Bismarck Creek Prospect, Iditarod District, Yukon-Koyukuk Borough, Alaska, USA
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Geology: This prospect is an east to northeast-trending stockworks of oxidized quartz veins and veinlets that contains cassiterite, axinite, and tourmaline in hornfels derived from the Upper Cretaceous, Kuskokwim Group (Miller and Bundtzen, 1994; Bundtzen and Miller, 1997). The deposit is within a 1.7 by 2 mile aureole of hornfels. The area is cut by north-northeast striking faults, one of which apparently controls the course of Bismarck Creek. The mineralized zone can be traced in rubble for more than 1,000 feet and varies from 6 to 100 feet wide. The sulfides have been completely oxidized leaving behind a heavy residue of reddish to orange gossan. Black cassiterite has been confirmed in polished sections from the zone. The deposit is associated with brecciated blocks of altered sandstone and siltstone floating in a matrix of quartz and gossan (Bundtzen and Miller, 1997). No intrusive rocks have been identified at the site; however, a large area centered on VABM 2424 is underlain by biotite-pyroxene hornfels that suggests a buried pluton here (Miller and Bundtzen, 1994). Chip-channel samples contain up to 2.80 percent tin, 63 parts per million (ppm) silver, 590 ppm copper, 4,600 ppm lead, more than 1.00 percent zinc, 1,100 ppm arsenic, and 310 ppm antimony (Bundtzen and Miller, 1997; McGimsey and others, 1988). Selected samples also contained up to 118 ppm indium (Miller, Bundtzen, and Gray, 2005). Based on extensive surface sampling, Bundtzen and Miller (1997) estimated that this prospect near Bismarck Creek contains an inferred resource of about 550,000 tons of material that contain 0.14 percent tin, 50.0 ppm silver, and about 0.75 percent combined base metals.
Workings: Samples were collected by the U.S. Geological Survey and Alaska Division of Geological and Geophysical Surveys in the 1980s. Bundtzen and Miller (1997) provide a geologic sketch and analyses from the prospect.
Alteration: Axinite-tourmaline-quartz 'greisen'.
Reserves: Based on extensive surface sampling, Bundtzen and Miller (1997) estimated that this prospect near Bismarck Creek contains an inferred resource of about 550,000 tons of material that contain 0.14 percent tin, 50.0 ppm silver, and about 0.75 percent combined base metals.
Commodities (Major) - Ag, Sn; (Minor) - As, Cd, Cu, F, In, Pb, Zn
Development Status: None
Deposit Model: Sn-polymetallic vein or polymetallic vein (Cox and Singer, 1986; models 20b and
6 entries listed. 3 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. McGimsey, R.G., Miller, M.L., and Arbogast, B.F., 1988, Paper version of analytical results, and sample locality map for rock samples from the Iditarod quadrangle, Alaska: U.S. Geological Survey Open-File Report 88-421-A, 110 p., 1 sheet, scale 1:250,000. Miller, M.L., and Bundtzen, T.K., 1994, Generalized geologic map of the Iditarod quadrangle, Alaska showing potassium-argon, major oxide, trace element, fossil, paleocurrent, and archeological sample localities: U.S. Geological Survey Miscellaneous Field Studies Map MF-2219-A, 48 pages; 1 sheet, scale 1:250,000. Miller, M.L., Bundtzen, T.K., and Gray, J.E., 2005, Mineral resource assessment of the Iditarod quadrangle, west-central Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2219-B, scale 1:250,000, pamphlet.