Unnamed Occurrence (ARDF - ID146; near hill 2825 on Granite Mountain), Iditarod District, Yukon-Koyukuk Borough, Alaska, USA
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Geology: This occurrence, also referred to as the Granite Mountain prospect (Bundtzen and Miller, 1997) consists of several sizable bodies of metasomatic axinite, anatase, and tourmaline at the contact between the Granite Mountain pluton and hornfelsed rocks derived from the Upper cretaceous Kuskokwim Group. The occurrence includes: 1) zones of tourmaline-bearing sheeted veins, 2) tourmaline-cemented breccias in hornfels; and 3) axinite-tourmaline-sulfide replacement bodies in both intrusive rocks and in hornfels. The nearby Granite Mountain pluton has a 40K/40Ar age of 62.6 Ma (Bundtzen and Miller, 1997). One large area of boron-rich tourmaline-axinite-anatase greisen is 1,300 feet long, about 50 feet wide, and cylindrical in cross section; it may contain 5.5 million tons of boron-enriched material of unknown grade. This zone also contains disseminated gray sulfide minerals. Grab samples from the occurrence contained 100 parts per million (ppm) tin, 500 parts per billion (ppb) silver, 50 ppb gold, 9.0 ppm bismuth, 28 ppm antimony, 80 ppm arsenic, and more than 2,000 ppm boron (McGimsey and others, 1988; Bundtzen and Miller, 1997).
Workings: Samples were collected by the U.S. Geological Survey and Alaska Division of Geological and Geophysical Surveys in 1984 (McGimsey and others, 1988; Bundtzen and Miller, 1997).
Age: The nearby Granite Mountain pluton has a A 40K/40Ar age of 62.6 Ma (Bundtzen and Miller, 1997).
Alteration: Development of axinite-tourmaline-quartz greisen.
Commodities (Major) - Ag, Sn; (Minor) - As, Au, Bi, Sb
Development Status: None
Deposit Model: Sn-polymetallic deposit or Polymetallic vein (Cox and Singer, 1986; model 20b o
4 entries listed. 2 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.