Unnamed Occurrence (ARDF - MG037), McGrath District, Yukon-Koyukuk Borough, Alaska, USA
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Location: This unnamed occurrence is located at the base of a cliff face on the east side of an unnamed, glacially carved, second order tributary of the Middle Fork of the Kuskokwim River. The occurrence is about three kilometers upstream from the mouth of the unnamed tributary and immediately downslope from an active valley glacier terminous at an elevation of 3,950 feet (1,204 m) in the NE1/4 sec. 32, T. 23 N., R. 26 W., of the Seward Meridian. The reporter briefly visited the site in 1982.
Geology: The occurrence consists mainly of several feldspar-hematite-quartz veins infilling joints and fractures in the Windy Fork composite pluton about one mile from the contact zone with sandstone and limestone of the Lower to mid-Paleozoic Dillinger subterrane (Reed and Miller, 1980; Gilbert and others, 1988; Bundtzen, Harris, and Gilbert, 1997). The individual joint fillings are 3 to 6 centimeters thick and continue for 2 to 3 meters along strike. Much of the mineralization trends N65W. Feldspars are typically altered to a clay matrix; quartz is the 'smokey' variety due to radioactive(?) decay (Gunter and others, 1993). Green-to-purple fluorite is common in talus but was not recognized in place. Composite grab samples of quartz-feldspar-hematite joint-filled material reported by Reed and Miller (1980) contain up to 490 ppm uranium and 376 ppm thorium.
Workings: The occurrence was found by the late Bruce Reed while conducting mineral resource investigations and regional geologic mapping for the U.S. Geological Survey in the western Alaska Range (Reed and Miller, 1980). Composite grab samples of quartz-feldspar-hematite joint-filled material as reported by Reed and Miller (1980) contain up to 490 ppm uranium and 376 ppm thorium.
Age: Late Tertiary, based on age (25Ma) of Windy Fork composite pluton (Solie and others, 1991).
Alteration: Feldspars are altered to clay minerals; quartz is a 'smokey' variety, possibly due to radioactive decay.
Reserves: None estimated.
Commodities (Major) - Th, U; (Minor) - Sn, Zr
Development Status: No
Deposit Model: Thorium-Rare Earth veins (Cox and Singer, 1986; model 11b).
5 entries listed. 3 valid minerals.
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Bundtzen, T.K., Harris, E.E., and Gilbert, W.G., 1997, Geologic Map of the eastern McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 97-14, 34 p., 1 sheet, scale 1:125,000. Gilbert, W.G., Solie, D.N., and Kline, J.T., 1988, Geologic map of the McGrath A-3 quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Professional Report 92, 2 sheets, scale 1:63,360. Gunter, M.E., Johnson, N.E., Knowles, C.R., and Solie, D.N., 1993, Optical, x-ray, and chemical analyses of four eudialytes from Alaska: Mineralogical Magazine, v. 57, p. 743-744. Reed, B.L., and Miller, T.P., 1980, Uranium and thorium content of some Tertiary granitic rocks in the southern Alaska Range: U.S. Geological Survey Open-File Report 80-1052, 16 p. Solie, D.N., Bundtzen, T.K., and Gilbert, W.G., 1991, K/Ar ages of igneous rocks in the McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Public Data File 91-23, 7 p., 1 sheet, 1:250,000 scale.