depleted in LILE which resulted in a composition equivalent to keratophyre.These volcanics may have formed in the waning stages of the 1850-1900 Ma Penokean Orogenyor they may be related to the proposed 1750 Ma Central Plains Orogeny (Bickford et al., 1986)
Analysis of the combined data indicates the presence of primitive to slightly depletedmantle material to the southern margin of the Superior Craton at approximately 2.9 Ga. Varyingdegrees of partial melting of this mantle produced lamprophyric dikes and contemporaneousmafic magma bodies, including one which crystallized to form the Otter Creek layered complex.The entire complex subsequently underwent low-temperature alteration, resulting mainly in theformation of hydrous minerals. Subsequent tectono-magmatic activity involving partial meltingof rocks derived from the 2.9 Ga mantle source occurred in this region at approximately 2.5 Gaand 1.75 Ga resulting in a quartz monzodiorite piutonic rock and felsic volcanics, respectively.
REFERENCES
Bickford, M. E., Van Schmus, W. It, and Zeitz, 1986. Proterozoic history of the midcontinentregion of North America. Geology, 14:492-496.
Smith, E. I. 1978. Precambrian rhyolites and granites in south-central Wisconsin: Fieldrelations and geochemistry. Geological Society of America bulletin, 89:875-890.
Van Schmus, W. R. 1978. Geochronology of the southern Wisconsin rhyolites and granites.Geosciences Wisconsin, 2:19-24.
Van Schmus, W. R., Bickford, M. E., and Zeitz, I. 1987. Early and middle Proterozoic Li
provinces in the central United States. In International Lithosphere Program. Edited byA. Kröner. American Geophysical Union Geodynamics Series, 17:43-68. j
Wallin, E. T., and Van Schmus, W. R. 1988. Geochronological studies of the Archean-Proterozoic transition, north-central United States. Geological Society of AmericaAbstracts with Programs, 20:131. —
J
J100
-4
formed at approximately 2523 Ma (Van Schmus et al., 1987). A mantle separation age of 2.9 Ga has been determined for this rock, indicating that it could have formed by partial melting of material derived from the same mantle rocks that produced the Otter Creek layered complex. Tectonic uplift tilted the layered igneous body and the quartz monzodiorite and exposed them to erosion.
Fdsic volcanism occurred at approximately 1775 Ma (Wallin and Van Schmus. 1988) and covered the exposed rocks. These volcanics yield a depleted mantle separation age of 2.7 Ga. They are similar in age and stratigraphic position to rhyolites found in central Wisconsin (Smith, 1978; Van Schmus, 1978), but have been more strongly altered than the Wisconsin rhyolites, being depleted in LILE which resulted in a composition equivalent to keratophyre. These volcanics may have formed in the waning stages of the 1850-1900 Ma Penokean Orogeny or they may be related to the proposed 1750 Ma Central Plains Orogeny (Bickford et al., 1986)
Analysis of the combined data indicates the presence of primitive to slightly depleted mantle material to the southern margin of the Superior Craton at approximately 2.9 Ga. Varying degrees of partial melting of this mantle produced lamprophyric dikes and contemporaneous mafic magma bodies, including one which crystallized to form the Otter Creek layered complex. The entire complex subsequently underwent low-temperature alteration, resulting mainly in the formation of hydrous minerals. Subsequent tectono-magmatic activity involving partial melting of rocks derived from the 2.9 Ga mantle source occurred in this region at approximately 2.5 Ga and 1.75 Ga resulting in a quartz monzodiorite plutonic rock and felsic volcanics, respectively.
REFERENCES
=&ford, M. E., Van Schmus, W. R., and Zeitz, 1986. Proterozoic history of the midcontinent region of North America. Geology, 14:492-496.
Smith, E. I. 1978. Precambrian rhyolites and granites in south-central Wisconsin: Field relations and geochemistry. Geological Society of America bulletin, 89:875-890.
Van Schmus, W. R. 1978. Geochronology of the southern Wisconsin rhyolites and granites. Geosciences Wisconsin, 2: 19-24.
Van Schmus, W. R., Bickford, M. E., and Zeitz, I. 1987. Early and middle Proterozoic provinces in the central United States. In International Lithosphere Program. Edited by A. Kroner. American Geophysical Union Geodynamics Series, 17:43-68.
, . Wallin, E. T., and Van Schmus, W. R. 1988. Geochronological studies of the Archean-
Proterozoic transition, north-central Un , , .. " ~ b s t r a c t s with Programs, 20: 131.
