robert h. moench and avery ala drake, jr

8/3/2019 Robert H. Moench and Avery Ala Drake, Jr

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Economic Geology of theIdaho Springs DistrictClear Creek andGilpin Counties, ColoradoGEOLOGICAL SURVEY BULLETIN 1208Prepared on behalf of th eU.S. Atomic Energy Commission

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* Economic Geology of theIdaho Springs DistrictClear Creek and- i; i? i Gilpin Counties, ColoradoBy ROBERT H. MOENCH an d AVERY AL A DRAKE, JR.

GEOLOGICAL SURVEY BULLETIN 1208

Prepared on behalf of theU.S. Atomic Energy Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1966


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WUTED STATES DEPARTMENT OF THE INTERIORST E W A R T L . U D A L L , Secretary

GEOLOGICAL SURVEYWilliam T. Pecora, Director

Library of Congress catalog-card N o. GS 65-368

For sale by the Superintendent of Documents, U.S. Government Printing OfficeWashington, D.C. 20402


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CONTENTSPageAbstract_ - _______-_________-_-_--________--___-__-________-_'-_.-. 1Introduction. _______________._________________________ _______-___ 3Geography. __________________.__._________________________ ___ 6Purpose and scope of report ___________________________________ 6

Mines and prospects in the Idaho Springs district ________________ 7Previous studies.________________________________________ __!__ 13Fieldwork____.._________.__..__._______________ ____-- 13Acknowledgments. _________________-_--__-_______-_-_-_--__--- 1 4History, production, and future__--___-__-___---_-----------_---l--- 1 4General geology._____________________________________ _ 1 ___________ 18Precambrian rocks.______________________________________i_____ 18

Gneissic rocks__________________-_-_______--_^----__-___-- 20Biotite gneiss. _____________ _:__-_____________ __ _ i-__- 20Granite gneiss an d pegmatite.-------------_-_I-__':!_---_ 2 1Microcline gneiss. _________________________L___ J.______ 2 1Quartz gneiss.____________________________L__-_-_._-__ '22Amphibolite and associated calc-silicate gneiss..____i___i__' 22Calc-silicate gneiss-___--__-_-------______-_--____------ 22- Origin of the gneissic rocks____-----__---_--__-_-----.-_ 23Granitic rocks_______.______--____--_______._-_______-.__ 23Granodiorite-__-_------____--------------------------- 24Quartz diorite_____-_--__________-____-__--__-__-___-__ 24Biotite-muscovite granite..___------_-_--_--_______----_ 24

Pegmatitic rocks._________________________________________ 24Te rtiary intrusive rocks_____-_______---__--____--_-_---_-_-_--_ 25Quaternary deposits..___________________________ __________-_^_ 28Structure____________________________________________ 28Foliation and lineation_______________--____^_______________ 29

Folda..................... .... ----- _ " _ _ _ _ _____________ 30Faults ------_-_---------- .------_-_-------------.--- 33Faults of Precambrian (? ) age____-------_-______.!l___'-___--- 35Faults of Laramide age___________________________________ 36Origin of the faults___________-.-____..__.____.___._._.__ 38Joints.___________________________________________________ 39Primary igneous joints._______-___-_________-___-__---- 40Joints related to Precambrian folds______________________ 40Regional joint system_______-__-__.__._^__._-_____-.-_- 40Geologic history summarized__________________________________ 42Ore deposits.______________________________________________________ 43Mineralogy._ ________-____-_--_____-_--_--_____________-_-_--- 45Pyrite____________ __________________________________ 45Sphalerite. _________.__ -_______---_-_-____________'___-_--- 47Galena. __________________________ ______________ __________ 48Chalcopyrite.____________.__:_____________________________ 48

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IV CO N T E N T SOre deposits ContinuedMineralogy Continued Page

Marcasite_ _____-_____--_---__-_-_____-_______-_-_________ 48Bornite_ _-___________-___-____-___-_--______-_-___--__-_ 48Covellite________________________..________ 48Chalcocite... -.........-.-.-.......--.-......-.__________ 48Argentite.________________ ____1___________________________ 49Tennantite.____-__-_---_---_-_--_-___-______------_----_- 49Enargite_____-_______--_-____-___----__---___-__-------_ 49Pearceite.__-_-_________-__-_-_--------__---_-__-_---____- 49Polybasite.__-_--_-_-_---------------------_------_------- 50Proustite-pyrargyrite_ ____________________________________ 50Gold__-_--_-----__-___-_-------------_------------------- 50Saver............_._._ . 0Copper-___----______-_-------_-_------__--------------__- 50Pitchblende and coffinite-___-______-___.________--_____-.__ 50Hematite_-_-------_----------------_----_-----_---------- 51Hydrous iron and manganese oxides.________________________ 51Wolframite_-----_-_-__--------_-----------_--------_-.. 51Telluride minerals.______-----___--___-_______,._____-______ 52Fluorite.._________________________________ 52

; Quartz and cryptocrystalline silica.__________________________ 52Uranophane.___--_____------_---_--______-___-___________ 53Uranium phosphate minerals______________________________ 53Primary carbonate minerals_______________________________ 53Secondary carbonate minerals_______._______________________ 53Bayleyite_-__-__-_-_--_--_--__________.___________________ 54Barite I 4Anglesite.__---____-------_.._--___-___-________--__-______ 54Hydrous uranium sulfate minerals.__-____..__.____._________ 54Copper sulfate minerals____________________________________ 54Epsomite(?)_-__..____----.-_-_-__.________.____._____._ 54Classification of the veins____-_-_-__________.___________________ 54Pyrite veins ______________________________________________ 55Pyritic copper veins--------_------------_-_-__-------___ 58Pyritic lead-zinc veins_____________________________________ 60Lead-zinc veins-__________--__-__-________________-____-__ 64Local variance in the vein ores.__-____-_-___________-__-.___ 65

Paragenetic sequence of primary vein minerals ____________________ 67Pyrite stage_______________r ______________________________ 69Base-metal stage_______-___________________.______________ 7 0Zonal distribution of vein types _________________________________ 73Areal zoning______________________________________________ 73' : Depth zoning.___________---______^_______________________ 75

Relation of silver-gold ratios to zonal pattern _________________ 76O re bodies_____________-- ---__----________^_________. 77Localization ___________------______----____________-_---_- 7 8O re bodies along deflections in strike or dip of veins.._____ 79O re bodies at vein intersections _________________________ 80Ore bodies related to rock competency.._________________ 80O re bodies at the intersections of veins and fold axes_____ 81Wallrock alteration.__________________-____-____.._-__--___----- 81Supergene alteration.__________________________________________ 83


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CONTENTSO re deposits ContinuedGenesis of the veins____________Sequence of ore formation.___.

Hypogene zoning.____________Environment of ore deposition.References cited..___.____.__-____.__.

ILLUSTRATIONS

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[Plates are in pocket]PLATE 1 . Map of the Idaho Springs district, Colorado, showing locationof mines and distribution of veins and igneous rocks ofTertiary age.2. Generalized geologic map and sections of the Idaho Springs

district.3. Geologic section along the Big Five tunnel. PageFIGURE 1 . Map showing location of the Idaho Springs district._________ 42. Index map showing the Idaho Springs district and adjacentmining districts.______________________________________ 53. Map showing major veins in the Idaho Springs district---..-- 344. Stereodiagram showing average attitudes of joints in the regionaljoint system-_________________________________________ 415. Diagram showing vein classification based on quantitativemineralogy ___________________________________________ 566. Map showing distribution of different classes of veins._______ 577. Diagram showing the paragenetic sequence of the primary oreand gangue minerals.____---_______-__-____----___----_ 688. Map showing hypogene mineral zones, sample localities, andmolecular percentage of FeS in sphalerite.____---______-__ 7 4

T A B L E SPageTABLE 1 . O re produced from the Idaho Springs district, 1904-59.___-._ 172. Petrography of the Tertiary igneous rocks.--___----._______ 273. Primary and secondary ore and gangue minerals_______--__ 464. Tenor of ore from some pyrite veins.______________________ 585. Tenor of ore from some pyritic copper veins_____-_,---_____ 596. Assays of selected ore samples from the Phoenix and Donaldson

mines. _ ____________________________________________ 607 . Tenor of ore from some pyritic lead-zinc veins._____________ 628. Assays of selected ore samples from the Alma Lincoln and BaldEagle mines._________________________________________ 639 . Tenor of ore from some lead-zinc veins._______-__--_____.-_ 6510. Iron content of sphalerite.______________________________ 88


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E C O N O M I C G E O L O G Y O F T H E I D A H O S P R I N G S D I S T R I C T ,C L E A R C R E E K A N D G I L P I N C O U N T I E S , C O L O R A D O

B y ROBERT H . MOENCH and A V E R Y A L A DRAKE, Jr.ABSTRACT

T he Idaho Springs district is in the central part of the Front Range mineralbelt, a northeast-trending zone of porphyritic intrusive rocks and hydrothermalveins of early Tertiary age. From 1860 through 1959 about $65 million worthof gold, silver, lead, copper, and zinc w as mined from the veins of the district.The bedrock of the district is composed largely of conformably layeredgneissic rocks and small bodies of granitic and pegmatitic rocks of Precambrianage. T he most abundant gneissic rocks are biotite gneiss, granite gneiss, andmicroline gneiss, which form five large conformable layers. In addition, quartzgneiss, amphibolite, and calc-silicate gneiss occur in small bodies. T he graniticrocks form many small bodies that are largely concordant but locally discordant. Three recognized varieties of the rocks, in the order of their em placement, are granodiorite, quartz diorite, and biotite-muscovite granite.Pegmatitic rocks in part form dikes and sills in and near the granitic bodies,to which most of these small intrusives m ay be related. One variety of pegmatite, possibly related to the biotite-muscovite granite, has been prospectedfor its uranium content.

