Bearskin Gulch

Occurrence, Inactive

Commodities and mineralogy

Main commodities Ag; Au; Cu; Mo
Ore minerals chalcopyrite; gold; molybdenite; pyrite

Geographic location

Quadrangle map, 1:250,000-scale CG
Quadrangle map, 1:63,360-scale B-3
Latitude 56.25
Longitude -158.95
Nearby scientific data Find additional scientific data near this location
Location and accuracy 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.

Geologic setting

Geologic description

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 ubiquitous 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.
Geologic map unit (-158.95202090419, 56.2492111635414)
Mineral deposit model Porphyry copper; porphyry copper-gold; porphyry copper-molybdenum (Cox and Singer, 1986; models 17, 20c, 21a).
Mineral deposit model number 17, 20c, 21a
Age of mineralization Tertiary?
Alteration of deposit 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.

Production and reserves

Workings or exploration 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.
Indication of production None

Additional comments

This site is on lands selected by the Bristol Bay Native Corporation.

References

MRDS Number A012447
Reporters S.H. Pilcher (Anchorage)
Last report date 2/8/2000