STRUCTURAL ANALYSIS OF THE MIDCONTINENT RIFT BASED ONSLICKENSIDE ANALYSIS
Kathleen Witthuhn, University of Minnesota
ABSTRACT
The Midcontinent Rift is a 1.1 billion year feature which has beendefined primarily by geophysical studies. Many studies have ex-amined the rift from a petrologic view but few have examined thestructural geology in any detail. This study examines the structuralaspects of the rift, elucidates the directions of opening and closing,and constrains the timing of the system, by utilizing paleostressstratigraphy. Preliminary field work in Minnesota, Canada, and theUpper Penninsula of Michigan has yielded data on fault and jointpopulations, from which stress tensors were calculated. Classifi-cation of faults and establishment of the sense of movement wasaccomplished by analyzing slickensides, recrystallization on the faultplane, and Reidel fractures as primary movement indicators. Faultswith opposite sense of movement (dextral vs. sinistral, normal vs.reverse) have similar attitudes indicating a 1800 reversal of stressdirection. This study proposes the direction of opening and closing ofthe rift was constrained by the geometry of the fault systems and bysimUar directions of the far field stresses.
101
I STRUCTURAL ANALYSIS OF THE MIDCONTINENT RIFT BASED ON SLICKENSIDE ANALYSIS
Kathleen Witthuhn, University of Minnesota
I I ABSTRACT . . ,.,.,.
,,^i ,a& Midcdri*l"e"* Rif
I 1 billion year feature which has been ned primarily by geophysical studies. Many studies have ex-
amined the rift from a petrologic view but few have examined the
I . .: structural geology in any detail. This study examines the structural aspects of the rift, elucidates the directions of opening and closing, and constrains the timing of the system, by utilizing paleostress stratigraphy. Preliminary field work in Minnesota, Canada, and the Upper Penninsula of Michigan has yielded data on fault and joint populations from which stress tensors were calculated. Classifi- cation of faults and establishment of the sense of movement was accomplished by analyzing slickensides, recrystallization on the fault plane, and Reidel fractures as primary movement indicators. Faults with opposite sense of movement (dextral vs. sinistral, normal vs. . . reverse) have similar attitudes indicating a 180Â reversal of stress
I. direction. This study proposes the direction of opening and closing of the rift was constrained by the geometry of the fault systems and by similar directions of the far field stre
I I I I I I
jA STUDY OF THEMATIC MAPPER LINEAMENTS IN NORTHWEST NEVADA
Woodzick, Thomas L., Orbital Technologies Corporation, Madison, WI JMurdock, Gary P., Battelle Memorial Institute, Columbus, OHPride, Douglas E., The Ohio State University, Columbus, OH
JAnalysts of the 1970's were able to identify major linear patterns (>250km long) such J
as the Midas Trench and Rye Patch lineaments in the area between Reno and Winnemucca
on single-band Landsat-] images and relate them to the geomorphology and structure of the
Basin and Range province. Multiband Thematic Mapper (TM) data with an improved
(30m) resolution provide an opportunity to re-examine the area in terms of the spatial
distribution of its numerous shorter linear features, 68,000 lineament segments were initially
interpreted (primarily from the deeply incised and shadowed ranges) on a 1:100,000 scale —
band-234 TM image. Subsequent reprocessing of the data with an intensity-hue-saturation
(IHS) transformation and edge-enhancement enabled another 36,000 lineament segments
to be interpreted from the topographically subdued intervening basins. Digitization of the
x-y coordinates of the lineament endpoints was accomplished at a digitizing tablet, using a
puck with a head-tail audio prompt and a domain-clearing technique to accommodate jlineament clusters.
Once digitized, the TM lineament set became amenable to visualization,
manipulation, and analysis as well as coregistration and integration with other spatially
addressable data sets such as aeromagnetics, known mining activity, and Digital Elevation
Model (DEM) data- -- via processing with a geographic information system (GIS). The
results of such treatments with the PANACEA 015 have been the focus of presentations by
Murdock, Pride, Woodzick and others at meetings in both Canada and the United States.
Selected examples from this body of work (e.g., lineament intersection enhancement and
comparison of TM/DEM lineament sets) will be discussed with respect to their significance
for mineral exploration and in light of the advanced capabilities offered by the next
generation of orbital monitoring systems such as RADARSAT and Eos.
