Red Shale Occurrence, McGrath District, Yukon-Koyukuk Borough, Alaska, USA
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Geology: The Red Shale occurrence is a stratiform, ferricrete gossan interbedded with quartzite, algal limestone, and dolomite of probable Cambrian or Late Proterozoic (?) age (Wilson and others, 1998). The section is exposed near the axis of an upright anticline that is structurally uplifted in a horst. The ferricrete zone occurs mainly as rubble crop and talus in a poorly exposed, shallow water, dolomite-carbonate dominant sequence that is interpreted to be the oldest exposed part of the Nixon Fork subterrane in the McGrath quadrangle (Babcock and others, 1994; Decker and others, 1994). The mineralized gossan strikes N15E, dips approximately 8E, ranges from 10 centimeters to 40 centimeters thick, and can be traced for about 140 meters along strike. Vegetation conceals the extensions of the mineralized area in both directions. Although poorly exposed, the mineralization is parallel to bedding. Minor late calcite incrustations were found in the ferricrete gossan. Three chip-channel samples (82BT219, 82BT221, 82BT222) taken at equally-spaced intervals along the exposed strike of the gossan zone contain up to 62.60 percent iron, 231 ppm copper, 485 ppm lead, 169 ppm nickel, and 2,250 ppm zinc (T.K. Bundtzen, written communication, 1983). No sulfides were recognized.
Workings: Three chip-channel samples (82BT219, 82BT221, 82BT222) taken at equally spaced intervals along the exposed strike of the gossan zone contain up to 62.60 percent iron, 231 ppm copper, 485 ppm lead, 169 ppm nickel, and 2,250 ppm zinc (T.K. Bundtzen, written communication, 1983).
Age: Late Proterozoic or Cambrian if mineralization is syngenetic.
Alteration: Strong ferricrete replacement of earlier sulfides.
Reserves: None.
Commodities (Major) - Pb, Zn; (Minor) - Cu
Development Status: No
Deposit Model: Uncertain; either Southeast Missouri Pb-Zn (?) or sedimentary exhalitive lead-z
Mineral List
3 entries listed. 2 valid minerals.
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References
Babcock, L.E., Blodgett, R.B., and St. John, James, 1994, New Late(?) Proterozoic age formations in the vicinity of Lone Mountain, McGrath quadrangle, Alaska, in Till, A.B., and Moore, T.E., eds., Geological studies in Alaska by the U.S. Geological Survey, 1993: U.S. Geological Survey Bulletin 2107, p. 143-155. 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. Wilson, F.H., Dover, J.H., Bradley, D.C., Weber, F.R., Bundtzen, T.K., and Haeussler, P.J., 1998, Geologic map of central (interior) Alaska: U.S. Geological Survey Open-File Report 98-133, 76 p., 3 sheets, scale 1:500,000.
