Unnamed Prospect (ARDF - SI012; north shore of outer Bertha Bay), Chichagof District, Sitka Borough, Alaska, USA
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Location: This prospect is at sea level at the north mouth of Bertha Bay. The prospect is at the head of a small bay just east of Pt. Dougherty, and 0.4 mile southwest of the center of sec. 9, T. 47 S., R. 56 E. It is location P-41 of Bittenbender and others (1999), location 15 of Cobb (1972, 1978), and MAS no. 0021140014 (U.S. Bureau of Land Management, 2002). The location is accurate within about 0.5 mile.
Geology: Johnson and Karl (1985) map the rocks in the area of this prospect as Cretaceous graywacke that is intruded by Tertiary gabbronorite and tonalite, and cut by high-angle faults that mainly strike northwest. The graywacke is thermally metamorphosed to hornfels near the contact of the gabbronorite and tonalite. The deposit consists of stringers of chalcopyrite and pyrrhotite in mafic intrusive rock (gabbronorite?) near its contact with the metamorphosed country rock. Bittenbender and others (1999) interpret the prospect as a magmatic segregation deposit, presumably similar to the one at Bohemia Basin (SI001).
Age: Assuming that the deposit is a magmatic segregation, it is Tertiary, the age of the gabbronorite(?) host rock.
Alteration: Thermal metamorphism of country rocks.
Commodities (Major) - Cu
Development Status: None
Deposit Model: Duluth Cu-Ni-PGE? (Cox and Singer, 1986; model 5a).
2 entries listed. 2 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. 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]