Pyramid Prospect, Alaska Peninsula District, Aleutians East Borough, Alaska, USA

This prospect is on land selected or patented by, or interim-conveyed to, the Aleut Corporation.
Location: This prospect is located on the Alaska Peninsula on the southeast flank of Pyramid Mountain approximately 4 miles northwest of Balboa Bay (Christie, 1974, color anomaly 90; MacKevett and Holloway, 1977, locality 6; Nokleberg and others, 1987, locality AP6; Wilson and others, 1988, locality 6; Young and others, 1997, locality 36). The map site is at an elevation of about 1,200 feet, in the S1/2 of sec. 35, T. 52 S., R. 74 W., of the Seward Meridian. The location is accurate.
Geology: The Pyramid copper-molybdenum deposit is associated with a quartz diorite pluton that cuts Paleocene to Eocene sedimentary rock of the Tolstoy Formation (Wilson and others, 1995). The stock has been dated at 6 million years (Wilson and others, 1996). The prospect is a classic copper-molybdenum deposit having a potassic core surrounded by a phyllic zone and an outer propylitic zone. The central potassic core contains secondary biotite after mafic minerals and 2 to 10 percent magnetite as fracture fillings and disseminated clots and grains. The core, entirely in the intrusive, is roughly 800 by 1,700 feet. It is essentially barren, with total sulfide content less than 0.25 percent and copper and molybdenum values in the 0.00X percent range (Christie, 1975). The phyllic zone is characterized by sericite, quartz, and andalusite in a zone surrounding the potassic core. It occurs mostly in the intrusive rocks and measures 700 to 2,500 feet in width. The propylitic zone is characterized by chlorite, magnetite, epidote, and calcite. The highest total sulfide content (5 to 10 percent) occurs in the inner part of the propylitic zone and outer part of the quartz-sericite zone. Pyrite:chalcopyrite ratios are 50:1 or greater and are associated with copper grades of 0.15 percent or less (Christie, 1975). Toward the inner part of the phyllic zone pyrite:chalcopyrite ratios decrease and copper grades increase to 0.3 to 0.4 percent; molybdenum grades are 0.03 to 0.04 percent. In this copper-rich part of the system the sulfides occur as disseminations and thin fracture fillings. The deposit has been oxidized to depths of 0 to 450 feet and exhibits a blanket of secondary copper enrichment as much as 300 feet thick. The enriched zone contains chalcocite and covellite, as well as some chalcopyrite. The best grades of copper, as much as 0.8 percent, occur in the upper 100 feet of the blanket. The thickest enrichment zones are not necessarily associated with the thickest zones of oxidation. Lead, antimony, and zinc are also reported in some assays. The Quintana-Duval-Aleut Joint Venture mapped, sampled, and drilled this deposit in 1974 and 1975. They diamond-drilled 19 holes for a total of 5,565 feet. As a result of this work they outlined an estimated resource of 126,000,000 tons of ore grading 0.403 percent copper and 0.025 percent molybdenum. Potential exists for another 49 million tons of chalcocite-enriched ore (Christie, 1975).
Workings: Quintana-Duval mapped, sampled, and diamond-drilled 19 holes totalling 5,565 feet in 1974-1975. The U.S. Geological Survey sampled the deposit in the 1980s.
Age: Six million years or younger.
Alteration: The alteration consists of a barren, potassically-altered core zone, an intermediate zone characterized by pervasive quartz and sericite, and a propylitic outer zone.
Reserves: The resource is estimated at 126 million tons of ore averaging 0.403 percent copper and 0.025 percent molybdenum. There is potential for an additional 49 million tons of chalcocite-enriched ore.

Commodities (Major) - Cu, Mo; (Minor) - Ag, Au, Pb, Sb, Zn
Development Status: None
Deposit Model: Porphyry Cu, Porphyry Cu-Mo (Cox and Singer, 1986; models 17, 21a)

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. Armstrong, R.L., Harakal, J.E., and Hollister, V.F., 1976, Age determinations of late Cenozoic porphyry copper deposits of the North American Cordillera: Institute of Mining and Metallurgy Transactions, Section B, v. 85, p. 239-244. Bundtzen, T.K., Green, C.B., Deagen, J.R., and Daniels, C.L., 1987, Alaska's mineral industry, 1986: Alaska Division of Geological and Geophysical Surveys Special Report 40, 68 p. 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) Christie, J.S., 1974, Aleut-Quintana-Duval 1974 joint venture, final report: Unpublished Quintana Minerals Corporation report, 24 p., 3 appendices, 2 maps. (Report held by the Aleut Corporation, Anchorage, Alaska.) Christie, J.S., 1975, Pyramid Project, Aleut-Quintana-Duval joint venture report on 1975 drill program: Quintana Minerals Report, 17 p. (Report held by the Aleut Corporation, Anchorage, Alaska.) Eakins, G.R., Bundtzen, T.K., Robinson, M.S., Clough, J.G., Green, C.B., Clautice, K.H., and Albanese M.A., 1983, Alaska's mineral industry, 1982: Alaska Division of Geological and Geophysical Surveys Special Report 31, 63 p. 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.) Hollister, V.F., 1978, Geology of the porphyry copper deposits of the Western Hemisphere: Society of Mining Engineering, American Institute of Mining, Metallurgy, and Petroleum Engineers Inc., 218 p. MacKevett, E.M., Jr., and Holloway, C.D., 1977, Map showing metalliferous mineral deposits in the western part of southern Alaska: U.S. Geological Survey Open-File Report 77-169-F, 38 p., 1 sheet, scale 1:1,000,000. Nokleberg, W.J., Bundtzen, T.K., Berg, H.C., Brew, D.A., Grybeck, D.J., Robinson, M.S., Smith, T.E., and Yeend, W., 1987, Significant metalliferous lode deposits and placer districts of Alaska: U.S. Geological Survey Bulletin 1786, 104 p., 2 plates, scale 1:5,000,000. 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. Wilson, F.H., White, W.H., Detterman, R.L., and Case, J.E., 1996, Maps showing the resource assessment of the Port Moller, Stepovak Bay, and Simeonof Island quadrangles, Alaska Peninsula, with a section on Geology of the Pyramid porphyry copper deposit, Alaska Peninsula, Alaska, by W.H. White, J.S. Christie, M.R. Wolfhard, and F.H.Wilson, and a section on Description of the Shumagin epithermal gold vein deposit, by W.H. White and L.D. Queen: U.S. Geological Survey Miscellaneous Field Studies Map MF-2155-F, 46 p., 2 sheets, scales 1:500,000 and 1:250,000. Wolfhard, F.H., 1974, Pyramid prospect preliminary evaluation: Quintana Minerals Corporation unpublished report, 9 p., 6 map sheets, various scales. (Report held by the Aleut Corporation, Anchorage, Alaska.) Wolfhard, F.H., 1976, Final report, Pyramid project 1976 work: Quintana Minerals Corporation unpublished report, 2 p. (Report held by the Aleut Corporation, Anchorage, Alaska.) Young, L.E., St. George, P., and Bouley, B., 1997, Porphyry copper deposits in relation to the magmatic history and palinspastic restoration of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral deposits of Alaska: Economic Geology Monograph 9, p. 306-333. Singer, D.A., Berger, V.I., and Moring, B.C. (2008): Porphyry copper deposits of the world: Database and grade and tonnage models, 2008. US Geological Survey Open-File Report 2008-1155.