supplement to u. s. geological survey professional the ajo ... · supplement to u. s. geological...

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 fanglomerate 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.

U.S

. D

EP

AR

TM

EN

T

OF

T

HE

IN

TE

RIO

R

GE

OL

OG

ICA

L

SU

RV

EY

PR

OF

ES

SIO

NA

L

PA

PE

R

20

9

RE

VIS

ED

P

LA

TE

23

V

20

00

'

10

00

'" 10

15

20

E

E

XP

LA

NA

TIO

N

1 K

cv

-

Qol

"" \

^-*

"'' -"

\ ^

Jem

. *"

/--'' \

/ ,'''

--"'

! fr''T

cm

/ '/'

Kcv

/ / J

-'''

^-1

Kcv

/,

'?

^-'~

i

/'"''" K

cv

^ - "

Tlf

tern

^- \

('

T

. ^-

Tcm

\

/'

. ^'" K

cv

/X

^

^/

Tcm

i i

- -

RE

VIS

ED

S

EC

TIO

N

TH

RO

UG

H

CO

OR

DIN

AT

E

B

0

000

10

00

F

EE

T1

i j

Al

Loco

motiv

Corn

elia

q

(Mo

Co

rne

lia

q (D

ioritic

Co

nce

ntr

Qa

l]

luviu

m

Tlf

e fa

ng

lom

era

te

Tcm

uo

rtz

monzo

nite

in

fa

cie

s )

Ted

uart

z

mo

nzo

nite

bord

er

facie

s)

Kcv

ato

r

]vo

lca

nic

*

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

-- -

f ..- '

g^ss^S^- v XV VS. v s \ \ x

Sea level

3000' -I

2000

1000

Sea level

Gibson Arroyo

Cornelia Arroyo

Tlf QalQal Kcv

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

TedQol

Tlf

1000

Tcm

TcmKcv

L_____I______L.

REVISED SECTION THROUGH COORDINATE 9

2000,V

1000'

Qol

Tcm

_J_______I_______L.

-i i i r

Tcm

Kcv

Ted

Tcm

Kcv

Ool


i r-D

Tlf

-F EXPLANATION

Alluvium

Locomotive fanglomerale

Cornelia quartz monzonite (Main facies)

Cornelia quartz monzonite (Dioritic border facies)

Concentrator volcanics

2000-D -F

i r

Ool

IOOO

Tcm

Tcm

Kcv

I I_______L_

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

2000,v -i i r

Ool

1000'

Tern

Kcv

i r B -P

Tern

I______L


2000"-D

Kcv

IOOO

I I 1_

Tcm

i : r

J____i_____I

Kcv

I_____I_____L

-i r

Qal

Tlf

Kcv

Qol


20001"KCV

IOOO

J___I I

-i i i r

Kcv

Tcm

i i L

Kcv

J_______I______I_______L_

Kcv

Tcm

J____L

Ool-

Tlf

J_____i_____L

-F

EXPLANATION

Al luvium

Locomotive fonglomerote

Cornelia quartz monzonite (Main facies)

Cornelia quartz monzonite (Dioritic border facies)

Concentrator volcanics

-F


REVISED SECTIONS, COORDINATES 12, 13, AND 14

IOOO 2000 3000 FEET

supplement to u. s. geological survey professional the ajo ... · supplement to u. s. geological...

Documents