During the Laramide orogeny, in early Tertiary time, th e Precambrian rockswere invaded by a sequence of nine varieties of porphyritic intrusive rocks.The older rocks of this sequence, with few exceptions, form irregular plutons,thick dikes, and some thickly lenticular concordant masses, whereas theyounger rocks typically form thin, long dikes. A ll but the youngest memberof the sequence were emplaced before the formation of the metalliferous veins.The gneissic rocks were folded at least twice during Precambrian time. Thefirst deformation produced north-northeast-trending major folds that form thestructural framework of the district. This deformation w as plastic in character and was accompanied by recrystallization at high temperatures andpressures. The second deformation produced northeast-trending folds andgranulation and w as most intense in a northeast-trending zone about 2 mileswide in the southeast part of the district. This zone of deformation is partof the Idaho Springg-Ralston shear zone, which extends more than 20 milesnortheastward to the front of the range. In the relatively incompetent rocksthe second deformation produced many small folds that trend N . 55 E. in aremarkably consistent pattern. The more competent units, however, were notfolded but were locally intensely granulated. T he second deformation wascataclastic in character and took place at somewhat shallower depths thanthe first.

Northwest-trending faults of the so-called "breccia reef" system may haveoriginated in Precambrian time, after the later of the tw o major deformations.


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Arching of the Front Range highland during the Laramide orogeny probably produced the regional joint system, and porphyritic intrusive rocks werethen emplaced, some as dikes along this joint system and others as plutons.Before the last member of the intrusive sequence was emplaced, regional shearstresses formed a northeast-trending network of faults and locally reopenedparts of the northwest-trending faults of the "breccia reef" system.

Laramide strike-slip faults of small displacement are distributed in three principal sets that trend northeast, east-northeast, and east; most of them dip northat medium to steep angles. Individual faults are fairly straight and traceablefor as much as 2.miles at the surface. The three principal sets were probablyformed almost contemporaneously, for the order of their formation changesfrom place to place, and their movement patterns are consistent with a single,east-northeast-oriented compressive stress system.

The veins of the district typically fill fault fissures and are similar in structure and mineralogy to those classed as mesothermal by Lindgren. Pyrite,sphalerite, galena, chalcopyrite, and tennantite are the principal ore minerals, and quartz and local carbonate minerals are the principal gangue minerals.The walls enclosing some veins are indistinct and consist of fractured zonesas much as 30 feet wide but typically less than 5 feet wide. These fracturedzones have been cemented with ore and gangue minerals. Other veins, rarelymore than a foot wide, have sharp walls and were formed by the filling of asingle fissure. The veins are classified on the basis of their mineral contentas pyrite veins, pyritic copper veins, pyritic lead-zinc veins, and lead-zinc veins.Pyrite veins (composed largely of pyrite and quartz) are valued only fortheir gold content, which is low grade; few veins have been mined profitably.The veins have indistinct walls and show evidence of recurrent movements.Pyritic copper veins contain abundant pyrite, smaller amounts of chalcopyrite and tennantite, and subordinate amounts of galena and sphalerite.Quartz is the principal gangue mineral; carbonate minerals are sparse. Thebase-metal minerals typically occupy fractures that cut through quartz andpyrite. The veins are valued chiefly for their content of gold and subordinatelyfor copper, silver, and, locally, lead. Only a few veins have been minedprofitably.

Pyritic lead-zinc veins contain pyrite, galena, sphalerite, and subordinatechalcopyrite and tennantite. Quartz is the principal gangue mineral, andcarbonate minerals are locally abundant. Some veins are symmetricallybanded; the base-metal minerals form the central band and pyrite forms theouter bands. In other veins the ore minerals occupy fractures that cut throughthe quartz and pyrite. The veins are of value chiefly for their gold and silver,but they also yield lead and copper. The most economically important mines ofthe district are in veins of this type.

Lead-zinc veins contain galena, sphalerite, and subordinate amounts ofchalcopyrite, tennantite, and pyrite. Quartz and various carbonate mineralsare the principal gangue minerals. The veins typically have sharp walls andare characterized by massive intergrowths of ore and gangue minerals. Lead-zinc veins are mined chiefly for silver, gold, and lead, and subordinately forcopper. Zinc, though abundant, has not always been recovered. The primaryores are richer in lead and silver and poorer in gold and copper than those ofother vein types.

Alteration of the wallrocks adjacent to the veins varies both in intensityand in lateral extent. Typically, an inner zone of hard sericitized rock isbordered by an outer zone of argillized rock that grades outward to fresh rock.


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INTRODUCTION 6/The width of the zones does not vary with the width of the veins, and in many

places the veins cut across the alteration zones.A zonal distribution of the ores in the district is indicated by the patternof vein types. Pyritic copper veins form a broad belt along the west side ofthe district. This belt grades eastward to a belt of pyritic lead-zinc veins and,farther to the east, to areas where the lead-zinc veins dominate. In generalaccord with this zonal pattern away from the zone of pyritic copper veinssilver, lead, and zinc increase in quantity and copper decreases: in the areasof lead-zinc veins, gold decreases and is less systematically distributed.

Because the veins are fissure filling, most ore bodies have been localized bythe same factors that controlled the width of the original openings. The majorfactors are deflections in dip and strike of the fault surfaces, and the amountand type of movement along the faults or fissures. Deflections may be relatedto intersections of two or more faults, faults and layers of competent wall-rocks, and, possibly, faults and fold axes.A sequence of events, which starts with fracturing and proceeds throughwallrock alteration and deposition of pyrite and then of base metals, is postulated as an explanation of the origin of the veins in the district. Districtwidefracturing may have allowed fluids to escape upward and outward from amagmatic source at depth. These fluids altered the wallrocks, and muchpyrite was formed by sulfidation of iron that was released from iron-bearingminerals of the wallrocks. At this stage pyrite veins formed throughout almostall of the district, and most fractures became clogged with pyrite and alteration products. During recurrent tectonic movements most pyrite veins werereopened and some new fractures were probably formed. Mineralization thenresumed, and the base-metal ores were deposited. The zonal pattern indicatesthat the source probably centered beneath the central pyritic zone in theCentral City district, and that at greater depth it extended southwestwardalong the west side of the Idaho Springs district.

About 1 35 mines and prospects in the district have workings that range inlength from about 1 00 feet to several miles; however, many are now partlyor completely inaccessible and have not been worked for many years.

During the district's most active period, from the 1860's into the 1890 's , themost accessible and locally richer near-surface ores were mined. Mining from1 9 0 0 through 195 9 was affected by periods of nationwide economic depression,when costs generally were lower and the labor supply greater. In the 1950 ' scosts increased disproportionately to metal values; this increase, combinedwith deterioration of the mines, inhibited mining activity. These trends canbe expected to continue. Unless new low-cost large-tonnage methods can beapplied in mining the ores, mining in the district probably will not producesignificant quantities of base and precious metals in the future.

INTRODUCTIONT he Idaho Springs mining district (figs. 1 , 2) is a segment of th eFront Range mineral belt, a northeast-trending zone of intrusiverocks and hydrothermal ore deposits of early Tertiary age (Loveringand Goddard, 1950, p. 72-73, pis. 2, 3, fig. 2 1 ) . This belt extends adistance of about 50 miles from the region just northwest of Boulder southwestward across the Front Range.


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ECONOMIC GEOLOGY, IDAHO SPRINGS DISTRICT, COLORADO106 105"

DANO SPRINGSDISTRICT.C L E A R C R E E K !COUNTY_1I

FIGURE 1. The Front Range, Colo., showing the location ofthe Idaho Springs district.


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39 45 '

CENTRAL C IT Y DISTRICT

LAWSON-DUMONT-FALLRIVER ARE A

FREELAND-LAMARTINEDISTRICT

FIQUBE 2. The Idaho Springs district and adjacent mining districts.

From 1859, when placer gold was discovered in Idaho Springs andlode gold w as discovered in Central City, through 1959 , ores valuedat about $200 million were shipped from a 50-square mile area thatincludes-the Idaho Springs and adjacent districts to the north, west,and southwest. The adjacent Central City district, which producedores valued at more than $100 million, w as clearly the leading producer in the mineral belt. Through 1959 ores of a value totalingabout $65 million were shipped from the Idaho Springs district.This exceeded the value of ores shipped from the districts to thewest and southwest during that time. In most areas the most valuable metal in the ores was gold, in some areas it w as silver, and inmany areas copper, lead, and zinc were subordinately valuable.

Mining activity in the Idaho Springs and adjacent districtsreached its peak in the late 1800's; it declined sharply after 1 9 1 4 ,renewed somewhat during the 1930's, and declined greatly duringWorld War II. In the 1950 's uranium prospecting stimulated somemining activity. N o uranium ore was produced, however, and atthe close of the decade only one mine the Bald Eagle was beingworked for its precious- and base-metal ores.


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6 ECONOMIC G E O L O G Y , I D A H O SPRINGS DISTRICT, COL ORAD OGEOGRAPHY

The Idaho Springs district encompasses an area of about 1 0 squaremiles in Clear Creek and Gilpin Counties, Colo. Idaho Springs, theprincipal town in the district, is on U.S. Highway 6 and 40, a segment of a major east-west interstate highway system. Most of thedistrict is easily accessible from this highway and from many goodgraded roads. State Highway 27 9 extends from Idaho Springsnorthward up Virginia Canyon to Central City, and State Highway1 03 follows Chicago Creek southward.

The Idaho Springs district lies athwart Clear Creek Canyon;hence, it is characterized by rugged topography. The gently rollingupland surface that extends southward from the Central City district is deeply incised by the canyon and its tributaries. Slopes inthe canyon average nearly 35, and the maximum local relief is about2,000 feet. Altitudes range from slightly more than 7,500 feet alongClear Creek to 9,925 feet on Pewabic Mountain. Clear Creek, aneastward-flowing tributary of the Platte Kiver, has, as tributarieswithin the district, Fall Kiver, Trail Creek, Chicago Creek, andVirginia Canyon. Spring Gulch joins Chicago Creek about a quarter of a mile south of Idaho Springs.

The climate of the region is temperate and dry, becoming morerigorous at higher altitudes. The mean annual temperature in IdahoSprings is 43 F., and the mean annual precipitation is 1 6 inches.Winters are cool, but snow is rarely more than a few inches deep,and, except on shaded slopes, rarely does it remain more than a fewdays. Summers are characterized by regular afternoon thunder-showers and are moderately cool and sunny.

The district w as apparently denuded of forest cover during earlymining, but since then a new forest has grown locally. South-facingslopes are sparsely timbered, but north-facing slopes are well-covered with conifer and aspen.

