|Quadrangle map, 1:250,000-scale||SI|
|Quadrangle map, 1:63,360-scale||D-3|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||Bittenbender and others (1999) report that the Big Ledge prospect is in a gully at an elevation of 500 feet, 1.6 miles west-southwest of East Point, but this location does not seem to agree in elevation with the D-3 topographic map at that site. For this record, the site is plotted along a logging road at the west edge of the SW1/4 sec. 12, T. 47 S., R. 64 E. It is location P-24 of Bittenbender and others (1999), location 48 of Cobb (1972, 1978), and MAS no. 0021140015 (U.S. Bureau of Land Management, 2002). The location is accurate within about 0.2 mile.|
Geologic descriptionThe rocks in the vicinity of the Big Ledge prospect are part of the Devonian and/or Silurian, Kennel Creek Limestone which consists of limestone, minor dolomite, limestone breccia, shale, siltstone, and conglomerate (Lathram and others, 1965). A gabbro or diabase dike about 20 feet thick cuts conglomerate at this prospect (Buddington, 1925; Bittenbender and others, 1999) . About 6 feet of the center of the dike is faulted and fractured, and rare quartz and calcite stringers cut it. The dike contains stringers of pyrrhotite up to 6 inches thick as well as disseminated pyrrhotite. There is small amount of pyrite and chalcopyrite along fractures. One sulfide-rich sample contained pyrrhotite, chalcopyrite, pentlandite, a secondary nickel mineral, a little pyrite, and a trace of sphalerite. Bittenbender and others (1999) sampled the deposit. Several samples of the mafic dike rock contained up to 0.91 percent copper, 0.84 percent nickel, and 910 parts per million cobalt.; a select sample with about 50 percent sulfides contained 7.02 percent copper, 4.4 percent nickel, and 910 parts per million cobalt. A claim was staked on the prospect but there was no development as of 1924 (Buddington, 1925).
|Geologic map unit||(-134.986577602203, 57.8037613146006)|
|Mineral deposit model||Gabbro dike with copper and nickel.|
|Age of mineralization||Silurian or younger based on the age of the host rocks.|
|Workings or exploration||A claim was staked on the prospect prior to 1923 but there was no development as of that date.|
|Indication of production||None|
Berg, H.C., and Cobb, E.H., 1967, Metalliferous lode deposits of Alaska: U.S. Geological Survey Bulletin 1246, 254 p.
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.
Buddington, A.F., and Chapin, Theodore, 1929, Geology and mineral deposits of southeastern Alaska: U.S. Geological Survey Bulletin 800, 398 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.
Lathram, E.H., Pomeroy, J.S., Berg, H.C., and Loney, R.A., 1965, Reconnaissance geology of Admiralty Island, Alaska: U.S. Geological Survey Bulletin 1181-R, p. R1-R48.
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||Donald Grybeck (U.S. Geological Survey)|
|Last report date||5/5/2005|