REC'D. JUN 13 El63 - Alaska

UNITED STATES DEPARTMENT OF THE INTERIOR

GEOLOGICAL SURVEY

REC'D. COLLEGE

JUN 13 El63 DIV. MINE6 & GZOLOGY

GEOCHEMICAL MAPS OF AN AREA NORTHWEST OF THE CHULITNA

RIVER, CENTRAL ALASKA RANGE

BY

C. C . Hawley and A l l e n L. Clark

Open-File Report

1969

This report is preliminary and has not been e d i t e d or reviewed for conformity with Geological Survey standards

.GEOCREMICAL MAPS OF AN AREA NORTIIWEST OF THE CHULITNA

RIVER, CENTRAL ALASKA RAWGE

By C. C. Hawley and Allen L. Clark

ABSTRACT

An area northwest of the Chulitna River i n west-central Alaaka Range loca l ly shims loca l anomalous concentrations of gold, s i l v e r , a rsenic , copper, zinc, and lead i n stream-sediment samples. Most stream sediments showing anomalous concentrations of metals can be corre la ted with e i the r known or newly discovered deposits o r occur- rences described i n Circular 617.

The s i x geochemical maps of t h i s repor t show the composition of stream-sediment samples from an area northwest of the Chulitna River, described i n Geological Survey Circular 617 by Hawley and others (1969).

The area is i n the western pa r t of the cent ra l Alaska Range ( f ig . I), and it includes the mineralized a rea described by Capps (1919), Ross (1933) , and Hawley and Clark (1968) . It has one well-known lode deposit--the Golden Zone.

GEO(;fIFMISTRY AND GEOCClXMZCAL DATA

The main mineral deposits and occurrences of the area a r e charac- ter ized by arsenic , copper, and gold and subordinately by zinc, s i l v e r , lead, t i n , bLsmuth, and antimony. Molybdenum occurs local ly . Of the elements l f s t e d above, six-copper, a rsenic , gold, zinc, s i l v e r , and lead--are su f f i c i en t ly abundant i n stream-sediment samples t o be pa r t i - cu la r ly useful i n geochemical prospecting. Arsenic, zinc, and s i l v e r have high l imi t s of analyt ica l detection r e l a t i v e t o t h e i r abundance bu t w i l l be more useful for geochemical prospecting i n the area with ana ly t i ca l methods capable of detecting smaller concentrations.

Concentrations of metals considered anomalous i n stream-sediment samples were determined with reference t o the mean concentration estimated from frequency d i s t r ibu t ion diagrams ( f igs . 2 and 3). Listed

below a re the mean concentrations, the lower l i m i t of ana ly t i ca l detection, and concentrations considered moderately and s t rongly anomalous.

Table 1.--Geochemical data on the concentration of copper, a rsenic , gold, zinc, s i l v e r , and lead i n stream-sediments, calculated from a group of about 165 samples.

Parts pe r d l l i o n Moderately Strongly Analytical

Approximate anomalous anomalous l i m i t of mean concentration concentration determination

Ai3 ( 0.5 0.5 2 0.5 Bs < 200 I f detected 200 200 Au 4 .02 .02 .07 .02 Cu 70 150 200 5 Pb 15 30 50 10 Zn (200 200 300 200

Reading from the top of the diagrams shows t h a t a concentration charac- ter ized as strongly anomalous w a s found i n 5 percent or l e s s of the samples. These concentrations are shown as a bull 's-eye pat tern on the geochemical maps ( f igs . 4, 5, 6 , 7, 8, and 9) . Concentrations characterized as moderately anomalous were found i n about 5 t o 15 per- cent of the samples; these concentrations a r e sham as so l id or i n some cases ha l f - f i l l ed c i r c l e s on the maps.

