Ivanof; Kawisgag Prospect, Alaska Peninsula District, Lake and Peninsula Borough, Alaska, USA
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Location: This site is located on an unnamed peninsula between Humpback and Ivanof Bays. It presents a wide area northwest of triangulation station Short (MacKevett and Holloway, 1977, locality 1; Nokleberg and others, 1987, locality AP 8; Green and others, 1988, locality 88; Wilson and others, 1988, locality 1; Young and others, 1997, locality 35). The map site is at an elevation of 1,250 feet, at the midpoint of the boundary betweeen secs. 1 and 36, T. 49 and 50 S., R. 65 W, of the Seward Meridian. The location is accurate.
Geology: At this prospect sandstones, grits, and conglomerates of the Eocene to Paleocene Tolstoi Formation are cut by an intrusive complex. (Fields, 1977; Wilson and others, 1995). Biotite from the complex has been dated at 7 million years (Wilson and others, 1994). The sedimentary rock near the complex exhibits intense thermal metamorphism resulting in biotite-quartz hornfels. The intrusive complex, best exposed in two cirque basins, consists of an early diorite porphyry phase and a later, more extensive, quartz porphyry phase. The quartz porphyry intrudes the diorite as well as the Tolstoy Formation. Peripheral sills and dikes similar in composition to the intrusive complex cut the sedimentary units. Both intrusive phases are mineralized, but only the diorite contains copper-molybdenum values. The sulfide mineralization, which includes arsenopyrite, pyrite, pyrrhotite, chalcopyrite, bornite, and covellite, covers an area of approximately 2 by 2.5 miles. Pyrite is pervasive as disseminations and fracture fillings in the quartz porphyry and in the sedimentary rocks throughout a 3-square-mile area. Two zones of copper-molybdenum-quartz stockwork mineralization were outlined by Bear Creek Mining (Fields, 1977), each extending over an area of approximately 2,000 by 600 feet and occurring in the diorite as well as in the surrounding sedimentary rock. Base metal values in surface rock samples ranged from 500 to 1,700 ppm copper and 20 to 240 ppm molybdenum, along with weakly anomalous gold values. Fields describes the pyrite:chalcopyrite ratio as 1:1 to 2:1 in the main mineralized areas and 5:1 to 10:1 in surrounding areas. Rock samples collected by Resource Associates of Alaska in 1979 in the northern cirque contained as much as to 790 ppm copper, 2,400 ppm molybdenum, and 2.4 ppm silver. Farnstrom (1991) associates the mineralization with two separate, intersecting quartz-sulfide stockworks. The older stockwork consists of quartz-sulfide-chlorite veinlets in the diorite that are traceable for as much as to 3,000 feet into the sedimentary rock. These veinlets contain pyrite, pyrrhotite, and chalcopyrite. Pyrite:chalcopyrite ratios exceed 5:1. The younger stockwork consists of veinlets as much as 0.4 inches wide containing pyrite, molybdenite, and only trace amounts of chalcopyrite, covellite, bornite, and malachite. According to Farnstrom (1991), the distribution of the copper-rich stockwork is spotty; the best deposits are at the contact of the diorite and the sedimentary country rocks. In 1979, Resource Associates of Alaska discovered quartz veins containing gold and arsenopyrite in areas peripheral to the copper-molybdenum mineralization (Moller and others,1982). Some of these veins are as much as 30 feet wide and can be traced for as much as 2,400 feet along strike. Typical widths are approximately 5 to 8 feet. Their gold content ranges from 0.08 to 0.20 ounce per ton. Strong secondary biotite is localized in and around the diorite over an area approximately 1,000 feet in diameter. Widespread sericitic alteration surrounds the biotite zone and coincides with the pyritic halo. Propylitic alteration forms an outer zone (Fields, 1977). Farnstrom (1991) described the alteration in and around the diorite as generally relatively slight. Mineralized areas, however, are marked by intense bleaching and silicification. According to Farnstrom, chlorite replaces mafic minerals, and also occurs in stockwork veinlets in both intrusive phases. In the diorite the chlorite is accompanied by epidote. Feldspars in both intrusive phases are altered to sericite, but this type of alteration tends to be more intense in the quartz porphyry. In contrast to Field's (1977), Farnstrom's alteration description does not indicate any zonal pattern. Some of the alteration she describes may be deuteric in origin rather than hydrothermal.
Workings: Bear Creek Mining mapped and sampled this prospect for Bristol Bay Native Corporation in the 1970's (Fields, 1977). Copper values in rock samples ranged from 500 to 1,700 ppm and molybdenum from 20 to 240 ppm. Minor gold values were also detected. Resource Associates of Alaska mapped and sampled the deposit in the mid 1980's, and ran approximately 9 line miles of VLF-EM and magnetics on the prospect. The U.S. Geological Survey sampled the deposit in the mid-1980's. Their rock samples 83APk7-12, 83AAi15, and 84AGe28-32 showed multiple element anomalies (Wilson and others, 1988). Cominco mapped and sampled the deposit in 1990 (Farnstrom, 1991).
Age: Seven million years or younger.
Alteration: According to Fields (1977), the alteration exhibits a zonal pattern with a central potassic core surrounded by a phyllic zone and an outer propylitic zone. Farnstrom (1991) describes sericitic (phyllic) alteration that is related in part to mineralization.
Commodities (Major) - Ag, Au, Cu, Mo; (Minor) - As, Pb, Zn
Development Status: None
Deposit Model: Porphyry Cu, Porphyry Cu-Mo, Porphyry Cu-Au (Cox and Singer, 1986; models 17, 2
Mineral List
9 entries listed. 9 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. Farnstrom, H.E., 1991, Ivanof Project, 1990 final report: Cominco Alaska, 12 p. (Report held by Alaska Earth Sciences, Inc., Anchorage, Alaska .) 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.) Frisken, J.G., 1992, Interpretation of reconnaissance geochemical data from the Port Moller, Stepovak Bay, and Simeonof Island quadrangles, Alaska Peninsula, Alaska: U.S. Geological Survey Bulletin 1968, 47 p., 3 sheets, scale 1:250,000. Green, C.B., Bundtzen, T.K., Peterson, R.J., Seward, A.F., Deagan, J.R., and Burton, J.E., 1989, Alaska's mineral industry, 1988: Alaska Division of Geological and Geophysical Surveys Special Report 43, 79 p. 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. Moller, S. A., Bernt, J., Farnstrom, H. E., Toupe, W., Hanneman, Nancy, 1982, Exploration and evaluation of precious metals potential of Bristol Bay Native Corporation lands, SW Alaska, vols. IV and V: Resource Associates of Alaska, Fairbanks, Alaska. (Report on file, Bristol Bay Native Corporation, Anchorage, Alaska). 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 Peninsula, Alaska: U.S. Geological Survey Miscellaneous Investigation Series Map I-2272, 2 map sheets, scale 1:250,000. Wilson, F.H., Shew, N., DuBois, G.D., and Bie, S.W., 1994, Sample locality map and analytical data for potassium-argon ages in the Port Moller, Stepovak Bay and Simeonof Island quadrangles, Alaska Peninsula, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2155-E, 18 p., 1 map 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. 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.
