Koby; Shepard Prospect, Chichagof District, Sitka Borough, Alaska, USA
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Geology: Johnson and Karl (1985) map the rocks in the area of this prospect as part of a batholith of Cretaceous or Jurassic tonalite and gabbro. The rocks are cut by numerous high-angle faults of varying, but mainly northwest, strike. The most prominent of these is the regional-scale Peril Strait Fault that follows the Lisianski River. The prospect is on or near this fault. Cobb (1972, 1978), citing Reed and Coats (1941) and Rossman (1959), reports that the prospect consists of quartz lenses as much as 7 feet thick along a fault zone in quartz-chlorite schist. The quartz contains about one percent sulfides, including arsenopyrite, pyrite, sphalerite, and galena; in 1959, the owners claimed that it also carries free gold. The deposit was explored by pits, trenches, and a 280-foot adit and crosscut that did not intersect mineralization. Bittenbender and others (1999), mainly citing Kimball (1982), describe the prospect as a quartz vein in greenstone schist. It was explored by a 300-foot adit, and by surface cuts for about 300 feet along strike. Samples across 3- to 6-foot widths of the vein assayed up to 0.02 ounce of gold per ton and 0.5-0.9 ounce of silver per ton. Selected dump samples contained up to 2.96 ounces of gold per ton, 52.5 ounces of silver per ton, and 1 percent lead. They report that the deposit also contains copper, bismuth, and cadmium. There is no record of any production, but the extent of the workings suggests that some gold may have been produced.
Workings: According to Cobb (1978), the deposit was explored by pits, trenches, and a 280-foot adit and crosscut that did not intersect mineralization. Bittenbender and others (1999) report that it was explored by a 300-foot adit and by surface cuts for about 300 feet along the strike of the vein.
Production: There is no record of any production, but the extent of the workings suggests that some gold may have been produced.
Commodities (Major) - Au, Pb, Zn; (Minor) - Bi, Cd, Cu
Development Status: Undetermined.
Deposit Model: Polymetallic vein (Cox and Singer, 1986; model 22c).
6 entries listed. 6 valid minerals.
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Bittenbender, P., Still, J.C., Maas, K., and McDonald, M., Jr., 1999, Mineral resources of the Chichagof and Baranof Islands area, southeast Alaska: Bureau of Land Management, BLM-Alaska Technical Report 19, 222 p. Cobb, E.H., 1972, Metallic mineral resources map of the Sitka quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-467, scale 1:250,000. Cobb, E.H., 1978, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Sitka quadrangle: U.S. Geological Survey Open-File Report 78-450, 124 p. Johnson, B.R, and Karl, S.M., 1985, Geologic map of western Chichagof and Yakobi Islands, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1506, 1 sheet, scale 1:125,000. Kimball, A.L., 1982, Mineral land assessment of Yakobi Island and adjacent parts of Chichagof Island, southeastern Alaska: U. S. Bureau of Mines Mineral Land Assessment report, MLA 97-82, 199 p. Reed, J.C., and Coats, R.R., 1941, Geology and ore deposits of the Chichagof mining district, Alaska: U.S. Geological Survey Bulletin 929, 148 p. Rossman, Darwin, 1959, Geology and ore deposits of the northwestern part of Chichagof Island, Alaska: U.S. Geological Survey Bulletin 1058-E, p. 139-216. U.S. Bureau of Land Management, 2002, Alaska mineral locations database report (Sitka quadrangle), July 2, 2002, 205 p. [http://imcg.wr.usgs.gov/dem.html]