GEOCHEMICAL MAPS

Analytical data on the stream-sediment samples are shown i n the following s i x maps, commented on b r i e f l y below:

Besides showing the d i s t r ibu t ion of copper, f igu re 4 shows the location of known mines and prospects and of newly iden t i f i ed occur- rences. It a l s o shows the approximate locat ions of two major f a u l t s and two areas containing mineralized rocks described i n Circular 617.

Arsenic and antimony

Arsenic and antimony a r e considered anomalous i f they a r e detected by the spectrographic method used. Arsenic was detected i n about 18 percent of the samples from the area, but antimony in l e s s than 6 per- cent.

The highest a r sen i c concentrations, exceeding 1,000 ppm, a r e i n the Golden Zone mine area ( f ig . 5). A concentrat ion of 700 ppm arsenic found i n upper Pa r t i n Creek is re la ted t o the P a r t i n Creek copper occur- rence ( f i g . 4). A l l antimony-bearing sites a re i n the southern p a r t of t he area; the sites i n Pa r t i n and Ohio Creeks a r e near known mineral- ized rocks, but those i n a t r ibutary t o McCallie Creek a r e of unknown or ig in .

Gold

Gold, l i k e a r sen i c and antimony, is anamalous i f detected even a t the minimum concentrat ion (0.02 ppm). The a r eas near anomalous sample sites i n upper Long Creek and Coal Creek a r e not near knmn occurrences and should be prospected ( f ig . 6) .

Zinc and s i l v e r

Zinc and s i l v e r a r e shown on the same map ( f ig . 7). The Coal Creek area shows the highest s i l v e r content (3 ppm) of any area where t he exis tence of nearby mineralized rocks is current ly unknown.

Lead

Lead ( f i g . 8) i s not s trongly enriched i n most depos i t s of the region, but an area centered approximately about t h e Golden Zone mine ( f i g s . 4 and 8) s h w s r e l a t i ve ly high concentrat ions of lead i n stream sediments.

Molybdenum, t i n , and tungsten

These t h ree elements were detected i n only a few stream-sediment samples although, a s discussed i n Circular 617 (Hawley and o thers , 19691, t i n occurs i n unusual amounts i n severa l mineral occurrences of the area . Tin was detected i n stream-sediments i n the Ohio Creek drainage where i t was found below tin-bearing gre isen i n a g ran i t e s tock and a t Canyon Creek where i t is known t o occur i n arsenopyrite- r i ch ve ins and o ther mineralized rocks. The map a l s o shows the loca- t i o n of two o ther g r a n i t e plutons which contain t i n . The t i n occurrence near the West Fork i s i n a small copper-bearing zone near the contact of a s tock. South of Costel lo Creek abnormal amounts of t i n (30 ppm) were detected with beryllium and s i l v e r i n a small and apparently umaineralized g ran i t e plug.

REFERENCES CITED

Capps, S. R . , 1919, Mineral resources of the Upper Chuli tna region [Alaska]: U.S. Geol. Survey Bull. 692, p. 207-232.

Hawley, C. C. , and Clark, A. L . , 1968, Occurrences of gold and o ther metals i n t h e Upper Chuli tna d i s t r i c t , Alaska: U.S. Geol. Survey Circ. 564, 21 p.

Kawley, C. C., Clark, Allen L. , Herdrick, M. II., and Clark, S. H. B. , 1969, Results of geologica l and geochemical i nves t iga t ions i n an area northwest of the Chulitna River, Alaska, 1968: U.S. Geol. Survey Circ. 617.

Ross, C. P., 1933, Mineral depos i t s near the West Fork of the Chuli tna River, Alaska: U.S. Geol. Survey Bull. 8 4 9 4 , p . 289-333.

Molybdenum occurs i n t r a c e amounts a t s i t e s a l s o general ly anoma- lous i n o ther metals. On lower Long and Copeland Creeks and i n a t r i bu t a ry t o Colorado Creek, i t is i n samples which a l s o contain de tec t - ab le t o moderately anomalous amounts of s i l v e r , z inc , and arsenic.

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