Unnamed Prospect (ARDF - ID179; east of George River), Iditarod District, Yukon-Koyukuk Borough, Alaska, USA
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Geology: This prospect consists of a sulfide- and hematite-bearing breccia in hornfels derived from the Upper Cretaceous, Kuskokwim Group The prospect is adjacent to a small granite-porphyry intrusion (Miller and Bundtzen, 1994; Miller, Bundtzen, and Gray, 2005). Breccias with fragments of both sedimentary and volcanic rocks exist, which suggest that remnants of a volcanic field are also present. The granite porphyry has a 40K/40Ar age of 71.3 Ma (Miller and Bundtzen, 1994). The sulfide breccia covers an area at least 1,000 feet by 1,000 feet in area. The best mineralization occurs in the creek at the east side of the deposit, where stibnite-quartz gash veins cut hornfels. Other areas contain quartz breccia with masses of goethite produced by the oxidization of sulfide minerals. Grab samples of mineralization contain up to 250 parts per million (ppb) gold, 1.00 percent antimony, 200 parts per million (ppm) tin, 200 ppm tungsten, more than 14.0 ppm mercury, 5.0 ppm molybdenum, 610 ppm arsenic, and 20.00 percent iron. (McGimsey and others, 1988).
Workings: The stibnite-quartz gash veins exposed at creek level was first noticed by the helicopter pilot David Blair in 1985. Samples were collected by the U.S. Geological Survey and Alaska Division of Geological and Geophysical Surveys in the 1980s (McGimsey and others, 1988). In 1996, Ventures Resources, Inc. acquired Alaska State prospecting sites over the prospect and continued to explore the site through 2003.
Age: Unknown; the intrusion adjacent to the hornfels that hosts the deposit has a 40K/40Ar age of 71.3 Ma (Miller and Bundtzen, 1994).
Alteration: Iron oxide alteration.
Commodities (Major) - Au, Sb; (Minor) - As, Hg, Mo, Sn, W
Development Status: None
Deposit Model: Polymetallic vein (Cox and Singer, 1986; model 22c); or peraluminous granite po
4 entries listed. 4 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.