Four Bear; PMRGX-28 Prospect, Alaska Peninsula District, Aleutians East Borough, Alaska, USA
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Location: The Four Bear prospect is located on the Alaska Peninsula approximately 5 miles southeast of Mount Dana. The map site is at an elevation of about 2,250 feet, near the center of sec. 8, T. 53 S., R. 77 W., Seward Meridian. It is referred to as PMRGX-28 in Wilson and others (1988, locality 76). The location is accurate to within 1,200 feet.
Geology: At the Four Bear prospect, siltstone, shale, sandstone, and conglomerate of the Upper Cretaceous Hoodoo Formation are cut by intrusive rocks that include diorite and rhyolite-rhyodacite quartz porphyry (Wilson and others, 1995). The diorite exhibits propylitic, argillic, and sericitic alteration. The younger rhyolite-rhyodacite quartz porphyry forms the central part of the prospect area. It contains as much as 20 percent disseminated pyrite and is pervasively sericitized. Locally, it is silicified and contains small stockworks of thin pyrite veins. Quartz and quartz-calcite veins 0.25 to 2 inches wide occur in widely scattered shears (Freeport Exploration Company, 1985). Pebble and breccia dikes occur in the intrusives and in the surrounding sedimentary rock. Opal and sphalerite occur in the dikes. Tourmaline is present in a breccia pipe. The sedimentary rocks are intensely hornfelsed and iron stained up to 1,000 feet from the intrusive rocks. Resource Associates of Alaska outlined a gold soil anomaly area of approximately 1,000 by 2,000 feet and an arsenic soil anomaly area of 3,000 by 3,500 feet (Trujillo and others, 1983). Their soil samples contained as much as 0.21 ppm gold, and their rock samples contained as much as 0.08 ppm gold, 5.5 ppm silver, 273 ppm copper, 450 ppm lead, 544 ppm zinc, 595 ppm arsenic, and 0.08 ppm mercury. Freeport Exploration Company collected six rock samples that had anomalous gold values. The tourmaline-bearing breccia contained 0.11 ppm gold and 1.4 ppm silver.
Workings: In 1983, Resource Associates of Alaska mapped the prospect and collected 87 rock and soil samples. The highest metal values in their rock samples were as much as 0.08 ppm gold, 5.5 ppm silver, 273 ppm copper, 450 ppm lead, 544 ppm zinc, 595 ppm arsenic, and 0.08 ppm mercury. In 1984, Freeport Exploration Company mapped the prospect and took 41 rock and 106 soil samples. Six samples contained anomalous gold values.
Age: Tertiary.
Alteration: Alteration ranges from propylitic to argillic to sericitic, along with local silicification.
Commodities (Major) - Ag, Au, Cu, Pb, Zn; (Minor) - As, Hg
Development Status: None
Deposit Model: Porphyry Cu, Porphyry Cu-Au (Cox and Singer, 1986; models 17, 20c)
Mineral List
6 entries listed. 5 valid minerals.
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References
Angeloni, L.M., Wilson, F.H., and Sutley, Stephen, 1985, Map and tables showing preliminary rock geochemical data, Port Moller, Stepovak Bay, and Simeonof Island quadrangles, Alaska: U.S. Geological Survey Open-File Report 85-470, 179 p., 1 sheet, scale 1:250,000. Freeport Exploration Company, 1985, 1984 report of activities, Canoe Bay joint venture: Freeport Exploration Company, 25 p. (Report held by the Aleut Corporation, Anchorage, Alaska.) Trujillo, R.S., Tapper, C.J., Alvarez, T.J., Porterfield, Ben, and Toupe, W.M., 1982, Exploration and evaluation of precious metal potential of portions of Aleut Corporation lands, southwest Alaska 1982: Resource Associates of Alaska unpublished report, 91 p, 33 maps, various scales. (Report held by the Aleut Corporation, Anchorage, Alaska.) Wilson, F.H., Detterman, R.L., Miller, J. W., and Case, J.E., 1995, Geologic map of the Port Moller, Stepovak Bay, and Simeonof Island quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigation Series Map I-2272, 1 sheet, scale 1:250,000. Wilson, F.H., White, W.H., and DuBois, G.D., 1988, Brief descriptions of mines, prospects, and mineral occurrences in the Port Moller and Stepovak Bay quadrangles, Alaska Peninsula: U.S. Geological Survey Open-File Report 88-666, 128 p., 1 plate, scale 1:250,000.