PURPOSE AND SCOPE OF REPORT. The Precambrian bedrock, Tertiary veins and porphyritic intrusive rocks, and all accessible mines were mapped during an investigation of the uranium and associated ore deposits of the district, aspart of a larger study in the central part of the Front Range mineralbelt. This report describes the general character, structure, andmineralogy of the metalliferous vein deposits of the Idaho Springsdistrict, and summarizes their relations to the Tertiary intrusivesand the Precambrian rocks and structure. A report on the Precam-;brian geology of the district w as published separately (Moench,1964); the Tertiary intrusive rocks have been described by Wells( 1960) . S ims and others ( 1963 ) comprehensively reported on the


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INTRODUCTIONuranium deposits of the region, including the few small deposits inthe Idaho Springs district. Tooker (1956; 1963) studied the alteredwallrocks adjacent to the veins in the region. The results of thesespecialized investigations within the district by Tooker, Wells, andSims and others are summarized in this report.

MINES AND PROSPECTS IN THE IDAHO SPRINGS DISTRICTT he mines and prospects whose locations are shown on plate 1 areidentified in th e following list. Detailed descriptions and maps ofindividual mines and prospects are not included in this bulletin because of the high cost of publication and the limited interest in them.

A separate report entitled "Mines and Prospects, Idaho SpringsDistrict, Clear Creek and Gilpin Counties, Colorado Descriptionsand Maps," by E. H . Moench and A . A . Drake, Jr., which presentedinformation supplemental to this bulletin, has been released for openfile. The following list also indicates which mines were describedand illustrated in the open-file report. Copies of part, or all, of theopen-file report may be obtained at cost from the U.S. GeologicalSurvey's Denver library, Federal Center, Denver, Colo., 80225. x

Key to mines a n d prospects shown o n plate 1Opening of mine or prospect

A ce of Diamonds shaft_____--_-_--__...___-Aduddell shaft...... -_-------_____Allan shaft. _________ _____________________Alma Lincoln mine (adit level), _____________Alpha shaft____--__-_-_--_____--___._____Alpine adit. _______________ ________________Amy shaft_____.____-__-_-________.___-.Arizona shaft- -_-___--__--___________-___.-Atlantic shaft___________ _______________Bald Eagle shaft.__---__-____.______ ...Bald Eagle Extension shaft. ___._.___--_____Banta Hill shaft.....---..--......... ____.Bell adit. _---.---------------__--__--__--.Belle V ue shaft.. .____.---_______-_.._____.Belman shaft. _______ _____________________Bertha shaft.______________________________B ig Chief shaft-.....-B ig 51 shaft. ________________B ig Five (Central) tunnel___-____-_-______--Birtley adit____________________________Borealis shaf __ . --___----___-_______-_____.

Location on pi.1 , this report(fig. 1 , open-filereport)

E-II, 1 3G-I, 3G-II, 1 1D-IV, 2H-II, 6C-III, 6G-II, 1 5C-IV, 1C-III, 5H-IV, 1G-II, 2F-II, 5E-IV, 1E-IV, 7E-II, 9E-II, 5I-I, 1C-V, 4G-III, 7C-IV. 2G-II, 8G-II,7E-I, 1 4G-I, 5B-V, 2C-V, 9E-IV, 19F-V, 1E-IV. 15

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8Key to mines and, prospects shown o n plate1Continued

Opening of mine or prospect

Bullion adit- ________ ___ _ _____ ___Bullion King N o. 3 adit__ _ ______Bullion King shaft. _ _____ __ ____Camp Valley adit. __ _Carlin shaft. ___-_ - -Casino shaft__-_-_ - _ _._ I

Champion (Bellevue) shaft. _______ .Champion Dirt N o; 1 adit:_______ _Clarissa shaft. ___________ ______ _ _ _.Clarissa adit (lower)..-.-- --_ ___Clear Creek shaft. ____-^----___ ___ ___ _ _

Columbia adit. __ -- ____- _ _____ ___Columbine adit. _____ ___ ___ _ _ _______

Comstock shaft. _ _ -__._ ---__________ _..Crocket shaft__-------_----_- _______ ___ _Crown Point and Virginia shaft. _____________Crystal adit_ _______ .__-_____ __-___-___De Lesseps shaft. -___--_- ______________ _Diamond adit__-._ _ ____ _____ ___ _Donaldson No. 6 Level adit. ___. _-_.-_._-Donna Juanita adit_________ _______._..__Dover prospect. _ _... _---__--_________Doves Nest shaft _ ____ - ______________ _Druid shaft____ _______ _____________East shaft_.__ ____________ _________ _East Hukill shaft______________________Eclipse shaft_----____-_--_____ ___ _Edgar adit and shaft. ___---____ __-__ ___Edeardine shaft..------ _ - _ _ ____ _____

Location on p i . "1 , this report(fig. 1 , open-filereport)F-II, 1 4C-I, 2F-II, 1 1F-II, 1 0D-III, 1 2E-III, 11C-V, 8F-III, 5D-V, 1D-IV, 23F-II, 3C-V, 1 0G-I, 7D-IV, 25C-V, 6G-III, 4G-III, 3D-I, 5D-I, 1 2B-IV, 4D-II, 2B-IV, 2E-III, 1 0E-I, 1 5E-I, 1 6E-I, 1 3F-V, 3G-I, 6B-V, 6E-I, 1D-III, 13D-IV, 22C-IV, 8E-IV, 5F-II, 6F-V, 4E-IV, 1 0E-I, 4F-III, 6G-II, 26G-II, 27A-V, 1E-II, 2C-IV, 15D-IV, 1 7D-IV, 1 8C-III, 8F-II, 7G-I, 2C-III, 1B-V, 4E-IV, 3G-III, 1E-III, 13G-III. 12

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Key to mines and prospec ts sh o w n o n plate1Continued


Edgar Extension adit__ ________ __ _____Edna Fannie adit. -_ _-__ ___ ____ --_-_Edward shaft____-___ -____-___ ____ _ _Elkhorn shaft_____ _...__._ _ ___ _Elliot and Barber adit. _ _ ^_ _ __


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10 ECONOMIC GEOLOGY, IDAHO SPRINGS DISTRICT, COLORADOKey to mines and prospects shown o n plate1Continued


Grover Cleveland shaft. _--__--__. _ ______Grover Cleveland shaft- ___________________Happy Easter (Queen Elizabeth) mine__ ______Helen adit_____________________________Highlander claim. _________________________Hot Pot shaft_________-_. _______Houston shaft -_---_______-__-___---__.____Hudson adit______-__--___.__-__- _____ ___Hughes shaft _____..__________-_---__--_---_Hukill shaft: ._ _ . ____ _____ _ ___

Irene adit____-__ _____ _ _________________Jackson shaft________ _____________________Jennie Lind No. 1 adit__-_____-_-__-____--_.J. L . Emerson shaft_____________________.__J. Warner shaft and adit. ___________________John L . shaft. . ____________________________John Paul Jones adit ____________ _________Jones shaft_______-___ ____________________Josephine shaft and adit--____-__---___ __Jumbo adit___________ _________________Kangaroo shaft. -_-____--___-________ _ _ _ _Kelly No. 4 level adit _____________________Kelly shaft____________ ______Kinda-U.P.R. mine____________ _________Lafayette adit_-_-_-___ ____________________Lawrence L . (Philadelphia) mine. ________ ___Lead Belt adit. _-_-_________-___.--____-___Liberator shaft_________________________Little Albert No. 5 adit. ___.______--____--_.Little Annie adit. __________ _______________Little Cub adit_____.__________Little Ella shaft........ -._.__-_-__-___ _Little Emma adit-_-________--____-_-_.--__Little Six adit- ______ _____ _._ ___________Loeber shaft____ _----___---__--_-_-_-__Lord Byron shaft-________________________.Lost Summit shaft. _ _________________ _____Lower East Lake adit_-_________________-__Lower Lake adit_-_________-_-__________-__Lucania tunnel. _________ _________________MAB adit___ ___________________M and E adit__-__--_-__-_-__----___-_--.


B-V, 7D-I, 1 5E-I, 5G-III, 1 4G-III, 1 3F-II, 1 5C-1,1C-III, 3D-I, 8C-IV, 4E-II, 7D-I, 1 3E-IV, 2E-IV, 1 2F-III, 7D-V, 7G-II. 23E-III, 9D-I, 1 4G-II, 25E-I, 11G-IV, 1G-II, 1 4D-IV, 4E-IV, 1 8F-II, 1 2B-IV, 6C-IV, 1 6E-II, 1 4C-I, 4E-IV, 1 7C-III, 7F-II, 8B-V, 3B-IV, 1D-II, 6D-V, 5E-IV, 8F-II, 9D-IV, 1 5D-III, 1 0E-I, 9E-III, 1 4C-V, 7E-III, 1 5D-IV, 7F-II, 2E-II, 1 0C-II, 1D-III, 1 5D-V. 6


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INTRODUCTION 11Key to mines and prospec ts shown o n plate1Continued


Manhattan shaft_____ ____ ________________Manhattan adit_ _ __________________________Martha Perks adit---__-_------_---_---___-Mastedon adit. ______-____--_____-.-----__-Maude Munroe mine_-____ ________________M ax shaft________________._--_-__.May Queen Annex adit_____-__-_---_-_-____McMickle adit__________-__----__--_____Merrimac adit _____________________________Metropolitan tunnel. _______________________Metropolitan adit .__-_---__--___-___-______Miami tunpel_____----__-___---_________Minnie shaft_____--__---__--_-_----__-_Minott shaft___________ .____-__-__-__.MIXadit--__-_-___-------------____-._M K shaft. _--____._--_-____--__-_--__.Monte Cristo adit__ ________________________Moose shaft _______________________________Morgan shaft__ ______--___-___----__---____Morning Star shaft__ _ ______________________Morning Star shaft. ________________________Mount Etna adit__---_----__----__---____Mount Vesuvius adit____-_--____--__-___-__Myra shaftNashville shaft _-__-_--_-_-___----__-______Needham adit_ ____________________________N ew Bedford adit. ____.-__--__________-____Niagara shaft.__-____--___-_-___--_________Nighthawk shaft. __________________________Nonpareil adit. ____________________________No. 12 adit (Alma Lincoln mine)__________October shaft. _-__---_-___--___---__-______O ld Settler adit___________________O ld Settler shaft._.._____ _ _____O ld Stanley shaft________ _____....Oliver shaft. ______________________________Oregon shaft___________________________O ro Fino adit_-__-_-_____-__.-__________O ro adit_ ___________________________ _____Ottawa shaft. __-_-----_----_--_--____-___-Owatonna shaft__ _ ._. _____________________Patten adit_______________ _____________Pennsylvania adit_ ._______--____-_____-___.Phillips shaft. _--__--_--__--__-____________Phoenix adit_______ _---_.__ ____________