102
A STUDY OF THEMATIC MAPPER LINEAMENTS IN NORTHWEST NEVADA
Woodzick, Thomas L., Orbital Technologies Corporation, Madison, WI Murdock, Gary P., Battelle Memorial Institute, Columbus, OH Pride, Douglas E., The Ohio State University, Columbus, OH
Analysts of the 1970's were able to identify major linear patterns (>250km long) such
as the Midas Trench and Rye Patch lineaments in the area between Reno and Winnemucca
on single-band Landsat-1 images and relate them to the geomorphology and structure of the
Basin and Range province. Multiband Thematic Mapper (TM) data with an improved
(30m) resolution provide an opportunity to re-examine the area in terms of the spatial
distribution of its numerous shorter linear features. 68,000 lineament segments were initially
interpreted (primarily from the deeply incised and shadowed ranges) on a 1:100,000 scale
band-234 TM image. Subsequent reprocessing of the data with an intensity-hue-saturation
(IHS) transformation and edge-enhancement enabled another 36,000 lineament segments
to be interpreted from the topographically subdued intervening basins. Digitization of the
x-y coordinates of the lineament endpoints was accomplished at a digitizing tablet, using a
puck with a head-tail audio prompt and a domain-clearing technique to accommodate
lineament clusters.
Once digitized, the TM lineament set became amenable to visualization,
manipulation, and analysis as well as coregistration and integration with other spatially
addressable data sets such as aeromagnetics, known mining activity, and Digital Elevation
Model (DEM) data - via processing with a geographic information system (GIs). The
results of such treatments with the PANACEA GIs have been the focus of presentations by
Murdock, Pride, Woodzick and others at meetings in both Canada and the United States.
Selected examples from this body of work (e.g., lineament intersection enhancement and
comparison of TM/DEM lineament sets) will be discussed with respect to their significance
for mineral exploration and in light of the advanced capabilities offered by the next
generation of orbital monitoring systems such as RADARSAT and Eos.
GEOSTATISTICAL AND 615 EVALUATION OF BIOGEOCHEMICAL AND ECOLOGICAL DATA FROMTHREE MINERALIZED SITES (Au & Cu-Ni-PGE), NORTHEASTERN MINNESOTA:IMPLICATIONS FOR MINERAL EXPLORATION IN A BOREAL FOREST
L. Zanko, A. Gokee, B. Dewey, S. Hauck, and J. Pastor
Natural Resources Research InstituteUniversity of Minnesota, Duluth
Duluth, Minnesota 55811
Detailed biogeochemical and ecological studies of vegetation wereconducted in the boreal forest of northeastern Minnesota. These studieswere implemented over three mineralized sites:
1) Raspberry Prospect: Archean (2.7 Ga) gold mineralization in theShagawa Shear Zone (SSZ), Vermilion greenstone belt, 5 miles west ofEly, Minnesota;
2) Spruce Road (SR): basal Cu-Ni mineralization of the Duluth Complex;
3) South Filson Creek (SFC): "cloud zone" Cu-Ni-PGE mineralization ofthe Duluth Complex.
The two Keweenawan age (1.1 Ga) Duluth Complex sites are 15 miles southeastof Ely. The SR site consists of low grade Cu-Ni sulfide mineralization atthe basal contact of the Duluth Complex, whereas the SEC site has discreet,fracture-controlled secondary Cu-Ni-PGM "cloud zone" mineralization.Glacial overburden thickness at the three sites varies from 0-40 ft.
Over 1300 vegetation samples were collected from 17 plant speciesduring 1989 and iggo at the three sites. Sampling took place over two weeksin late August and early September of both years. Tissue samples (1100)submitted for multi-element INAA and DCP analysis included leaves, twigs,needles, and outer bark.
Analysis of 500 vegetation samples from the Raspberry site in 1989shows Abies balsamea (balsam fir) twigs, Pop'ilus tremuloides (quaking aspen)leaves, Corylus cornuta (beaked hazel) leaves and twigs, Acer spicatum(mountain maple) twigs, and Aster macrophyllus (large-leaved aster) leavescontain anomalous gold values. Many of the gold anomalies in vegetationoverlie a zone of sericite-iron carbonate alteration associated with theSSZ. A. balsamea twigs recollected in 1990 verify the gold anomaly.
At the two Cu-Ni sites, 600 tissue samples from A. balsamea (twigs),Picea marl ana (black spruce - twigs and bark), Pinus banksiana (jack pine -bark), Ledunigroenlandicum (labrador tea - twigs), Chan,aedaphne calyculata(leatherleaf - twigs), Alnus crispa (green alder - twigs and leaves), Alnusrugosa (speckled alder - twigs and leaves), and A. macrophyllQs (leaves)were analyzed for base and precious metals. Anomalous levels of Ni, Co, Au,and Pd were found in A. balsamea (twigs), P. mariarza (twigs), L.
groenlandicum (twigs), and A. inacraphyllus (leaves). These anomalies appearto correspond closely to the bedrock mineralization.
