Old Man Breccia Prospect, Yentna District, Matanuska-Susitna Borough, Alaska, USA
Location: Alaska Department of Natural Resources claim records show the prospect in a stream bed of a northern tributary to Old Man Creek, four to five miles up stream from the confluence of Old Man Creek with the Skwentna River in Sections 1 and 2, T. 21 N., R. 17 W.
Geology: A float sample of pyrrhotite-chalcopyrite-quartz-pyrite, that contained several ounces of gold, lead to the discovery of a breccia in siltstone, part of Upper Jurassic and Lower Cretaceous flysch (KJs) of the Kahiltna terrane. The breccia is exposed over an area approximately 105 feet x 90 feet. Within the breccia angular clasts of argillite from less than one inch up to a few inches long occur in a matrix of well-healed milky quartz and lesser carbonate. The quartz-argillite breccia forms sinuous, sub-vertical zones around large argillite blocks. Sulfides in breccia are sparse (generally less than 1% total) and consist of pyrrhotite pods +/- pyrite and chalcopyrite, and rare coarse-grained arsenopyrite. Veins of quartz-pyrrhotite-pyrite-chalcopyrite crosscut the breccia zone and vary from a few inches to 2 feet in thickness and contain occasional specks of visible gold. No obvious alteration occurs in the hanging wall (Cominco American Incorporated, unpublished reports,1986-1988).
Workings: Exploration work included mapping, rock chip geochemistry, grid soil sampling, grid magnetic and VLF geophysical surveys, and diamond drilling.
Commodities (Major) - Au, Cu; (Minor) - Fe
Development Status: None
Deposit Model: Gold-bearing, sulfide breccia
ReferencesReed, B.L., and Elliott, R.L., 1970, Reconnaissance geologic map, analyses of bedrock and stream sediment samples, and an aeromagnetic map of parts of the southern Alaska Range: U.S. Geological Survey Open-file Report 70-271, 24 p., 4 sheets, scale 1:250,000.
5 entries listed. 5 valid minerals.
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