C-V, 1C-V, 12C-III, 2D-III, 11D-W, 8G-II. 12E-IV. 1 6D-III, 4D-III, 3E-IV, 20C-IV, 1 2C-III, 4F-II, 1 3C-V, 3F-IV, 3D-I, 1G-II, 22F-III, 1D-I, 7E-II, 1D-IV, 1H-I, 1A-IV, 2C-IV, 7D-III, 6G-II, 1C-IV, 1 4C-IV, 1 3C-V, 5E-I, 10G-II, 18D-III, 16E-IV, 11C-V, 1 1H-II, 15F-IV, 1F-IV, 2D IV , 20D-IV, 21C-V, 14A-IV, 3A IV 4D-V, 2E-IV, 1 4D-I, 4H-II, 2D III, 1 4G-III, 9F-I, 1F-III, 3D-IV, 6D-I, 9B-IV. 5


XXXXXXXXXXXXX

0

XXX

X

XXXXXy


XXXX

XXX

X

( ' )X

X

X

ySee footnote at end of table.783-337 65 2


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12 ECONOMIC GEOLOGY, IDAHO SPRINGS DISTRICT, C O L O R A D OKey to mines and prospects shown o n plate1Continued


Pine Tree shaft ______ _______ _ _ _ _

Quartermaster shaft- .___ _. _ __-.Red Jacket adit^_ _ ___ _ _- ___Red Lyon adits______ __ _ _Refuge shaft_____Reilly (? ) shaft ___________.__-_-- ____.-___..Richmond shaft. _ _ ____Rio Grande shaft__ _. ______----. - ._Road Level adit (Stanley mine)-Rockford tunnel-__- ___ __ _ _ ____ _ _ __St. 'Joseph shaft. _. _ _ _________ _____Santa Fe shaft. _ _ ______

Shafter adit _- __________ _ _______Ship Ahoy shaft. _ _ ____ __ __Silver A ge adit __________ _ _South Lincoln and Ruby adit. ___ _.__ _Spear adit...- _______ ______ _____Specie Payment shaft_ _________ ______Squaw shaft____ ____ ___. _ ___ _____Stanley (Gehrman) shaft.... _ ___ _______Star adit. _______ __ ______ _ _ _ _ _Summit mine________ ______Sunnyside adit _____-_--__ _ ______Syracuse mine. ____ ________ _ - ____ _

Tom Boy adit. _____________ _ ___Torpedo mine__ _ _ _ _________ ._ __ ___Treasure Vault shaft. _ _ _________Treasure Vault adit__ __ _ - _ __._Trio adit____________.._-....____.--.__

See footnote at en d of table.

Location on pi .1 , this report(fig. 1 , open-filereport)

C-V, 13G-II, 20H-II, 3D-I, 3D-IV, 1 4E-III, 1 7E-III, 1 2F-IV, 4F-III, 4D-V, 4E-III, 7H-II, 8F-III, 2D-II, 1G-II, 1 9E-I, 7D-IV. 1 2B-III', 1H-II, 4D-IV, 1 0G-II, 9G-II, 1 6C-II, 3D-IV, 1 9E-III, 3E-III, 1 6H-II,- 1 0H-II, 9H-II, 11D-V, 8D-IV, 3D-II, 7D-II, 3D-II, 5C-IV, 1 0D-IV, 11D-V, 3E-III, 1E-III, 6H-II, 1 3G-II, 3A-IV, 1G-II, 21D-III, 5D-II, 9G-III, 6G-III, 1 5D-V, 9D-III, 7G-III, 11G-III, 1 0E-I, 6G-II, 1 7


X

X

X-X

X- X-X

X -XXXXXXXXXX

XX 0)X

- X 0 )XXXXXXXXX


X

XXX

XXX

XX 0 )X ( ' )XXXXXX


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INTRODUCTION 13Key to mines and, prospects s h o w n on plate1Continued


Tropic tunnel. ___ _-__-. ________ _ _Tw o Brothers tunnel. ___ _ _ - _ _ ___ _ _ _Tyson shaft______________ ____________:___Union adit_ _________ ______ ______ _ _United Gold Adit_____ _____U. S. adit _____________________________Unknown adit_ _______ _*___ _ _ _ _ _ ___Upper. East Lake adit__ _ ' _ _ _ _ _ ___ ____Veto.shaft__________ ________ _______Vida shaft___________________ .-_._ __.'_-_Waltham shaft- -____ ___ __ _ _ ____'_Ward adit_____________ _ _ - _ _ _ ___. _Welch(?) shaft..... -__--_ -.__West Doves Nest shaft___ _ _ _.__ . ____West Santa Fe shaft_________..-____--_._Whale adit (Stanley mine) ___ .-.. _ _ _Wheatland adit___ -__-__-__ _____ _____:: .Wild Rose shaft______ .______-.___-___.__Williams shaft ___________ _ _ _ _ _ _ . _ _Willis Gulch shaft -indsor Castle shaft. _____ ______ ________Wyandotte mine. ________ _ _ _ ____ _____York adit (Stanley mine) ___ ____ _______


G-III, 5E-II, 8C-IV, 6B-IV, 3C-II, 4E-IV, 4 .D-I, 2F-II, 4E-II, 12G-II, 30D-I, 11D-I, 10F-V, 2F-V, 5D-II, 4F-II. 1G-II, 10D-IV, 13G-III, 9C-V, 2E-I, 3G-I, 1E-II, 11E-II, 3D-III, 8D-IV, 26

Open-file reportIncludesdescriptionXXx.XXX

X

XXX

XX

IncludesillustrationXXX .

X

XX

Xi In Prof. Paper 3 7 1 (Sims an d others, 1 9 6 3 ) .

PREVIOUS STUDIESThe Idaho Springs district was studied during the early 1900 ' s

by the U.S. Geological Survey. Spurr and Garrey ( 1 9 0 8 ) and Ball(1908), during study of the Georgetown quadrangle, mapped thesouthern part of the district on a scale of 1 : 62,500 and studied manyof the most productive mines. Bastin and Hill ( 1 9 1 7 ) mapped thenorth half of the Idaho Springs district (scale 1:12,000) during theirstudy of the Central City quadrangle. Lovering and Goddard( 1950 ) summarized the information on the district in their report onthe mining districts of the Front Range, and Goddard ( 1 9 4 7 ) prepared a separate brief summary.

FIELDWORKFieldwork for the present investigation was done during the sum

mers of 1953 and 1954. The surface geology was mapped on a scaleof 1:6,000 on a special topographic base map prepared by the U.S.


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14 ECONOMIC G E O L O G Y , I D A H O SPRINGS DISTRICT, COLORA DOGeological Survey from aerial photographs taken in 1951 . Theaccessible mines were mapped on scales of 1:480, 1 : 600, and 1 : 1 , 2 0 0 .Approximately 4 man-years were devoted to the study of the districtby the writers and their associates. The areas mapped by the writersand those mapped by their colleagues are shown on the index onplate 1 .Concurrently with the study of the Idaho Springs district, otherparties of the U.S. Geological Survey were working in the adjacentmining districts (fig. 2). The Freeland-Lamartine district and theChicago Creek area, southwest of the Idaho Springs district, werestudied by Harrison and Wells (1956,1959). The Lawson-Dumont-Fall River area to the west w as studied by C. C. Hawley and F. B.Moore, and the Central City district to the north w as studied bySims, Drake, and Tooker ( 1 963 ) . Specialized studies of the uraniumdeposits (Sims, Drake, and Tooker, 1963; Sims and others, 1963;Sims and Sheridan, 1 9 6 4 ) , Tertiary igneous rocks (Wells, 1 9 6 0 ) , andwallrock alteration (Tooker, 1963) were also made.

ACKNOWLEDGMENTSWe thank our colleagues P. K. Sims, J. E. Harrison, J. D. Wells,

C. C. Hawley, and F. B. Moore, of the U.S. Geological Survey, w homapped parts of the surface and some of the mines in the IdahoSprings district. We were ably assisted in the fieldwork by MaxSchafer, A. E. Dearth, Alien F. Moench, J. R . McDonald, and PeterBuseck. Many thanks are also extended to Mr. Charles L. Harring-ton, U.S. Mineral Surveyor, Idaho Springs, Colo., w ho furnishedmany mine maps and much useful data, and Mr. J. Price Briscoew ho allowed the writers to publish assay records of the IdahoSprings Sampling Works, a now-defunct company that Mr. Briscoeowned from 1 9 1 9 to 1936. Mr. Carl Belser of the U.S. Bureau ofMines furnished a useful unpublished report on the Lucania tunnel,and Mr. Frank Jones of Idaho Springs provided assay data andmaps of many mines. Mr. A . J. Martin of the U.S. Bureau of Minesprovided production figures of individual mines of the district whichare published with permission.

HISTORY, PRODUCTION, AND FUTURE O n January 7 , 1859, George A . Jackson, a native of Missouri,washed gold from gravels near the mouth of Chicago Creek. Thisw as one of the first discoveries of gold in Colorado and it precipi-

1 Many of the facts presented here were obtained from reports by Bastin and Hill( 1 9 1 7 , p. 67-99) and by Spurr and Garrey ( 1908, p. 172-174).


