Fortyseven Creek Lode Prospect, Aniak District, Bethel Borough, Alaska, USA
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Geology: The rocks in the vicinity of the Fortyseven Creek lode prospect are sandstone, siltstone, and shale of the Upper Cretaceous, Kuskokwim Group (Cady and others, 1955) that are intruded by small alaskite and granite porphyry bodies (Robinson, 1984 [RI 84-7]; Decker and others, 1995). Detailed geologic mapping of the prospect area (Bundtzen, unpublished. field data, 1983, 1998) indicates that a 3,300-foot section of the sedimentary rocks is complexly folded. Sub-isoclinal folds are overturned with vergence to the east-southeast. The prospect area is cut by a N25E-trending, vertically dipping fault zone that can be traced for a distance of at least 15 miles; the fault is sub-parallel to the trace of the Denali-Holitna fault zone which is about 2.4 miles to the southeast. This fault zone, informally called the Fortyseven Creek fault zone, contains a pronounced zone of shearing that ranges in width from about 160 to 3,000 feet. A zone of hornfels averaging about 1,000 feet thick parallels the southeast edge of the Fortyseven Creek fault zone. A small alaskite stock cuts the shear zone near the southern boundary of the prospect area (Bundtzen and Nokleberg, 1987). Decker and others (1995) reported a K-Ar age of 63.0 Ma from sericite in the mineralized portion of the Fortyseven Creek fault zone. However, Gray, Gent, and others (1997b) and Miller and others (2002) reported 40Ar/39Ar sericite age of 70.3 Ma from sericite in the mineralized portion of the Fortyseven Creek fault zone. Mineralization in the Fortyseven Creek fault zone consists of conjugate, extensively sheared, quartz-carbonate-sulfide veins that strike N75E and N50W, and dip steeply. The individual veins range from 8 inches to 10 feet thick. Ore minerals occur in discontinuous pods in individual quartz-carbonate veins. The most abundant sulfides are arsenopyrite and pyrite, followed by argentite, jamesonite, scheelite, stibnite, wolframite and free gold. The scheelite grains in the quartz veins are locally quite large; individual grains and masses are up to 1.5 inches in diameter. Selected samples of mineralization contain up to 34 parts per million (ppm) gold (Hawley, 1989). Robinson (1984 [RI 84-7]) reported grades of up to 2.4 ppm gold and 16.7 ppm silver from grab samples at the Fortyseven Creek lode. Six chip-channel samples collected by T.K. Bundtzen along a 1.2 mile of the fault zone (unpublished data, 1998) contained up to 332 parts per billion (ppb) gold, 65.2 ppm silver, 3.54 percent arsenic, 287 ppm bismuth, and 62 ppm tin.
Workings: The lode deposit was found by Russell Schaeffer in 1947, when he also discovered nearby placer deposits (SM074). Holitna Mining and Exploration, Inc. trenched several of the quartz veins in the main shear zone in 1985 (Hawley, 1989). Additional sampling and trenching also occurred in the 1990s (T.K. Bundtzen, unpublished. data, 1998).
Age: A 40Ar/39Ar date of 70.3 Ma was obtained on sericite in a quartz vein (Gray, Gent, and others, 1997; Miller and others, 2002). A K-Ar age of 63.0 Ma that was obtained from sericite in a quartz vein was reported by Decker and others (1995). This is the approximately the same age range as reported for mineralization at the Donlin Creek lode gold deposit in the Iditarod quadrangle (Bundtzen and Miller, 997).
Alteration: Sericite in quartz veins.
Commodities (Major) - Ag, Au, W; (Minor) - As, Bi, Sb, Sn
Development Status: None
Deposit Model: Polymetallic vein (Cox and Singer, 1986, model 22c).
11 entries listed. 9 valid minerals.
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Bundtzen, T.K., and Miller, M.L., 1997, Precious metals associated with Late Cretaceous-early Tertiary igneous rocks of southwestern Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 242-286. Bundtzen, T.K., and Nokleberg, W.J., 1987, Significant metalliferous lode deposits, west-central Alaska, in Nokleberg, W.J., Bundtzen, T.K., Berg, H.C., Brew, D.A., Grybeck, D., Robinson, M.S., Smith, T.E., and Yeend, Warren, Significant metalliferous lode deposits and placer districts of Alaska: U.S. Geological Survey Bulletin 1786, p. 23-31. Cady, W.M., Wallace, R.E., Hoare, J.M., and Webber, E.J., 1955, The central Kuskokwim region, Alaska: U.S. Geological Survey Professional Paper 268, 132 p. Cobb, E.H., 1972, Metallic mineral resources map of the Sleetmute quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-368, 1 sheet, scale 1:250,000. Cobb, E.H., 1976, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Dillingham, Sleetmute, and Taylor Mountains quadrangles, Alaska: U.S. Geological Survey Open-File Report 76-606, 92 p. Decker, J.E., Reifenstuhl, R.R., Robinson, M.S., Waythomas, C.F., and Clough, J.G., 1995, Geology of the Sleetmute A-5, A-6, B-5, and B-6 quadrangles southwestern Alaska: Alaska Division of Geological and Geophysical Surveys Professional Report 99, 16 pages, one sheet, scale 1:63,360. Gray, J.E., Gent, C.A., Snee, L.W., and Wilson, F.H., 1997, Epithermal mercury-antimony and gold-bearing vein lodes of southwest Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 287-305. Gray, J.E., Motooka, Jerry, and Theodorakos, P.M., 1997, Areas favorable for metallic resources and newly discovered mineral occurrences in the Buckstock Mountains area, southwestern Alaska, in Dumoulin, J.A., and Gray, J.E., eds, Geologic Studies in Alaska by the U.S. Geological Survey in 1995: U.S. Geological Survey Professional Paper 1574, p. 111- 124. Hawley, C.C., 1989, The Fortyseven Creek prospect, southwestern Alaska, summary of data and recommendations: Unpublished report prepared for Holitna Basin Mining and Exploration, Inc., 7 p. Miller, M.L., Belkin, H.E., Blodgett, R.B., Bundtzen, T.K., Cady, J.W., Goldfarb, R.J., Gray, J.E., McGimsey, R.G., and Simpson, S.L., 1989, Pre-field study and mineral resource assessment of the Sleetmute quadrangle, southwestern Alaska: U.S. Geological Survey Open-File Report 89-363, 115 p., 3 plates, scale 1:250,000. Miller, M.L., Bradley, D.C., Bundtzen, T.K., and McClelland, William, 2002, Late Cretaceous through Cenozoic strike-slip tectonics of southwestern Alaska: Journal of Geology, vol.110, p. 247-270. Robinson, M.S., 1984, Rock, pan-concentrate, and stream-sediment geochemistry, Sleetmute A-6 quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 84-7, 1 sheet, scale 1:40,000.