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The Survey collects, catalogs, and archives data on mineral properties regardless of its views of the veracity or accuracy of those data. ~ m C > cZ GEOLOGICAL SURVEY CIRCULAR 569 .....m g..... ~ I G'> gm ::x: m n~ >..... >z o .....~ C/)m > cz .. m~ Geochemical Anomalies and >.....,.... :;; m ;iIO Metalliferous Deposits Between oc C/) C Windy Fork and Post River m ~ oC/) =i Southern Alaska Range ,C/) oC/) C -I ::x:m ;iIO Z .....> (I'> ~ ;iIO> Z G'> m I G'> om o..... nG'> >..... Geochemical Anomalies and Metalliferous Deposits Between Windy Fork and Post River Southern Alaska Range By Bruce L. Reed and Raymond L. Elliott G E 0 LOG I CAL 5 U R V Eye IRe U L A R 569 'M:Jshington 1968 United States Department of the Interior STEWART l. UDAll, Secretory Geological Survey William T. Pecora, Director Free on application to the U.S. Geological Survey, Washington, D.C. 20242 CONTENTS Page Page Abstr act --- ---- - 1 Areas of anomalous metal Introduction - - -- - - concentrations - -Continued Geologic environment- - Ar ea 3 --- - - -- - 4 Sedimentary rocks 1 Area 4 - - 5 Igneous rocks - 2 Area 5 --- 5 Structure ----- 2 Area 6 - 5 Geochemical data --- 2 Area 7 - - - 5 Areas of anomalous metal concentrations - 3 Age of mineralization -- - 5 Area 1 - - 3 Summary and conclusions 8 Area 2 -- --- - - - ------ - - - - 4 References cited - --- - - 8 ILLUSTR ATIONS Page Figure 1. Index map showing report area and location of McGrath A-2, A-3, B-2, and B-3, quadr angles -------- - ---- - - - --------- ------- - - - - 11 2. Reconnaissance map showing generalized geology between Windy Fork and Post River, southern Alaska Range - -- - - - - - -- - -- - - ---- - -- - - - - 12 3. Map showing location of stream-sediment samples between Windy Fork and Post River, southern Alaska Range - -- --- - - -- -- -- - -- - -- -- - -- 14 4-9. Maps of- 4. Copper distribution showing location of samples with 100 ppm (parts per million) or greater ------ -- ------ -- -- -- - - ------ 15 5. Lead distribution showing location of samples with 50 ppm or greater --- - - - 16 6. Molybdenum distribution showing location of samples with 5 ppm or greater --- 17 7. Nickel distribution showing location of samples with 100 ppm or greater --- - - 18 8. Silver distribution showing location of samples with 1 ppm or greater - ----- 19 9. Zinc distribution showing location of samples with 200 ppm or greater - 20 10. Map showing areas with anomalous metal concentrations in stream- sediment samples - - -------- - - -- -- - - ----- - ------------ 21 11. Map of Bowser Creek showing location of stream-sediment samples and sulfide mineral deposits --- - - -- - - --- - ---- -- - -- -- ----- --- - 22 TABLES Page Table 1. Semiquantitative spectrographic analyses and gold analyses of bedrock samples from the Windy Fork-Post River area - - - - -- - - - -- - - -- ----- - - -- 6 2. Partial analyses of bedrock samples from areas 3, 4, and 5 - - - - - --------- 8 III Geochemical Anomalies and Metalliferous Deposits Between Windy Fork and Post River Southern Alaska Range By Bruce l. Reed and Rqymond l. Elliott Abstract the major north-south valleys (Kuskokwim River, Post River, Sheep Creek, and Windy Fork) in the winter and early spring. Small, properly equipped aircraft can Geochemical sampling between Windy Fork and Post River, land on gravel bars along the Post and Kuskokwim southern Alaska Range, defines seven areas with an 0 m a lou s concentrations of lead, zinc, silver, copper, or molybdenum 10 R i v e r s and Windy Fork, and on S mit h, Post, and stream-sediment samples, Five of these areas contain metal­ Valeska Lakes. liferous deposits that are spatially related to centers Of. hypabyssal igneous bodies, The deposits occur as replacement bodIes m I1me­ The region is in rugged terrain. Relief increases stone and as replacement bodies and fracture flllmgs m 10truslve from north to south and averages between 2,000 and rocks. Most of the deposits that crop out are small and dIscon­ 3,000 feet; peaks in the southern part of the region tinuous but the number of occurrences with locally high silver crest between 6,000 and 7,700 feet in height. Timber content: their possible