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HISTORY, PRODUCTION, A N D FUTURE 15tated a rush of prospectors into the region. Placer deposits weremined at first, but rich oxidized ores on th e outcrops of the veinswere worked as early as 1860. In 1864 the Whale Company wasorganized to work the Whale lode (now called the Stanley), one ofthe most productive in the district. Mining activity tapered off during the Civil War because of th e shortage of men and materials butresumed with greater fervor shortly thereafter. Added stimuluswas given to mining by the completion of th e Union Pacific Kailroadto Cheyenne in 1867 and by th e completion of th e Denver PacificLine between Cheyenne and Denver in 1870. In 1873 th e narrow-gage railroad between Denver and Floyd Hill, on Clear Creek, wascompleted; in 1877 it was extended to Georgetow n. M ost of thelode mining was confined to the oxidized ores during the first fewyears of activity. The advent of a satisfactory smelting process forsulfide ore in 1866 permitted' profitable lode mining of th e unox-idized ores. Though most of the early mining was for gold, silverbecame increasingly important from 1870 to 1873.After 1866 the history of the district was closely tied with thedevelopment of better milling and smelting techniques and withfluctuations of national economy. Excellent summaries of millingand smelting practices are given by Bastin and Hill ( 1 917 , p. 153-163), and by Sims, Drake, and Tooker ( 1963) .A study of production records provides some insight to th e economic factors that affected mining in the district. Records priorto 1904 are incomplete, but the production for the Idaho Springsdistrict was influenced by the same factors that influenced productionin all of Clear Creek County (Spurr and Garrey, 1908, p. 174-175) .In the county production increased rapidly from $40,500 in 1866 to$2,203,948 in 1874, and more gradually, with some fluctuation, to$3,560,000 in 1894. T he financial panic of 1873 caused a decline'inthe price of silver, and th e mining emphasis shifted from the silver-rich veins to those containing a higher proportion of gold. Actualtonnage of ore shipped increased, and the total value of ores minedremained about the same. Apparently the panic of 1893 had a similar effect, because the production for 1894 was greater than that forany preceding or subsequent year. After 1894, production for thewhole county gradually declined and, in 1903, was valued at only$1,792,203. This was probably due in part to a simultaneous declinein the price of silver from about $1.00 per ounce in 1892 to $0.60 per.ounce in 1902. ' .

Partial production records are available for the Idaho Springsdistrict. From 1904 to 1959 inclusive th e ore production from theIdaho Springs district (together with some of the mines outside


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16 ECONOMIC GEOLOGY, IDAHO SPRINGS DISTRICT, COLORADOthe district) w as valued at $14,987,314 (table 1), but the total production since mining began in the Idaho Springs district w as fargreater. From 1859 to 1903 the ore production from Gilpin Countywas valued at about $81 million (Bastin and Hill, 1 9 1 7 , p. 1 7 4 - 1 7 5 ;Sims, Drake, and Tooker, 1963) , or about 3i times the post-1903production. The same ratio of pre-1903 to post-1903 productionapplied to the Idaho Springs district gives it a total productionvalued at about $65 million.

Mining activity in the Idaho Springs district w as at its peak before 1 9 0 0 ; its decline, which began after 1894, leveled off in 1904when the Argo tunnel was driven to intersect at depth many of thevaluable veins in the Central City and Idaho Springs districts. Theveins did not prove to be as rich at depth as expected, however, andthe tunnel did not greatly stimulate mining in the district. From1905 through 1 9 1 7 the annual production value ranged from $416,282to $724,309. Production declined sharply because of World WarIfrom a value of $585,568 in 1 9 1 8 to $86,957 in 1921 . It reached analltime lo w of $17 ,277 in 1924 . Production increased again becauseof the additional labor supply that resulted from the financial crisisof 1929 and because of the increase in the price of gold in 1933 to$35 an ounce.

The value of ores produced in the district w as $347,580 in 1934,reached a high of $688,001 in 1940, and dropped to a lo w of $41,949in 1944 during World War II. During 1946-50 the annual production ranged from $11,850 to $198,362; during 1951-53 it decreasedas a result of the lower base-metal prices. Exploration w as stimulated by the demand for uranium in the early 1950's, but it did notresult directly in an increase in base- or precious-metal production,nor did it result in any shipments of uranium ore. The Bald Eaglemine consistently produced from 1955 through 1959 , however, andit accounted for most of the $328,000-worth of base- and precious-metal production from tihe district in 1956 the largest annual production of ore after 1941 . From 1956 through 1959 production fromthe Bald Eagle mine gradually declined.

Throughout the history of the district, only a few mines producedmost of the ore. Six properties that were discovered before 1874each reportedly yielded ore valued at more than $1 million before1 9 00 : the Stanley, Specie Payment, Sun and Moon, Frontenac, andGem mines, and the French Flag-Silver Age-Franklin group.Other important mines, whose production w as valued in the hundreds of thousands of dollars, include the Seaton, Aduddel, Druid,Crown Point and Virginia, Fraction, Champion Dirt, Shafter,Edgar, and Bald Eagle.


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HISTORY, PRODUCTION, A N D FUTURE 1 7TABLE 1. O re produced from th e Idaho Springs district, 1904-69

[Source of data: Years 1904-7 and 1952-59, U.S. Bur. Mines (unpub. compilation used( b y permission; total annual values based on average price of each metal for thatyear); years 1908-^31, 1946-51, U.S. Bur. Mines; y,ears 1932-45, Vanderwilt (1947).(Data for all years except 1954-59 m ay include production from some mines outsideithe Idaho Springs district]

Year

1904190519061907190819091910191119121913191419151916191719181919192019211922192319241 0 9 1 ;ICWfl1927192819291930193119321933193419351936193719381939194119421943194419451946 .194719481949. .._19501951195219531954 -19551956195719581959 .

Ore soldor shipped(short tons)3,70232, 039

30,16441, 98166, 14752, 56456, 60551 , 63746, 07 453, 41962,58758,64751,04440 , 83933, 33911 , 5689,267

8,2006,4972,5351,2432 ,1106, 5004,1041,4525,68523,642

13, 32611 ,47912, 13843, 16249 , 16555, 83566, 16846, 67150,37197, 50767 , 99739 , 5046,6642,0173,9901 9 , 6397,2225,7058,1269,2553,3477156,0641,3822,5058,3449 , 1 1 57,4026,438

Gold, fine(ounces)

3,63918, 33916, 20 316, 70825, 37 018, 76914, 71214, 82112, 89013,63518, 69 818, 16714, 77110, 69 39 ,4743,4241,476

1 ,4131,31773727 67793,5152,3356382,5454,4663,1375,3674,4828,4858,9766,6087,3765,827

10, 87 81 5 , 44 912 , 7856,17186022 63552,7812, 0212,2892,3112,494

9874851,60680521 44,4624,1511,2821,658

Silver, fine(ounces)

61 , 302252, 418185, 37 0193, 9 83263, 479182, 163171 , 24 8167,953161, 681214, 481193, 221236, 772231, 673260,346203, 23989 , 70970 , 800

33,94936, 19814, 42 210, 37 313, 65512, 50311, 9569,42927 , 94027 , 95621 , 63822, 69523, 82955, 91464, 44 173, 57 977, 12249,50068,441112, 34475,64830, 61 122 , 57 610,65811, 26411, 68314, 99210,28024,96129,41510, 21 52,3769,31114, 1 9 13,95139,30234,08823,86824,095

Copper(pounds)

6,30240 , 40 9108, 115140, 665212, 435176, 101314, 759333, 22 8255, 061309,885250, 744341,432409, 789424, 186293,300100, 61 828, 022

16, 69 03,9335,0212,68012,5608,56416, 3814,42437 , 06813, 00 05,4009,55058, 3009 8, 40 6

45, 10060, 80046, 20 0129, 700183 20 0106, 70056, 10034, 50010, 80011,600

14, 40 07,7003,00016, 00 012, 9002,5001,5001 6 , 00 04,4007 ,100

88,00080, 50050,40054,600

Lead(pounds)

1 5 6 , 5 4 8341, 27 2500, 89 2235, 227779, 709867, 59 7688, 114

1 , 571, 755897, 3411, 628, 3241 , 239, 57 21 , 059, 2322,029,0291,979,3601,606,211640, 001665, 075

481, 156510, 982162, 31455,36362, 00 068, 19088,707127, 984123, 00 068, 75060, 000103, 632276, 100442, 900

481, 800645, 100520, 500693, 400933, 900549, 000193, 300231, 40 0113, 600155, 00 0

58, 600159, 40 0174, 00 0415, 000237, 30098, 50020 , 00 073, 00 093,80069 , 300

625, 500685, 40 0528, 800404,800

Zinc(pounds)

1,279' 38, 33931 , 97 8

202, 19313, 4354,63912, 53013, 58026, 666

3,800

14, 00 0

17,000

153, 000139, 600202, 20 014 , 00 024,600129, 000211, 000125, 00 041 , 000

8008,50011,600

Total value

$ 1 1 8 , 20 05 5 , 2 8 0509, 74 4416, 282707, 838530, 302458, 703509, 358440, 639532,088575, 69 4607, 665698, 604724, 309585, 568223, 896166, 050

86, 95792,04939 , 16517 , 27 737, 02086, 89 761,48524 , 47 982 , 104111, 58874 , 126119 ,47 8105, 437347, 580386, 381

314, 588363, 218265, 2 1 1473, 266688, 001545, 153257, 49 284,51841, 94958, 584117 , 203111,850138,373198,362166, 38168,89822 , 70878,88422 , 32225, 485

328, 01 6296, 687144, 595143, 337


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18 ECONOMIC G E O L O G Y , I D A H O SPRINGS DISTRICT, COL ORAD OIn value of the total ore output of the district, gold accounts for59 percent, silver 18 percent, lead 15 percent, copper 7 percent, andzinc about 1 percent. Although sphalerite is almost as abundant as

galena in most ores, zinc was not reported for many years.In the future unless metal prices increase spectacularly and moreefficient means of mining are found the district cannot be expectedto produce significant quantities of precious and base metals. From1 9 1 0 through 1959 profits from most mining in the district weremarginal. Mining w as stimulated somewhat during periods of economic depression or recession, owing to lower mining costs andgreater labor supplies. This pattern may continue, but the cost ofrehabilitating the old mines may become prohibitive as the districtages. The district has been thoroughly prospected, and new orebodies are likely to be found only on known veins. Mo st of thevaluable veins contain much unexplored ground. Because the veinsare typically narrow and require extensive timbering, however, eventhe most valuable veins are not generally amenable to low-cost large-tonnage operations that would be required for success under presenteconomic conditions. Even under the most favorable circumstances,it may be impossible to amortize the costs of rehabilitation, exploration, development, and the construction of ore-treatment facilities.

