Lone Occurrence, McGrath District, Yukon-Koyukuk Borough, Alaska, USA
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Location: The Lone occurrence is located about 0.5 miles south of VABM Lone at an elevation of 2,500 feet (762 m) in the SE1/4 sec. 18, T. 28 N., R. 29 W., of the Seward Meridian. The reporter visited the site in 1982 and 1992.
Geology: A leucocratic, biotite granite radiometrically dated at 71.0 Ma (Solie and others, 1991) intrudes Lower Ordovician deepwater carbonate of the Nixon Fork Subterrane (Decker and others, 1994). The intrusion created a 760 meter wide thermal aureole, which locally has formed hornfels and small, discontinuous pods of skarn. The skarns at the Lone occurrence consist of magnetite, epidote, undetermined amphibole, and weakly disseminated pyrite and chalcopyrite. The magnetite bearing skarns crop out over a 10 meter by 12 meter area (Bundtzen, Harris, and Gilbert, 1997). Selected analyses (2) of the skarn zones at the Lone occurrence contain up to 0.50 percent copper, 35.00 percent iron, and 4.7 grams/tonne silver.
Workings: Selected analyses from surface samples at the Lone occurrence contain up to 0.50 percent copper, 35.00 percent iron, and 4.7 g/tonne silver.
Age: Inferred to be related to the 71.0 Ma granite intrusion (Solie and others, 1991).
Alteration: Epidote.
Reserves: None.
Commodities (Major) - Cu, Fe; (Minor) - Ag
Development Status: No
Deposit Model: Copper-iron skarns (Cox and Singer, 1986; model 18b).
Mineral List
5 entries listed. 4 valid minerals.
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References
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. Decker, J., Bergman, S.C., Blodgett, R.B., Box, S.E., Bundtzen, T.K., Clough, J.G., Coonrad, W.L., Gilbert, W.G., Miller, M.L., Murphy, J.M., Robinson, M.S., and Wallace, W.K., 1994, Geology of southwestern Alaska, in Plafker, G. and Berg, H.C., eds., The Geology of Alaska: Geological Society of America DNAG, The Geology of North America, v. G-1, p. 285-310. 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.