Bacillus cereus spore counts and other measures of microbial activityin soils were used as bioindicators of natural metal enrichments. S. cereuscounts, total carbon and nitrogen, nitrogen mineralization, and pH analyseswere done on the A horizon of soils collected at 200 ft. intervals acrosseach site. B. cereus counts ranged from 1,000 to 576,000 spores/gm soil,
103
GEOSTATISTICAL AND GIS EVALUATION OF BIOGEOCHEMICAL AND ECOLOGICAL DATA FROM THREE MINERALIZED SITES (Au & Cu-Ni-PGE), NORTHEASTERN MINNESOTA: IMPLICATIONS FOR MINERAL EXPLORATION IN A BOREAL FOREST
L. Zanko, A. Gokee, B. Dewey, S. Hauck, and J. Pastor
Natural Resources Research Institute University of Minnesota, Duluth
Duluth, Minnesota 55811
Detailed biogeochemical and ecological studies of vegetation were conducted in the boreal forest of northeastern Minnesota. These studies were implemented over three mineralized sites:
1) Rasoberrv Prosoect: Archean (2.7 Ga) gold mineralization in the Shagawa Shear Zone (SSZ), Vermilion greenstone belt, 5 miles west of Ely, Minnesota;
2) Spruce Road (SRI: basal Cu-Ni mineralization of the Duluth Complex;
3 ) South Filson Creek fSFC1: "cloud zone" Cu-Ni-PGE mineralization of the Duluth Complex.
The two Keweenawan age (1.1 Ga) Duluth Complex sites are 15 miles southeast of Ely. The SR site consists of low grade Cu-Ni sulfide mineralization at the basal contact of the Duluth Complex, whereas the SFC site has discreet, fracture-controlled secondary Cu-Ni-PGM "cloud zone" mineralization. Glacial overburden thickness at the three sites varies from 0-40 ft.
Over 1300 vegetation samples were collected from 17 plant species during 1989 and 1990 at the three sites. Sampling took place over two weeks in late August and early September of both years. Tissue samples (1100) submitted for multi-element INAA and DCP analysis included leaves, twigs, needles, and outer bark.
Analysis of 500 vegetation samples from the Raspberry site in 1989 shows Abies balsamea (balsam fir) twigs, Popu7us tremuloides (quaking aspen) leaves, Cory7us cornuta (beaked hazel) leaves and twigs, Acer spicatum (mountain maple) twigs, and Aster macrophy7lus (large-leaved aster) leaves contain anomalous gold values. Many of the gold anomalies in vegetation overlie a zone of sericite-iron carbonate alteration associated with the SSZ. A. balsamea twigs recollected in 1990 verify the gold anomaly.
At the two Cu-Ni sites, 600 tissue samples from A. balsamea (twigs), Picea mariana (black spruce - twigs and bark), Pinus banksiana (jack pine - bark), Ledum groenlandicum (labrador tea - twi'gs), Chamaedaphne ca7ycu7ata (leatherleaf - twigs), Ainus crispa (green alder - twigs and leaves), Alnus rugosa (speckled alder - twigs and leaves), and A. macrophy77us (leaves) were analyzed for base and precious metals. Anomalous levels of Ni, Co, Au, and Pd were found in A. balsamea (twigs), P . mariana (twigs), L. groenlandicum (twigs), and A. macrophyllus (leaves). These anomalies appear to correspond closely to the bedrock mineralization.
Bacillus cereus spore counts and other measures of microbial activity in soils were used as bioindicators of natural metal enrichments. 6. cereus counts, total carbon and nitrogen, nitrogen mineralization, and pH analyses were done on the A horizon of soils collected at 200 ft. intervals across- each site. 6. cereus counts ranged from 1,000 to 576,000 spores/gm soil,
matching spatial variation of soil metal anomalies. Initial results indicatethat B. cereus counts can be an effective indicator of concealed ore bodies
Uin northern Minnesota. The effectiveness of other soil properties asbiogeochemical prospecting tools is being evaluated.
Ecological characterization consisted of identification andquantification of tree, shrub, and herbal species present within 550 treeand herb plots and 1100 shrub plots. Geostatistical interpretation of thetree, shrub, and herb data will be used to map the various plant communitiesat each site as well as the spatial distribution of metals. A geographicalinformation system (GIS) will be used to determine what spatialrelationships exist between the geochemical, geological, ecological, andgeographical data. The ecological, nutrient uptake, geostatistical, and GISstudies are being used to better characterize and identify variables thataffect metal uptake and concentration by boreal forest vegetation overmetallic mineralization.
jJ
J
J
jU
Li
104 J
J
..%.
matching spatial variation of soil metal anomalies. Initial results indieate that 8. cereus counts can be an effective indicator of concealed ore bodies in northern Minnesota. The effectiveness of other soil properties as biogeochemical prospecting tools is being evaluated.
Ecological characterization consisted of identification and