GENERAL GEOLOGYThe Idaho Springs district is underlain dominantly by gneissic,

granitic, and pegmatitic rocks of Precambrian age (pi. 2), whichconstitute part of the core of the Front Eange. These rocks areintruded by numerous small porphyritic dikes and irregular plutonsof early Tertiary age and are cut by numerous faults that containthe ore deposits of the district. Some faults possibly originated inPrecambrian time, but most formed near the close of the emplacement period of the early Tertiary magma sequence.

Physical character and structure of the Precambrian and Tertiaryrocks had a marked influence on the formation of the fault patternsand on the localization of ore bodies. Accordingly, a brief description of the rock types and the structure of these rocks is given in

.the pages that follow. A more comprehensive report on the Precambrian rocks has been published separately (Moench, 196 4 ) . Thepetrography and structure of the Tertiary intrusive rocks in thisdistrict and adjoining ones were reported in detail by Wells ( 1 9 6 0 ) .

PRECAMBRIAN BOCKSThe Precambrian rocks are a generally conformable succession of

interlayered gneissic, granitic, and pegmatitic units. The gneissicrocks, which are dominantly metamorpliosed sedimentary rocks, are


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''"" P R E C A M B R f A N ' ROCKS 19th e oldest and by far the most widespread and abundant rocks inthe district. These rocks have been invaded by three varieties ofgranitic rock (from oldest to youngest): granodiorite, quartz diorit'e,and biotite-muscovite granite. Several types of small pegmatitebodies are interlayered with and locally cut th e gneissic and graniticrocks.T he major gneissic rock units are interlayered biotite gneisses,granite gneiss, and microcline-quartz-plagioclase-biotite gneiss,which, for convenience, in most of th e text are called, respectively,biotite. gneiss, granite gneiss, and microcline gneiss. T he biotitegneiss and granite gneiss are intermixed in layers that range from afraction of an inch to several hundred feet in thickness. T he thickerlayers can be mapped individually at 1:6,000, but on plate 2 thesetw o rock types are combined so that the major units shown are microcline gneiss and a mixture of biotite gneiss and granite gneiss,within th e latter mixed unit, granite gneiss increases in abundancesouthwestward across the district, apparently at the expense of th ebiotite gneiss. Thinly layered rocks, that consist of roughly equalproportions of biotite gneiss and granite gneiss are termed migmatite.Although migmatite was not noted at the surface, it was discerniblein some mines. Small bodies and layers of amphibolite, calc-silicategneiss, and quartz gneiss are associated with the major units; however, they are not shown on plate 2.The biotite gneiss and associated minor rocks were assigned to theIdaho Springs Formation by Ball (1906) and by Levering andGoddard (1950, p. 19-20); the microcline gneiss near Idaho Springswas mapped by Lovering and Goddard (1950, pi. 2) as quartz mon-zonite gneiss and -gneiss pegmatite. T he granite gneiss and associated pegmatite were mapped by Harrison and Wells (1956, p.50-53) in the adjoining Freeland-Lamartine district.

In contrast to neighboring areas, granitic rocks are sparse in th eIdaho Springs district. T he few bodies of granitic rocks largeenough to show on plate 2 are small and appear to be satellite tothe larger plutons or bathpliths that crop out to the southwest, west,and north of the district. T he granodiorite is similar to the Bo ulderCreek Granite (Lovering and Goddard, 1950, p. 25-27); the biotite-muscovite granite is similar .to the Silver Plume Granite from thetype locality at Silver Plume, Colo., about 16 miles southwest ofIdaho Springs (Ball, 1906 ) . Lithologic names rather than geographic formational names are used here because th e stratigraphyof the Precambrian metasedimentary rocks and correlations of theintrusive rocks are not fully established.


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20 E C O N O M I C G E O L O G Y , I D A H O SPRINGS D I S T R I C T , - C O L O R A D OGNEISSIC ROCKS

The gneissic rocks are divided into three major lithologic units:microcline gneiss (or microcline-quartz-plagioclase-biotite gneiss),biotite gneiss (or interelayered biotite gneisses), and granite gneiss(or granite gneiss and pegmatite). The biotite gneiss and granitegneiss are grouped on plate 2, and together they are conformablyinter-layered with units of microcline gneiss. If these units are assumed not to be overturned, a stratigraphic succession can be recognized as shown in section B-B' of plate 2. The lowermost unit, athick layer of mixed biotite gneiss and granite gneiss, forms the coreof the Idaho Springs anticline on the southeast side of the district.This lowermost unit is overlain by a thin and discontinuous layer ofmicrocline gneiss. The microcline gneiss, in turn, is overlain by athick layer of mixed biotite gneiss and granite gneiss. A thick unitof microcline gneiss is higher in the stratigraphic succession, and itis overlain by a thick layer of mixed biotite gneiss and granitegneiss the uppermost unit in. the district.

In addition to these major units, small bodies of amphibolite, calc-silicate gneiss, and quartz gneiss are exposed, but these small outcrops are not shown on plate 2. All the gneissic rocks are describedhere without regard to their apparent stratigraphic position.

. BIOTITE GNEISSTwo main varieties of biotite gneiss are recognized: biotite-quartz-

plagioclase gneiss and sillimanitic biotite-quartz-gneiss, both ofwfhich are locally garnetiferous. These rocks alternate with oneanother in layers that range from about an inch to several feet inthickness and probably represent the original bedding. In outcropsthe biotite gneiss is marked by its .dark-gray color, pronounced layering, and tendency to split parallel to the layering. Conformablelayers and lenses of granite gneiss, present in most exposures, emphasize the layered appearance of the unit.

Typical biotite-quartz-plagioclase gneiss is fine grained, equigran-ular, light to dark gray, and is faintly to intensely foliated. Thegneiss typically contains quartz and plagioclase in nearly equalamounts and 10-35 percent biotite. The feldspar composition generally ranges from oligoclase to andesine. Foliation is produced bya parallel alinement of biotite and locally by segregation of mineralsinto light and dark layers.The sillimanitic biotite-quartz gneiss is light to dark gray and iswell foliated; it is flecked with pods and smears of white fibrous sil-limanite and has a marked schistose structure. The rock containsabundant quartz, about 20 percent biotite, as much as 30 percent


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PRECAMfcRIAN ROCKS 21sillimanite (but generally much less), and, commonly, some mi-crocline and albite-oligoclase. Small amounts of muscovite can befound in most specimens, and garnet (almandine-spessartite) islocally abundant.

GRANITE GNE I S S A N D PEGMATITEGranite gneiss ( or granite gneiss and pegmatite) is exposed

throughout the district in layers and lenses that range in thicknessfrom less than an inch to several hundred feet. It is generally associated with biotite gneiss, and in many places relatively equalamounts of these two rock types are intimately mixed in thin alternating layers that form migmatite. Granite gneiss, some of whichis also associated with microcline gneiss, is most abundant in th esouthwest corner of the district. Northeastward, layers of biotitegneiss are more abundant, and layers of granite gneiss are less abundant and discontinuous.The granite gneiss is light colored and contains sparse to abundantwisps, laminae, and layers of biotite gneiss. Excluding the layersof biotite gneiss, most of th e unit is nearly devoid of dark mineralsand has the composition of a true granite; it contains abundantquartz and microcline and subordinate amounts of sodic plagioclase.T he rock is typically fine to medium grained, and the gneissic structure is produced mainly by layers of slightly different grain size aswell as by conformable inclusions of biotite gneiss. Locally, the rockis pegmatitic and contains feldspar crystals as much as 3 feet across.The coarse feldspar is white to pink and contains graphic inter-growths of quartz.

, . MICROCLINE GNE I S SMicrocline gneiss ( or microcline-quartz-plagioclase-biotite gneiss)is exposed in thin discontinuous layers near Idaho Springs and ina major layer that extends northward and eastward far beyond th elimits of the mapped area. This extensive microcline gneiss layerwedges near the south margin of th e district (pi. 2) . The contactsbetween microcline gneiss and biotite gneiss are typically sharp,traceable for long distances, and provide the best structural "markers" in th e district.T he microcline gneiss is a fine- to medium-grained light-gray or

light-tan rock that is characteristically thinly laminated and wellfoliated. Laminae are typically 1 mm or less thick and are in alternating layers that are characterized by abundant or sparse biotite.Biotite-rich layers rarely exceed 1 inch in thickness. The rock contains 25-50 percent quartz, 30-55 percent oligoclase, as much as 35percent microcline, and typically less than 10 percent biotite.


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22 ECONOMIC GEOLOGYp IDAHO ;:SPR;I:N;GS DISTRICT, COLORA DO The microcline'gneiss" contains many-small conformable layers andlenses of amphibolite and also some lenses of biotite gneiss andgranite gneiss that are large enough to show on plate 2.

QUARTZ GNEISSSeveral thin layers of quartz gneiss are exposed along the south

east side of the area, directly north and south of Idaho Springs.The layers are rarely more than about, 15 feet thick, but they maybe traced as far as 1 mile along their strike.The quartz gneiss is light colored, fine to medium grained, andtypically has a glassy luster. The gneissic structure of the rockresults from slight differences in grain size in the laminae parallelto the rock layers. The rock contains as much as 80 percent quartzand some feldspar. Dark minerals, such as biotite and magnetite,are either sparse or absent.

AMPHIBOLITE AND ASSOCIATED CALC-SILICATE GNEISSAmphibolite commonly is exposed along the contact between themicrocline gneiss and the biotite gneiss in the form of lenses -locally

more than 1 00 feet thick. -Amphibolite also forms smaller layers andlenses in the microcline gneiss and, less commonly, in biotite gneissand granite gneiss.The amphibolite is a dark-gray to black, fine- to medium-grainedrock that contains hornblende and andesine in various proportionsand small amounts of quartz. Biotite and pyroxene are common,though rarely are both present in tiie same specimen. Som e varietiesof amphibolite are massive and nearly structureless, others aregneissic and laminated. The gneissic structure is produced by alternating hornblende- and plagioclase-rich layers and by the planarorientation of hornblende.Calc-silicate gneiss locally forms irregular masses or crosscuttingveinlike structures in amphibolite. The contacts are ragged andthe calc-silicate gneiss appears to be an alteration product of theamphibolite.

