Syenite Porphyry Prospect, Iditarod District, Yukon-Koyukuk Borough, Alaska, USA
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Geology: The Syenite Porphyry prospect consists of fine-grained cinnabar and stibnite along fracture surfaces in an altered iron-stained roof pendant and intrusive rocks of the 70.3 Ma, Beaver Mountain stock (Bundtzen and Laird, 1982). The roof pendant is a mudstone that has been altered to a hard, dark-gray hornfels. The underlying intrusion is a potassically altered, syenite-porphyry phase of the Beaver Mountains intrusion. The sulfide-bearing fractures contain amethyst, and smokey and chalcedonic quartz (Szumigala, 1993). Grab samples of mineralization collected by the U.S. Geological Survey contained more than 10 parts per million (ppm) mercury and 200 ppm zinc (McGimsey and others, 1988, Miller, Bundtzen, and Gray, 2005). Grab samples collected by Battle Mountain Mining Company contain up to 1,960 parts per billion (ppb) gold, 3.9 parts per million silver, 896 ppm arsenic, 1,896 ppm antimony, and more than 50 ppm mercury. Six of of the nine samples contained greater than 50 ppm mercury (Szumigala, 1993).
Workings: The Alaska Division of Geological and Geophysical Surveys examined the Syenite Porphyry prospect in 1979 (Bundtzen and Laird, 1980 and 1982). Noranda Exploration looked at the occurrence in 1983 (John Dunbier, oral communication, 1983). Surface sampling was conducted by the U.S. Geological Survey in 1984 and 1985 (McGimsey and others, 1988). Battle Mountain Mining Company investigated the Syenite Porphyry prospect in 1990 (Szumigala, 1993).
Age: Undated; may be related to emplacement of the Beaver Mountains stock that has been dated at 70.3. Ma (Bundtzen and Laird, 1982).
Alteration: Iron staining; potassic alteration.
Commodities (Major) - Au, Hg, Sb; (Minor) - Ag, As, Zn
Development Status: None
Deposit Model: Hot springs Au-Ag (Cox and Singer, 1986; model 25a).
1 entry listed. 1 valid mineral.
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Bundtzen, T.K., and Laird, G.M., 1982, Geologic map of the Iditarod D-2 and eastern D-3 quadrangles, Alaska: Alaska Division of Geological and Geophysical Surveys Geologic Report 72, 1 sheet, scale 1:63,360. 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. Szumigala, D.J., 1993, Gold mineralization related to Cretaceous-Tertiary magmatism in the Kuskokwim Mountains of west-central and southwestern Alaska: Los Angeles, University of California Ph.D. dissertation, 300 p. Szumigala, D.J., 1996, Gold mineralization related to Cretaceous-Tertiary magmatism in west-central Alaska-A geochemical model and prospecting guide for the Kuskokwim region: Geological Society of Nevada Symposium Proceedings, p. 1317-1340.