Unnamed Occurrences (ARDF - KC149), 'Sink Lake', Annette Island, Ketchikan District, Prince of Wales-Outer Ketchikan Borough, Alaska, USA
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Geology: The country rocks in the area of this site include undivided, metamorphosed Silurian and Ordovician volcanic, sedimentary, and intrusive rocks; and a sequence of recrystallized, Upper Triassic carbonate, clastic, and rhyolitic rocks that unconformably overlies the Paleozoic rocks (Berg, 1972). The rocks are complexly folded and faulted, and were metamorphosed to greenschist-grade phyllite and semischist in Late Cretaceous time. The principal occurrence consists of sulfide-bearing quartz-calcite ladder and stringer veins in a 300-foot by 500-foot area at or near a faulted contact between dolomitic limestone and locally sericitized rhyolite (Karl, 1992, loc. 35c). The veins aggregate as much as 10% of the total rock volume in this area. A 10-foot-wide shear zone at the contact contains quartz veins that predate the shearing (Karl, 1992, loc. 35). The veins, which are mostly in the dolomitic limestone, contain galena, sphalerite, pyrite, hematite, tetrahedrite, stibnite, malachite, and barite. Samples collected in the 1930s by the USGS reportedly also contained chalcopyrite, covellite, chalcocite, and a trace of pyrargyrite ('ruby silver') (Berg, 1972, loc. 4). Sheared rhyolite sampled in the 1970s by private interests reportedly contained massive chalcopyrite up to 2 inches thick, malachite coatings, and up to 50% magnetite (Karl, 1992). Karl's samples of sulfide- and/or magnetite-bearing quartz veins and country rocks at the sheared contact of greenstone and rhyolite contained up to 300 ppm Zn and 20 ppm Mo (Karl, 1992, loc. 35a, b). Other samples of sulfide-bearing quartz veins and of mineralized country rocks collected during USGS and private investigations in the 1930s, 1970s, and the early 1990s assayed as much as 0.03 oz. Au/ton, 20.4 oz. Ag/ ton, more than 2% Cu, 12.43 % Pb, more than 1% Zn, more than 1% Sb, 3000 ppm Ba, and 15 ppm Mo (Karl, 1992, loc. 35c).
Workings: Sheared rhyolite sampled in the 1970s by private interests reportedly contained massive chalcopyrite up to 2 inches thick, malachite coatings, and up to 50% magnetite (Karl, 1992). Karl's samples of sulfide- and/or magnetite-bearing quartz veins and country rocks at the sheared contact of greenstone and rhyolite contained up to 300 ppm Zn and 20 ppm Mo (Karl, 1992, loc. 35a, b). Other samples of sulfide-bearing quartz veins and of mineralized country rocks collected during USGS and private investigations in the 1930s, 1970s, and the early 1990s assayed as much as 0.03 oz. Au/ton, 20.4 oz. Ag/ ton, more than 2% Cu, 12.43 % Pb, more than 1% Zn, more than 1% Sb, 3000 ppm Ba, and 15 ppm Mo (Karl, 1992, loc. 35c).
Age: Late Triassic or younger.
Alteration: Secondary copper and silver minerals suggest local supergene enrichment. Dolomitization of limestone; sericitization of rhyolitic volcanic rocks.
Commodities (Major) - Ag, Au, Cu, Pb, Zn
Development Status: None
Deposit Model: Polymetallic veins (Cox and Singer, 1986; model 22c)
Mineral List
14 entries listed. 14 valid minerals.
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References
Berg, H.C., 1972, Geologic map of Annette Island, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-684, 8 p., 1 sheet, scale 1:63,360. Elliott, R.L., Berg, H.C., and Karl, S.M.,1978, Map and table describing metalliferous and selected non-metalliferous mineral deposits in the Ketchikan and Prince Rupert quadrangles, Alaska: U.S. Geological Survey Open-File Report 78-73B, 17 p., 1 sheet, scale 1:250,000. Karl, S.M., 1992, Map and table of mineral deposits on Annette Island, Alaska: U.S. Geological Survey Open-File Report 92-690, 57 p., 1 map, scale 1:63,360.