The calc-silicate gneiss is mottled, light to dark colored, and fineto coarse grained. It contains calcium-rich garnet, quartz, abundantepidote, and some hornblende, plagioclase, and, locally, clinopy-roxene.

CALC-SILICATE GNEISSCalc-silicate gneiss, apparently unrelated to amphibolite, forms

large bodies on the north and east sides of Pewabic Mountain. Therock is mottled dark to light, crudely banded, and is fine to coarse


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,- -' R E C A M B R I A N ROCKS. .... 23grained. It is composed largely of diopside, epidote, quartz, scapo-lite, oligoclase, microcline, and arfvedsonite ( a sodium-rich amphi-bole) in various proportions.ORIGIN OF THE QNEISSIC ROCKS

T he gneissic rocks, with the exception of th e granite gneiss, probably represent a thick succession of sedimentary rocks that weremetamorphosed at high temperatures and pressures nearly equivalent to conditions of the upper range of the ahnandine-amphibolitefacies, as defined by Fyfe, Turner, and Verhoogen (1958, p. 230-232) . The biotite gneiss probably represents metamorphosed shaleand interbedded sandstone, for this unit is marked by alternatingsillimanitic and nonsillimanitic layers. The layers have the appearance of beds, and the sillimanite probably reflects the high aluminumcontent that characterizes most shales. T he origin of the microclinegneiss is more debatable. Units of microcline gneiss are conformable with units of biotite gneiss and contain abundant conformablelayers and lenses of amphibolite as well as some layers of biotitegneiss. These features are most easily explained as having resultedfrom sedimentary processes. Conceivably, th e microcline gneiss represents metamorphosed arkose. Amphibolite is most abundantlyexposed along the contacts between the microcline gneiss and biotitegneiss. This indicates that it was once a sedimentary rock. Thecomposition of the amphibolite suggests formation from impuredolomitic sedimentary rocks; it could also represent metamorphosedbasalt. O ne variety of calc-silicate gneiss probably formed by re-crystallization of amphibolite, but th e other varieties may representcalcareous sedimentary layers in the predominantly noncalcareousshales and sands that formed the biotite gneiss. T he quartz gneisslayers probably represent quartz-rich sandstone beds.Granite gneiss increases in abundance southwestward across theareas seemingly largely at the expense of biotite gneiss. This wouldindicate that th e granite gneiss formed largely by replacement ofthe biotite gneisses; the ragged contacts observed in th e outcropssupport this interpretation. However, the possibilities that the observed regional change in composition represents a sedimentaryfacies change or that the rock w as injected from a magmatic sourceare not precluded. GRANITIC ROCKS

T he granitic rock units are intrusive igneous rocks and were em-placed in th e following order: Granodiorite, quartz diorite, andbiotite-muscovite granite. The granodiorite and quartz diorite occur


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24 E C O N O M I C G E O L O G Y , I D A H O SPRINGS D I S T R I C T , C O LO R A D Oas small, nearly conformable bodies, and the biotite-muscovite graniteforms small sills, phacoliths, and a few thin crosscutting dikes.

GRANODIORITESeveral small bodies of granodiorite crop out in the southeasternpart of the district. The granodiorite exposed here is fine grained

and schistose and is similar to the granodiorite found near the borders of larger bodies in the Chicago Creek area (Harrison and Wells,1959 , p. 1 2 ) . The rock is dark gray and composed of about 15 percent quartz, 40 percent oligoclase-andesine, 15 percent microcline,20 percent biotite, and 10 percent accessory minerals sphene, mag^netite, apatite, allanite, and epidote.

QUARTZ DIORITEQuartz diorite forms several small nearly conformable bodies on

the southeast side of the district. The rock is similar in geologicoccurrence and texture to the granodiorite but is darker in color.The central parts of the larger bodies of quartz diorite are darkgray, medium grained, and equigranular, whereas the margins aretypically well foliated. The massive rocks contain as much as 7 0percent combined hornblende, clinopyroxene, and biotite, as muchas 40 percent plagioclase (andesine), and generally less than 15 percent quartz. Accessory minerals sphene, apatite, allanite, pyrite,magnetite, and zircon form as much as 10 percent of the rock.Some specimens of the well-foliated rocks contain as much as 1 5percent of untwinned orthoclase.

BIOTITE-MTISCOVITE GRANITEBiotite-muscovite granite forms small sills, phacoliths, and a few

thin dikes. Only two bodies of biotite-muscovite granite are largeenough to be shown on plate 2, but some small bodies are shown onmany of the mine maps. The rock is light tan or gray, fine to medium grained, equigranular to subporphyritic, and is characterizedby abundant tabular crystals of feldspar that are as much as 1 cn iin length. Near the margins of some bodies most of the tabularfeldspar crystals and biotite books are oriented about parallel to thecontacts. The biotite-muscovite granite contains approximately 30'percent quartz, 60 percent feldspar, and less than 1 0 percent biotiteand muscovite. Microcline predominates over plagioclase, whichis mainly oligoclase.

PEGMATITIC ROCKSSeveral types of pegmatitic rocks are exposed, but not all are

readily distinguishable from the coarser parts of the granite gneiss.With one exception the two rock types are mineralogically similar,,


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TERTIARY INTRUSIVE ROCKS 25

containing abundant quartz and microcline, subordinate amounts ofplagioclase feldspar, and locally abundant biotite and magnetite.Some pegmatite dikes cut bodies of granodiorite or quartz dioriteat various angles; they probably formed late in the cooling historyof these rocks. Other dikes are similarly associated with biotite-muscovite granite. Still others cut some of the youngest Precam-brian structural features of the district.A type of pegmatite, that contains disseminated uraninite is especially abundant in an area that trends northeast from the mouth ofFall Kiver through Virginia Canyon to Seaton Mountain and perhaps beyond. This rock is of special interest, for an attempt w asmade to mine it as a source of uranium at the Highlander claim inVirginia Canyon. The pegmatite consists of quartz, microcline, andsubordinate plagioclase .(oligoclase), and as much as 20 percentbiotite. The biotite occurs as randomly oriented books, laths, andknots. The uraninite is a primary mineral in the pegmatite; it isdisseminated through the rock but is concentrated mainly in biotite.Molybdenite, galena, and pyrite are rarely present. Additional dataon the pegmatite are given by Sims and others (1963, p. 1 0 - 1 2 ) .

TERTIARY INTRUSIVE ROCKSThe Idaho Springs district contains an intricate network of por

phyry dikes and irregular plutons of early Tertiary age (pi. 1).These rocks constitute part of a belt of porphyries that extendsnortheastward across the Front Range and, together with the Tertiary mineral deposits, constitutes the Front Range mineral belt.Lovering and Goddard ( 1950 , p. 4T) inferred that the porphyriesof the eastern part of the Front Range are early Tertiary in age.The bases of their inference were ( 1 ) the presence of interbeddedvolcanic rocks in the Upper Cretaceous and Lower Tertiary (Paleo-cene) Denver and Middle Park Formations, and (2 ) the relationof porphyry intrusives in different parts of the Front Range to thechronology of Laramide orogenic movements. This age is close toapproximate absolute age of 60 million years determined on uraninite from metalliferous veins of the region (Faul, 1954, p. 263), forthe veins formed during the waning stage of igneous activity.

Spurr and Garrey ( 1 90 8 ) , Ball ( 1 90 8 ) , and Bastin and Hill( 1 9 1 7 ) made the first comprehensive studies of the porphyries inthe parts of the Idaho Springs district dealt with in their respectivereports. Lovering and Goddard ( 1950 ) utilized these previous reports to aid their investigation of the whole Front Range mineralbelt. More recently, Wells ( 1960 ) made a detailed petrographicstudy of the porphyries of the Idaho Springs and adjacent mining


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26 ECONOMIC G E O L O G Y , I D A H O SPRINGS DISTRICT, C O L O RA DOdistricts. With some modifications in terminology, Wells followedthe classifications adopted by the earlier workers, and his classification is used in this report.

Wells ( 1 960 ) described 1 3 kinds of porphyries, of which 9 areexposed in the Idaho Springs district. These rocks are separableon the basis of color, texture of the groundmass, size, shape, andabundance of the phenocrysts, qualitative and approximate quantitative mineralogy, and the character of fractured surfaces. BecauseWells gave complete petrographic descriptions of all porphyriesexposed in the region, these rocks are not described in detail here.Their salient characteristics are summarized in table 2.

The Tertiary igneous rocks were emplaced as listed in table 2, fromoldest to youngest in ascending order. This sequence shown in table2 agrees with that of Wells ( 1960, fig. 58), except that he reversedthe emplacement order of trachytic granite porphyry and quartzbostonite porphyry; however, he noted (p. 229 ) that the intersectingrelationships between these rocks do reverse locally. The sequenceof intrusion w as determined by crosscutting relations and faultingrelations, some of which were observed in the Idaho Springs district.All varieties of porphyry except the biotite-quartz latite, the youngest of the sequence, are cut by the metalliferous veins at many placesand were emplaced before the veins formed. The biotite-quartz latite,on the other hand, cuts metalliferous veins in many places.The kinds of intrusive bodies formed by the porphyries changedas time passed. Several of the older porphyries the albite gran-odiorite, light-colored granodiorite, and alkalic syenite form irregular plutons and thick dikes in the northeast corner of the district,whereas the younger porphyries tend to form thin dikes throughoutthe district. The complex patterns of intersecting dikes in thenortheast part of the district (pi. 1 ) indicate tihat composite plutonsexist at some depth. Quartz monzonite prophyry, which is intermediate in age, forms one large lens and several small concordantlenses on the south end of Bellevue Mountain as well as many dikesthroughout the district. Biotite-quartz latite, the youngest porphyry recognized, tends to form small lenticular bodies south ofClear Creek, many of which were emplaced along preexisting veins.

Except for the dikes that have been intruded along the IdahoSprings fault and the biotite-quartz latite dikes that follow veins,the porphyries appear to have been intruded along joints, not alongfaults. Characteristically, the country rock on either side of a dikehas been separated but not offset, except locally where a dike hasguided a later fault.


