Von Frank - South Prospect, McGrath District, Yukon-Koyukuk Borough, Alaska, USA
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Location: The Von Frank - South prospect is situated on a southerly spur of Von Frank mountain at an elevation of approximately 2,550 feet (777 m) in Section 1, T. 23 S., R. 23 E., of the Kateel River Meridian. The reporter visited the site in 1990.
Geology: The Von Frank - South prospect is a chalcopyrite-pyrite-(moybdenite) quartz stockwork in a 69 Ma diorite stock, a cupola of the larger Von Frank Mountain monzonitic pluton (Patton and others, 1980; Moll and others, 1981; DiMarchi and others, 1994; Bundtzen and Miller, 1997). Much of the stockwork occurs in joint-fractures with regular N 55 E, 80 SE, and N 45 W, 75 SW orientations. Mineralized veinlets ranged from 1 inch to 6 inches (2.5 cm to 15 cm) thick and total about 15 per every 4 feet (1.2 m). About 90 percent of the stockwork is in plutonic rocks; the remaining 10 percent occurs in hornfels. Sulfides in stockwork from drill core include chalcopyrite, sphalerite, bournonite, stibnite, and pyrite. Bundtzen and Miller (1997) compare the Von Frank - South prospect with other Cu-Au Porphyry prospects in the Kuskokwim Mineral Belt of Southwest Alaska. Drill intercepts include 429 feet (131 m) of 0.13 ounces/ton (0.45 g/tonne) gold, and 135 feet (41 m) of 0.035 ounces/ton (1.2 grams/tonne) gold (Bundtzen and others, 1994). Up to 0.35 percent copper was also present in drill core. The best mineralization from surface samples yielded 0.559 ounces (19.1 grams/tonne) gold. Gold and copper have a correlation coefficient of 0.92 (DiMarchi and others, 1994).
Workings: ASA Inc. discovered the Von Frank-South prospect in 1992. In 1993, ASA Inc., an Alaskan-based exploration firm, completed 2,000 feet (610 m) of reconnaissance diamond drilling, geologic mapping, and geochemical sampling. Drill intercepts include 429 feet (131 m) with an average grade of 0.013 ounces/ton (0.45 grams/tonne) gold, and 135 feet (41 m) of 0.035 ounces/ton (1.2 grams/ton) gold (Bundtzen and others, 1994). Up to 0.35 percent copper was also detected in drill core, but analytical results from mineralized sample widths were not available. The highest grade surface sample yielded 0.559 ounces/ton (19.1 grams/tonne) gold. Gold and copper have a correlation coefficient of 0.92, with much lower values of gold and other metals (DiMarchi and others, 1994).
Age: Late Cretaceous (69.0 Ma), based on biotite age in host diorite (Bundtzen and others, 1994).
Alteration: Quartz+sericite+ankerite in diorite intrusion.
Commodities (Major) - Au, Cu; (Minor) - As, Mo
Development Status: No
Deposit Model: Porphyry Cu-Au (Cox and Singer, 1986; model no. 20c)
8 entries listed. 8 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. Bundtzen, T.K., Swainbank, R.C., Clough, A.H., Henning, M.W., and Hansen, E.W., 1994, Alaska's mineral industry, 1993: Alaska Division of Geological and Geophysical Surveys Special Report 48, 84 p. DiMarchi, J.J., Weglarz, T.B., Adams, D.D., Huber, J., and West, A.W., 1994, Annual Report for 1993--Doyon Option Lands, Volume 1: ASA Inc. Report, 187 p. (Report held by Doyon, Limited, Fairbanks, Alaska.) Patton, W.W. Jr., Moll, E.J., Dutro, J.T., Jr., Silberman, M.L., and Chapman, R.M., 1980, Preliminary geologic map of the Medfra quadrangle, Alaska: U.S. Geological Survey Open-File Report 80-811, one sheet at 1:250,000 scale.