Cobol; Mine Mountain; Cobol North Mine, Chichagof District, Skagway-Hoonah-Angoon Borough, Alaska, USA
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Location: The Cobol Mine is identified as an abandoned mine and marked by an adit symbol on the USGS D-7 topographic map (1997 ed.). It is at an elevation of about 750 feet at the northwest foot of Mine Mountain on northwest Chichagof Island. The mine is 0.2 mile north of the center of sec. 29, T. 46 S., R. 57 E. It is location P-34 of Bittenbender and others (1999), who call it 'Mine Mountain', or 'Cobol North'; location 11 of Cobb (1972, 1978); and MAS no. 0021140025 (U.S. Bureau of Land Management, 2002). The location is accurate.
Geology: Johnson and Karl (1985) describe the rocks in the area of the Cobol Mine as Mesozoic or Paleozoic siliceous metasedimentary and metavolcanic rocks that are intruded by an elongate stock of Cretaceous or Tertiary quartz diorite and tonalite. The mine is at the contact between the metamorphic and intrusive rocks. The rocks are cut by high-angle faults of diverse, but mainly northwest, strike. The most prominent is the Border Ranges Fault, a regional-scale, northwest-striking, steeply-dipping fault whose trace is about 3 miles southwest of the mine. Cobb (1972, 1978), citing Buddington (1925) and Reed and Coats (1941) reports that the Cobol Mine claims were located and development begun in 1922. The deposit consists of auriferous, sulfide-bearing, quartz fissure veins in albite-quartz diorite and greenstone that are intruded by an aplite dike. The wallrock is sericitized near the veins. The vein that was mined was about 2 feet thick and contained generally sparse sulfides, including arsenopyrite, sphalerite, and galena, with minor pyrite, chalcopyrite, and pyrrhotite. Free gold occurred in the vein and in gouge along the hanging wall. Mining and milling in 1933-35 produced about 1,000 ounces of gold from 135 tons of ore from a stope about 70 feet long and 40 feet high. The mining equipment was removed in 1936. Bittenbender and others (1999), citing Kimball (1982), call this property the Mine Mountain Mine and note that it was developed by an adit and 250 feet of crosscuts and drifts. Samples across a 1.2-foot-wide vein in a 70-foot stope assayed up to 2.45 ounces of gold per ton. Samples from 120 feet of other workings contained up to 0.15 ounce of gold per ton across veins 0.2 to 0.7 foot thick. Isotopic studies indicate that the gold-quartz veins in coastal southern and southeastern Alaska are Eocene, about 50 Ma in age (Haeussler, 1995; Goldfarb, 1997; Goldfarb and others, 1997).
Workings: Bittenbender and others call this property the Mine Mountain Mine and note that it was developed by an adit and 250 feet of crosscuts and drifts.
Age: Isotopic studies indicate that the gold-quartz veins in coastal southern and southeastern Alaska are Eocene, about 50 Ma in age (Haeussler and others, 1995; Goldfarb, 1997; Goldfarb and others, 1997).
Alteration: The wallrock is sericitized near the veins.
Production: Mining and milling in 1933-35 produced about 1,000 ounces of gold from 135 tons of ore that was mined from a stope about 70 feet long and 40 feet high. The mining equipment was removed in 1936.
Commodities (Major) - Au, Cu, Pb, Zn
Development Status: Yes; small
Deposit Model: Polymetallic vein (Cox and Singer, 1986; model 22c).
7 entries listed. 7 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. Buddington, A.F., 1925, Mineral investigations in southeastern Alaska: U.S. Geological Survey Bulletin 773-B, p. 71-139. 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. Goldfarb, R J., 1997, Metallogenic evolution of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 4-34. Goldfarb, R.J., Miller, L.D., Leach, D.L., and Snee, L.W, 1997, Gold deposits in metamorphic rocks in Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 151-190. Haeussler, P J., Bradley, D., Goldfarb, R., Snee, L., and Taylor, C., 1995, Link between ridge subduction and gold mineralization in southern Alaska: Geology, v. 23, no. 11, p. 995-998. 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. 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]