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28 ECONOMIC G E O L O G Y , IDAHO SPRINGS DISTRICT, COLORA DOQUATERNARY DEPOSITS

T he Quaternary deposits are composed of alluvium, colluvial creepdebris, and talus. Talus is common on the steep slopes below cliffs.Colluvial creep debris is widespread but was mapped only where itcompletely covers broad areas. T he debris sheets rarely exceed 10feet in thickness, but they effectively cover large areas of bedrock.These debris sheets are composed of a heterogeneous mixture ofangular rock fragments and fine-grained material, some of whichhas moved downhill a considerable distance. Ridges of creep debrisas high as 20 feet are common in many gullies that are flanked bydebris sheets. The ridges are probably the result of pressures created by the persistent downhill creep on both sides of the gullies.T he creep debris sheets may have formed partly in late Pleistocenetime because of the more intense frost conditions that prevailed then(Harrison and Wells, 1959 , p. 26) .Alluvium covers the floor of Clear Creek Canyon, parts of the valleys of Trail Creek and Spring Gulch, and, locally, the terraces thatare well above Clear Creek. The alluvium at the present drainagelevels consists of fine to coarse gravels, some of which is locallyderived and some of which is derived from several miles upstream.Ball (1908, p. 83-84) noted three sets of terraces near Idaho Springs;these are cut in bedrock at about 1 60 feet, 55 feet, and 25 feet respectively above Clear Creek. T he tw o higher terraces are cappedby about 20 feet of gravel, and the lower is capped by about 5 feetof gravel. T he terrace gravels are fine to coarse and contain well-rounded boulders and cobbles.Some of the gravels on the terraces or on the present valley floorsmay have been deposited in Pleistocene time by melt waters fromvalley glaciers, which are known to have existed in th e headwatersof Clear Creek and some of its tributaries. There is no evidence ofglaciation in the Idaho Springs district other than the gravel deposits.

STRUCTURET he structural framework of the Idaho Springs district is outlined by th e major units of comformable gneisses, whidh are foldedalong northeast-trending axes (pi. 2). During Precambrian timethese rocks, which now strike generally northeast, were deformed at

least twice, and may have been faulted. The first deformation waspervasive plastic folding that took place at considerable depth athigh temperatures and pressures and was accompanied by intenserecrystallization and th e emplacement of many small bodies ofgranitic rocks. The second deformation was characterized by intense granulation as well as by folding in a relatively narrow zone.


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STRUCTURE 29It apparently took place at a somewhat shallower depth throughouta 2-mile-wide zone, .termed the Idaho Springs-Kalston shear zone,which extends at least 20 miles northeastward to the margin of th eFront Eange (Tweto and Sims, 1963) . During the Laramide orogeny the rocks were jointed, intruded by a sequence of porphyriticigneous rocks, and cut by an anastomosing network of faults.T he Precambrian structure of the district was described in detailby Moench ( 1 964 ) , and the joint patterns and Precambrian structureof a larger area were described by Moench, Harrison, and Sims(1962) and by Harrison and Moenclh ( 1 9 6 1 ) . These facets of th egeology of the district are summarized here.

FOLIATION AND MNEATIONAll the gneissic Precambrian rocks and some of the granitic rocksrocks are characterized by well-developed foliation and lineation.Foliation in the gneissic rocks is expressed by compositional lay

ering in the rock and by preferred planar orientation of platy andtabular minerals. With few exceptions both features are parallel,and they are parallel to the major lithologic layers shown on plate 2.This type of foliation probably represents bedding in the originalsediments.Where the rocks were granulated by the younger Precambriandeformation, a cataclastic type of foliation formed. This foliationis characterized by a subparallel mesh of close-spaced fractures thatare healed mainly by quartz. T he cataclastic foliation typically isparallel to the older foliation described above, but locally it breaksacross it.T he granitic rocks commonly have a foliation that is termed aprimary flow structure. In th e granodiorite, primary flow structureis expressed as an alinement of elongate inclusions parallel to discordant contacts (Harrison and Wells, 1959, p. 12 ) . In the biotite-muscovite granite, tabular feldspar crystals commonly show a similar parallel alinement that has resulted from the flowage of partlycrystallized magma.Granodiorite and quartz diorite also show a secondary metamor-phic foliation that is similar in character to the foliation in th egneissic rocks. In the Chicago Creek area the secondary foliationin granodiorite is locally superposed on th e primary foliation andis continuous with the foliation in th e gneissic rocks (Harrison andWells, 1959, p. 12 ) .T he gneissic rocks of th e district are characterized by many kindsof lineation, but five categories are recognized: ( 1 ) th e axes of smallfolds and crinkles; (2 ) elongate minerals, such as sillimanite and


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30 ECONOMIC G E O L O G Y , I D A H O SPRINGS DISTRICT, COL ORAD Ohornblende, and elongate mineral aggregates, such as small pods ofgranitic material; (3) the axes of boudinage, or "pinch" structuresthat formed by stretching; (4) rodding (rod-shaped features thatresult from the rolling or shearing between layers); and (5) slicken-side striae.The lineations have systematic orientations that can be related toeach of the two Precambrian fold systems. Small folds, crinkles,and mineral alinements parallel the major folds of the older foldsystem throughout the district. Also, small warps, crinkles, boudinage, and sparse mineral alinements are oriented approximatelyat right angles to the major folds and probably formed at a latestage in the older deformation. Within the zone of younger Precambrian folding, small folds, crinkles, and sparse mineral alinements parallel the younger folds. Abundant rodding and slickensidestriae are oriented about at right angles to tfie trends of the youngerfolds; they formed by the shearing that accompanied the youngerfolding.

FOLDSThe gneissic rocks were deformed twice during Precambrian time.

The first deformation, which took place at considerable depth inthe earth's crust, w as pervasive and resulted in major folds thattrend sinuously north-northeast. These folds define the structuralframework of the district. The second deformation, which tookplace at somewhat shallower depth, folded the incompetent biotitegneisses and associated rocks along axes that trend N . 55 E. andsheared the more competent microcline gneiss. Folds and shears ofthe younger deformation, which are superposed on the older Precambrian folds, are largely restricted to the southeast half of theIdaho Springs district (pi. 2). These effects of the younger Precambrian deformation represent part of the Idaho Springs-Kalstonshear zone (Tweto and Sims, 1963, p. 998) .

The major folds of the older deformation are wide and largelyopen. Their axes trend sinuously nearly north to about N . 50 E.(pi. 2). The Idaho Springs anticline is the dominant fold in tihesoutheastern part of the district. This anticline is one of the majorfolds in this part of the Front Kange, for it is an asymmetric feature that marks the boundary between a large area of rocks thatstrike mainly northeast on the northwest side of the axis, and a largearea of rocks that strike west to northwest on the southeast side of theaxis. Tweto and Sims ( 1963) interpreted this anticline as an earlymanifestation of the Idaho Springs-Kalston shear zone. The anticlinalaxis trends' about N . 60 E. in the southern part of the area and turns to


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STEIUCTURE 31N. 45 E. in the central part; it plunges gently to moderately northeast. In the southern part of the district, the northwest limb of theIdaho Springs anticline is about 2 miles wide and dips steeplynorthwest; it is bounded on the northwest by th e Trail Creek syn-cline, a relatively small, open fold (pi. 2). In the central part ofthe district the northwest limb of the Idaho Springs anticline isabout iy% miles wide and is bounded on th e northwest by the Pewa-bic Mountain syncline northwest of which are the Bellevue Mountain anticline and the Central City anticline (pi. 2). T he four foldsin th e northwest part of the district are relatively gentle warps inrocks that are grossly flat lying. T he Central City anticline, whichenlarges northward, is the dominant fold in th e Central City district(Sims, 1964) .Although the major older Precambrian folds are open and simplein their gross aspect, some, locally, are closed and overturned. T heBellevue Mountain anticline and Pewabic Mountain syncline, forexample, are open warps near the contact between the microclinegneiss and biotite gneiss. Where these warps continue upward intothe biotite gneiss, the limbs steepen and the folds overturn to th esoutheast (pi. 2). This feature reflects th e incompetence of th ebiotite gneiss relative to the microcline gneiss- that is, the biotitegneiss flowed much more readily when it was deformed.In contrast to the generally open and simple character of the major folds, many small folds a few tens of feet wide or less areclosed and have axial plane that are subparallel .to the rock layering.Many of these are drag folds that have formed by slippage of successively higher layers toward the anticlines. M ost of the northwest-bearing small folds are open warps and apparently formed late inthe older deformation.Small folds and lineations that are related to th e major olderPrecambrian folds are of many kinds and are ubiquitous. T he axesof small folds, crinkles, and mineral alinements bear nearly northto N. 50 E., averaging about N. 25 E., parallel to the axes of themajor folds. The axes of many small folds and boudinage, however,bear northwest about normal to the axes of the major folds. Thesenorthwest-bearing lineations were observed mostly in the northwestern part of the district.T he effects of the younger Precambrian deformation are largelyrestricted to the southeast half of the district. Southeast of theboundary, which is shown on plate 2, the gneissic rocks are pervasively granulated; thin sections of most of the specimens obtainedfrom this zone show a fine network of anastomosing, subparallel


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32fractures. The biotite gneiss within this zone is completely foldedas well as granulated, whereas the microcline gneiss is granulatedbut rarely folded.

The younger folds are superposed on both limbs of the IdahoSprings anticline. They are small, have steeply dipping axialplanes, and are distinctly asymmetric. In contrast to the sinuousnortheast trend of the older folds, the younger folds trend N . 55 E.in a remarkably consistent pattern. Only a few folds exceed 1 00feet in width, but even the small ones may be traced for long distances. Their plunge is extremely variable; it ranges from nearlyhorizontal just south of Idaho Springs to steeply northeast in manyplaces to the north. The folds range in shape from structural terraces having nearly flat crests and steep northwest limbs to sharp-crested chevron folds and, locally, to nearly isoclinal folds. East-facing monoclines are locally present on the southeast limb of theIdaho Springs anticline. These shapes depend on the position ofthe minor folds on the Idaho Springs anticline and their origin maybe explained by differential movements in which the northwest sideshave been raised relative to the southeast sides (Moench. and others,196 2 ) .

Some of the large younger Precambrian folds are shown in sections on plates 2 and 3. P