|Quadrangle map, 1:250,000-scale||SI|
|Quadrangle map, 1:63,360-scale||B-6|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||The Cobol Mine in Slocum Arm is at an elevation of about 550 feet, about 0.5 mile north of the ruins of the town of Cobol. The mine is at an adit symbol on the USGS B-6 topographic map (1996), 0.2 mile north-northeast of the center of sec. 36, T. 50 S., R. 59 E. It is location P-91 of Bittenbender and others (1999), location 44 of Cobb (1972, 1978), and MAS no. 0021140026 (U.S. Bureau of Land Management, 2002). The location is accurate.|
Johnson and Karl (1985) map the rocks in the area of the Cobol Mine as Cretaceous melange and greenstone, and as the Freeburn assemblage, a collage of kilometer-scale, fault-bounded, lozenge-shaped blocks of Cretaceous and Cretaceous(?), metasedimentary and metavolcanic rocks. The rocks are cut by numerous high-angle, northwest-striking faults. They include a regional fault along Slocum Arm, and others that may be the extensions of the faults at the Hirst (SI087) and Chichagof (SI093) mines that control the mineralization there. There also is at least one steeply-dipping fault that strikes about north.
Reed and Coats (1941) describe the deposit as an auriferous, fault-controlled quartz vein. The fault strikes about N5E and dips nearly vertically; it cuts graywacke that strikes about N25W and dips about 75SW. A light-colored dike apparently intruded along the fault is offset by subsequent movement on the fault. The vein is up to about a foot thick and consists of gouge and milky quartz, locally with ribbon structure. The veins contain sparse pyrite, galena, and gold; gold also occurs in fractures in the dike rock. Post-ore fault movement is marked by smeared pyrite and gold along fault surfaces. Workings include a lower tunnel about 1,330 feet long and an upper tunnel about 540 feet long.
Bittenbender and others (1999), citing Stewart (1931) and Kimball (1982), report production of 100 ounces of gold from a mineralized fault zone, 57 feet long and 3 feet wide, that averaged 0.28 ounce of gold per ton. A rich sample of mineralized float below the workings assayed 8.74 ounces of gold per ton. They report two adits, 1,600 and 550 feet long, a winze, and a stope.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).
|Geologic map unit||(-135.869803252096, 57.492639945868)|
|Mineral deposit model||Low-sulfide gold-quartz vein (Cox and Singer, 1986; model 36a).|
|Mineral deposit model number||36a|
|Age of mineralization||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).|
|Workings or exploration||Reed and Coats (1941) report a lower tunnel about 1,330 feet long and an upper tunnel about 540 feet long. Bittenbender and others (1999) report two adits, 1,600 and 550 feet long, a winze, and a stope.|
|Indication of production||Yes; small|
|Production notes||Bittenbender and others (1999) report production of 100 ounces of gold from a mineralized fault- zone, 57 feet long and 3 feet wide, that averaged 0.28 ounce of gold per ton.|
Additional commentsThe mine is in West Chichagof-Yakobi Wilderness.
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.
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.
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.
Ship, C.B.P., and Shipman, E.M., 1938, Cobol prospect: Unpublished report, 14 p. (Report held by the Mineral Information Center, Bureau of Land Management, Juneau, Alaska.)
Stewart, B.D., 1931, Memorandum on the mineral resources of Chichagof Island: Alaska Territorial Department of Mines, Miscellaneous Report 114-3, 5 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]
|Reporters||H.C. Berg (U.S. Geological Survey)|
|Last report date||10/16/2004|