SUPPLEMENT TO U. S. GEOLOGICAL SURVEY PROFESSIONAL
THE AJO MINING DISTRICT, ARIZONA
by James Gilluly
In 1928 two diamond drill holes were put down on the Bluestone claims south of the New Cornelia pit at Ajo, Ariz. During the prepara tion of Professional Paper 209, "The Ajo min ing district, Arizona," Mr. Hoval A. Smith offered to make available to the U. S. Geological Survey the records of these drill holes. Unfortunately the information did not reach the author. Again in 1948, after the publication of the Ajo pro fessional paper, Mr. -Smith renewed his offer tc make the drill records available to the Survey, These holes furnish data that necessitate some modification of the inferred position of the ero sion surface on which the Locomotive fanglomer- ate was deposited. Accordingly, this supple mentary note has been prepared. In addition, the following revised sections are included: sections B-B' and C-C on plate 20; sections through coordinates 9 to 14 on plate 22; and the section through coordinate B on plate 23. Also a new map, plate 2lA, has been prepared to show structure contours .on the base of the Locomo tive fanglomerate and the location of the two drill holes on the Bluestone claims.
The cores of these drill holes were exam ined in October 1948 by C. A. Anderson and N. P. Peterson of the Geological Survey, and they report mineralized bedrock below the Loco motive fanglomerate. Hole No. 1 is 2,253 feet deep, and the upper 1,492 feet is in fanglomer ate. Hole No. 2 is 2,050 feet deep, and the upper 1,668 feet is in fanglomerat'e. The rock beneath the fanglomerate is highly silicified and sericitized, and locally brecciated. The upper part of the bedrock is oxidized and contains some native copper, limonite, and a few seams of chalcocite. At depths 200 feet below the bedrock surface, primary pyrite and chalcopyrite show no signs of enrichment or oxidation.
Most of the bedrock resembles the Con centrator volcanics, and examination of repre sentative thin sections of the cores confirms this identification. In hole No. 1, at a depth of 2,000 feet, specimens of core resemble diorite porphyry, and this rock may be related to the
monzonite intrusive bodies; ^hs^£ver, thisis so altered hydrothermally as totTTcTke specicdetermination impossible.
The bedrock at these depths, beneath the Locomotive fanglomerate, may be part of an upfaulted block or of a buried hill which, after the tilting of the fanglomerate, would now appear as a more gently sloping basal surface of that formation. Although faults have been mapped at intervals along the northern contact of the Loco motive fanglomerate, no evidence has been found for a continuous northern fault contact, and the continuity of geologic boundaries north of the fanglomerate shows that no large fault can there exist. Movement along a fault near the south west corner of section 27 (pi. 20) has brought the Concentrator volcanics against the Locomotive fanglomerate, but this fault seems to swing southeastward where it can be traced last near the boundary between sections 27 and 34. Hence it would have no effect on the altitude of the base of the fanglomerate as intersected in holes 1 and 2. Overlap of fanglomerate on bedrock is very evident where the contact is exposed. This is shown by the wedging out of the lower beds of the fanglomerate westward along the contact. The combined evidence now available from the drill holes in fanglomerate in this area (see pi. 21 A) suggests strongly that the erosion surface on which the fanglomerate was deposited not only has. irregularities but also, locally, has a relatively gentle slope. The presence of the bedrock in holes 1 and 2 at a higher elevation than would be inferred from the projection of the erosion surface is evidence that the irregular ities, so evident along the strike, also exist down the dip. Although a concealed fault may exist and account for the relations, no fault of adequate displacement has been found.
Possibly future exploration will reveal beneath the Locomotive fangiomerate other areas where the bedrock is relatively near the surface. Some of them may be underlain by bedrock suf ficiently mineralized in copper and close enough to the surface to constitute minable ore.
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CRETACEOUS(T) TERTIARY
QUATERNARY
U.S. DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY PROFESSIONAL PAPER 209
REVISED PLATE 20
Cornelia Arroyo
zooo'-i
1000 -
Sea level
NEW CORNELIA MINES
-- -
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g^ss^S^- v XV VS. v s \ \ x
Sea level
3000' -I
2000
1000
Sea level
Gibson Arroyo
Cornelia Arroyo
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s. \ V \ X \ V\
2000'
1-1000
^Sea level
REVISED SECTIONS B - B 1 AND C-C', AJO MINING DISTRICT, ARIZONA
1000
9000 FEET
EXPLANATION
Alluvium Batamote andesite Hospital porphyry Ajo volcanics (~\Q\,tuffaceous member)
Tlf,Locomotive fanglomerate
Feldspathic andesite porp hyry
Cornelia quartz
monzoni te
(Main fades)
Cornel i a q uartz
manzan ite
(Dioritic border fades)
Hornblende andesite
dikesConcentrator volcanics Cardigan gneiss
U.S. DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY
PROFESSIONAL
PLATE
209
E XPLANATION
Contact between Locomotive fanglomerate and older rocks
Fault , showing dip
Contours on base of Locomotive fanglomerate
Drill holes, also shown on key map, Plate 22 , with altitude of bedrock surface
Drill holes, Bluestone claims, with altitude of bedrock surface
22 23 24 25X'60
Coordinates 2 Key map
Plate 22
14 15 16 17 18 19 20 . 21
/ / / / /Locomotive
fang lomerate
Coordinate B Key map PI. 22
Coordinate -D Key map PI. 22
1000
R.6 W. H2°/92' '
MAP SHOWING CONTOURS ON BASE OF LOCOMOTIVE500 0 ________1000
FANGLOMERATE4000 FEET 59583
U.S. DEPARTMENT OF THE INTERIOR
GEOLOGICAL SURVEY
2000'
PROFESSIONAL PAPER 209 REVISED PLATE 22
SHEET I OF 2 SHEETS
B -D -F
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REVISED SECTION THROUGH COORDINATE 9
2000,V
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REVISED SECTION THROUGH COORDINATE 10
i r-D
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-F EXPLANATION
Alluvium
Locomotive fanglomerale
Cornelia quartz monzonite (Main facies)
Cornelia quartz monzonite (Dioritic border facies)
Concentrator volcanics
2000-D -F
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REVISED SECTION THROUGH COORDINATE II
REVISED SECTIONS, COORDINATES 9, 10, AND II
IOOO 2000 3000 FEET 59583
U.S DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY
PROFESSIONAL PAPER 209 REVISED PLATE 22
SHEET 2 OF 2 SHEETS
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-F
EXPLANATION
Al luvium
Locomotive fonglomerote
Cornelia quartz monzonite (Main facies)
Cornelia quartz monzonite (Dioritic border facies)
Concentrator volcanics
-F
REVISED SECTION THROUGH COORDINATE 14
REVISED SECTIONS, COORDINATES 12, 13, AND 14
IOOO 2000 3000 FEET