Bearskin Gulch Occurrence, Alaska Peninsula District, Lake and Peninsula Borough, Alaska, USA
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Location: This site represents an area of approximately 8 square miles in T. 45 S., R. 62 W., of the Seward Meridian, covering the headwaters of both the east and west forks of Bearskin Creek (Cox and others, 1981, locality 6). The map site is plotted at an elevation of 1200 feet at the head of the east branch of Bearskin Gulch, which is at the head of Bearskin Creek. The location is accurate to within 2 miles.
Geology: The Jurassic Naknek Formation is the main rock unit exposed at the site (Detterman and others, 1981). It consists of graywacke, siliceous siltstone, hornfels, quartzite, and minor grit. The strata strike N 20 to 45 E, and dip 28 to 40 southeast. A small (1000 feet diameter) weakly-mineralized diorite porphyry stock crops out within the western fork of Bearskin Creek. The sulfide system as exposed is approximately 2 by 4 miles (Fields, 1977). Pyrite and chalcopyrite occur in highly shattered and fractured rocks throughout the area as fracture fillings, veinlets, and disseminations. Some of the better copper mineralization appears to occur within the sandstone and grit units of the Naknek Formation. The sulfide content of the sedimentary rocks varies from 3 to 10 percent with a high pyrite to chalcopyrite ratio. Within the diorite sulfide content is 1 to 2 percent. Secondary biotite is present within the copper-bearing areas and beyond. Chlorite is ubiquituous throughout the area in all rock types. Sericite occurs as veinlet selvages and within the higher sulfide areas on the periphery of the system. Other peripheral alteration minerals include epidote, kaolinite, montmorillonite, and pyrophyllite. In the eastern fork of Bearskin Creek metal values ranged from 235 to 1500 ppm copper, 0.01 to 0.13 ppm gold, 4 to 30 ppm molybdenum, 0 to 4.1 ppm silver (Fields, 1977). These values outlined an area of 1500 by 3000 feet. In the diorite located in the western fork maximum values of 800 to 900 ppm copper, 0.02 to 0.62 ppm gold, and 2 to 26 ppm molybdenum were obtained from an area of 1000 feet by 2000 feet.
Workings: Bear Creek mapped and sampled this site during the period 1975 to 1976 (Fields,1977). Maximum metal values obtained were 1500 ppm copper, 30 ppm molybdenum, and small amounts of precious metals.
Age: Tertiary?
Alteration: Secondary biotite is present within the copper-bearing areas and beyond. Chlorite is present throughout the area in all rock types. Sericite is present as veinlet selvages and within the higher sulfide areas on the periphery of the system. Other periphery alteration minerals include epidote, kaolinite, montmorillonite, and pyrophyllite.
Commodities (Major) - Ag, Au, Cu, Mo
Development Status: None
Deposit Model: Porphyry copper; porphyry copper-gold; porphyry copper-molybdenum (Cox and Sing
Mineral List
4 entries listed. 4 valid minerals.
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References
Cox, D.P., Detra, D.E., and Detterman, R.L., 1981, Mineral resource maps of the Chignik and Sutwik Island quadrangles, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF- 1053-K, 2 sheets, scale 1:250,000. Fields, E.D., 1977, 1976 Annual report: Alaska search, Chignik area-Bristol Bay region: Bear Creek Mining Company, 44 p., 22 map sheets. (Report held by the Aleut Corporation, Anchorage, Alaska.)
