Cobol (Slocum Arm)

Mine, Undetermined

Commodities and mineralogy

Main commodities Au
Other commodities Pb
Ore minerals galena; gold; pyrite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale SI
Quadrangle map, 1:63,360-scale B-6
Latitude 57.493
Longitude -135.868
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.

Geologic setting

Geologic description

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).

Production and reserves

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 comments

The mine is in West Chichagof-Yakobi Wilderness.

References

MRDS Number A013289

References

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.
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.
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.)
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