Unnamed ('Sink Lake')

Occurrences, Undetermined

Commodities and mineralogy

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale A-5
Latitude 55.106
Longitude -131.403
Nearby scientific data Find additional scientific data near this location
Location and accuracy This site represents four occurrences near an unnamed lake (informally called Sink Lake), about midway between the heads of Purple Lake and Crab Bay. The occurrences are are in section 16, T. 78 S., R. 93 E., of the Copper River Meridian, and are at elevations of about 200-400 feet around the lake, mainly near its southeast corner, and above the north shore. The coordinates are for the approximate center of the estimated half-mile-square area that contains the occurrences. The site corresponds to loc. 4 in Berg (1972 [I 684]), loc. 146 in Elliott and others (1978), and loc. 35a-d in Karl (1992). The location is accurate within about 0.1 mile.

Geologic setting

Geologic description

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).
Geologic map unit (-131.404679909332, 55.1056597850438)
Mineral deposit model Polymetallic veins (Cox and Singer, 1986; model 22c)
Mineral deposit model number 22c
Age of mineralization Late Triassic or younger.
Alteration of deposit Secondary copper and silver minerals suggest local supergene enrichment. Dolomitization of limestone; sericitization of rhyolitic volcanic rocks.

Production and reserves

Workings or exploration 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).
Indication of production None

References