continuity beneath surficial deposits, and a is sparse and consists chiefly of spruce and scattered favorable geologiC environment suggest that thIS regIOn has an economic potential for base and precious metals. cottonwood that are restricted to the lower altitudes along the major north-south valleys. The only previous geological investigation in this INTRODUCTION region is that made by Brooks (1911) who traversed A new region of lead, zinc, and silver mineralization the Alaska Range via the South Fork of the Kuskokwim was ide n t i fie d in the summer of 1967 during field River in 1902. The present report describes the min­ geologic evaluation of the southern Alaska Range as eral occurrences and res u Its of stream-sediment part of the U.S. Geological Survey's Heavy Metals sampling in part of the region that was covered during Program. One lead~zinc-silver deposit in this region the 1967 field season. Between 15 minutes and an hour has already been described (Reed and Elliott, 1968). were spent e x ami n i n g each of the metalliferous The seven geochemical maps accompanying this report deposits described in this report; additional mapping delineate other areas of anomalous concentrations of and physical exploration are necessary to evaluate lead, zinc, silver, copper, molybdenum, and nickel in these occurrences. stream- sediment samples. Known min era I occur­ rences in these areas are briefly described. GEOLOGIC ENVIRONMENT The region described in this report is about 150 The part of the Alaska Range considered here is miles northwest of Anchorage, on the west side of the made up of Paleozoic sedimentary rocks intruded by upper reaches of the Sou t h FO,rk of the Kuskokwim granitic stocks, hypabyssal breccia bodies, and many River. The region covers about 560 square miles and felsic and mafic dikes. Tertiary extrusive rocks, in­ includes parts of the McGrath A-2, A-3, B-2, and cluding mafic flows and pyroclastic d ep 0 sit s with B-3 quadrangles (fig. 1). It lies south of Farewell, a interbedded sedimentary units, locally overlie the Federal Aviation Agency Station that m a i n t a ins a Paleozoic rocks. Mineral deposits are spatially related 5,000-foot gravel runway. McGrath, about 62 air miles to the intrusive bodies and ,occur as replacement de­ northwest of Farew'ell, is the nearest source of gasoline posits in limestone or in the intrusive rocks. and supplies. The region described herein is best reached by heli­ Sedimentary Rocks copter. The only road in the region is a jeep trail The P a leo z 0 i c rocks consist predominantly of which extends south from the Farewell airfield along interbedded shale, siltstone, and argillite, but they Sheep Creek for about 5 miles. From this point a four­ include prominent units of thin to massively bedded wheel-drive vehicle with large tires can continue south gray limestone. These rocks sh9w little or no effect along the creek for an additional 6 miles. This, how­ of regional metamorphism. Because of the strati­ ever, necessitates crossing the creek several times, graphic and structural complexity of the region, the and crossings can be accomplished only during periods sedimentary rocks are shown in figure 2 as one unit, of low water. Travel by tracked vehicle is possible in with the exception of prominent limestone units. The limestone units shown locally in the southeast part The north- to northeast-trending structural features of figure 2 may be Ordovician in age, whereas the are truncated to the north by the Farewell fault zone, limestone north of the Farewell fault is believed to a major right-lateral strike-slip crustal feature that be of Devonian age. Graptolites, including species of is part of the Denali fault s y s t e m (Grantz, 1966). Glyptograptus and Climacograptus of Middle or Late Immediately south of the fault zone, folds are dragged Ordovician age (Michael Churkin, oral commun., 1968), to the east -northeast, and the fault zone is broken into were found in carbonaceous shale at localities F -1 and numerous small segments. Small, h y d rot her mall y F -2 in figure 2. At Saint Johns Hill, north of the Fare­ altered, slightly serpentinized bodies of ultramafic well fault (locality F - 3, fig.