Unnamed Prospect (ARDF - ID036; west wall of Yankee Creek), Innoko District, Yukon-Koyukuk Borough, Alaska, USA

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Location: This prospect is west of Yankee Creek at an elevation of about 1,500 feet. It is near the northwest corner of section 1, T. 33 N ., R 38 W., of the Seward Meridian. The location is accurate.
Geology: This prospect is in a highly altered, northeast-trending mafic (ultramafic?) dike that intrudes sandstone and siltstone of the Upper Cretaceous, Kuskokwim Group (Bundtzen and Laird, 1983; Miller and Bundtzen, 1994). The dike is brown to tan in color and is almost completely altered to silica-carbonate minerals and mariposite. Rubble-crop suggests that the dike is 6 to 25 feet wide; it it can be traced along strike for about 500 feet in old prospect pits and rubble crop (Bundtzen and Laird, 1983). A grab sample of the altered dike contained 5.0 parts per million (ppm) silver, 200 ppm nickel, and 2,000 ppm chromium (Bundtzen and Laird, 1983). This occurrence is an example of chromium, nickel, and occasional precious metal enrichment that occurs in altered mafic and ultramafic? dikes in the Ganes -Yankee Creek fault zone and in other areas of the Iditarod quadrangle. The abundant chromite and magnesiochromite found in panned concentrates from Yankee Creek (ID037) and Ganes Creek (ID011) may be derived from occurrences of this type (Bundtzen, Cox, and Veach, 1987).
Workings: The altered dike at this unnamed prospect has been explored by several old prospect pits.
Age: Undated; similar mafic dikes near the DeCourcy Mountain Mine (ID158) have 40K/40Ar dates of 71 to 76 Ma (Miller and Bundtzen, 1994).
Alteration: Pervasive alteration of mafic to ultramafic dikes to silica and carbonate minerals and mariposite.

Commodities (Major) - Ag, Cr; (Minor) - Ni
Development Status: None

References

Bundtzen, T.K., and Laird, G.M., 1983, Geologic map of the Iditarod D-1 quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Professional Report 78, scale 1:63,360. Bundtzen, T.K., Cox, B.C., and Veach, N.C., 1987, Heavy mineral provenance studies in the Iditarod and Innoko districts, western Alaska: Process Mineralogy VII, The Metallurgical Society, p. 221-246. 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.

Mineral List

Phengite var: Mariposite

1 entry listed.