PMRGX-21 Prospect (Mud Bay), Alaska Peninsula District, Aleutians East Borough, Alaska, USA

This site is located on state land.
Location: This prospect is located on the Alaska Peninsula approximately 4 miles southwest of the head of Mud Bay on Port Moller and about 5.5 miles east of the head of Herendeen Bay. The map site is at an elevation of 1,550 feet, 0.2 mile southwest of the top of hill 1750, in the SE1/4 of sec. 26, T. 51 S., R. 73 W., of the Seward Meridian. It is referred to as PMRGX-21(Mud Bay) in Wilson and others (1988, locality 86). The location is accurate to within 500 feet.
Geology: This prospect was first staked by Resource Associates of Alaska in 1980. They outlined a mineralized system 3.5 miles long by as much as 1 mile wide that trends east-west. The system is hosted by sedimentary rocks of the Eocene to Oligocene Stepovak Formation (Wilson and others, 1995). The rocks are cut by dikes and sills varying from andesite to rhyodacite (Trujillo and others, 1982). The mineralized system is intensely silicified and is marked by widespread pyrite that occurs as fracture fillings and as disseminations. A carbonate-chlorite alteration halo as wide as 800 feet surrounds the silicified zone, which is characterized by irregular quartz-sericite masses and apparently randomly distributed quartz veins. The silicified zone also contains local zones of argillic alteration, especially adjacent to dikes and sills and to quartz-sulfide veins. The best mineralization found by Trujillo and others (1982) was along the northern edge of the silicified zone, where there are sulfide-bearing quartz veins a few inches to as much as 2 to 3 feet wide. These veins are mostly in the silicified zone and are as much as 300 feet long. They contain massive galena, coarse-grained sphalerite, blebs, and stringers of chalcopyrite, and rare tetrahedrite. A 13.8-foot channel sample assayed 15.4 ounces of silver per ton, 0.46 percent copper, 5.73 percent lead, and 4.37 percent zinc. Selected samples of 1- to 3-foot-wide veins assayed 0.01 to 0.098 ounce of gold per ton, 23.7 to 160 ounces of silver per ton, 0.2 to 0.4 percent copper, 2.9 to 5.2 percent lead, and 0.8 to 12.3 percent zinc (Trujillo and others, 1982). Other types of mineralization in the silicified zone include stockworks of galena-calcite veins as much as 1 inch thick, and pyrite disseminated in fractured rocks. The latter type contains only pyrite but assayed as much as 1.54 ounces of silver per ton. Disseminated sulfides in the carbonate-chlorite alteration zone also carry silver. Soil geochemistry indicates mercury enrichment in the carbonate-chlorite zone adjacent to the silicified zone. Gold anomalies occur sporadically and are most common in the area of mercury enrichment. Some base metal anomalies are related to dikes or dike swarms in the silicified zone. In 1984 Freeport Exploration Drilling Company geologists outlined an apparent eastward extension of the deposit. It was their opinion, however, that the rocks were hornfelsed rather than silicified. They were discouraged by this fact and by what they described as weak geochemisty and alteration and the consistently narrow veins.
Workings: Resource Associates of Alaska explored this prospect from 1980 to 1982. During that time they mapped and hand trenched the prospect, collected 222 rock and 423 soil samples, and conducted VLF-EM and magnetic surveys. Selected samples of 1- to 3-foot wide veins assayed 0.01 to 0.098 ounce of gold per ton, 23.7 to 160 ounces of silver per ton, 0.2 to 0.4 percent copper, 2.9 to 5.2 percent lead, and 0.85 to 12.3 percent zinc. In 1984 Freeport Exploration Drilling Company did additional mapping and trenching and collected 143 rock and 47 soil samples. Six rock samples collected by the U.S. Geological Survey in the mid-1980s were anomalous in copper, lead, zinc, silver, arsenic, bismuth, and antimony (Wilson and others, 1988).
Age: Eocene or younger.
Alteration: Alteration at this prospect includes silicification, argillization, sericitization, and carbonate-chlorite replacement.

Commodities (Major) - Ag, Cu, Pb, Zn; (Minor) - As, Bi, Cd, Sb
Development Status: None
Deposit Model: Polymetallic veins (Cox and Singer, 1986; model 22c)

References:
Anderson, G.L., Butherus, D.L., Fankhauser, R.E., Pray, J.C., Lindberg, P.A., and Hoffman, B.L., 1980, Exploration and evaluation of lands leased from Aleut Native Corporation 1980: Resource Associates of Alaska Report, 84 p, 16 map sheets, various scales. (Report held by the Aleut Corporation, Anchorage, Alaska.) Butherus, D. L., Gressitt, E.E., Pray, J., Corner, N.G., Lindberg, P.A., Fankhauser, R.E., 1979, Exploration and evaluation of the Aleut Native Corporation lands: Resource Associates of Alaska Report, 69 p, 90 map sheets, various scales. (Held by the Aleut Corporation, Anchorage) 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., Harris, E.E., and Bailey, E.A., 1987, Preliminary analytical results and sample locality map for rock samples collected in 1985 and 1986, Port Moller and Stepovak Bay quadrangles, Alaska: U.S. Geological Survey Open-File Report 87-497, 44 p., 1 plate, 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.