Unnamed Occurrences (ARDF - CR140), Coco Harbor, Dall Island, Ketchikan District, Prince of Wales-Outer Ketchikan Borough, Alaska, USA
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Location: These occurrences are along the shoreline at the northwest head of Coco Harbor; they are about 0.2 mile southeast of the center of section 5, T. 79 S., R. 82 E.
Geology: As described by Maas and others (1991), this deposit consists of a large area of hornfels and skarn with molybdenum and copper mineralization. The deposit is in marble interlayered with felsic and mafic metavolcanic rocks, metagraywacke, and chlorite schist, all of which are cut by diorite dikes. The hornfels contains disseminated pyrite, pyrrhotite, molybdenite, and minor chalcopyrite. Quartz veins with pyrite, pyrrhotite, minor molybdenite, and rare chalcopyrite, are locally abundant. In 28 samples, the highest molybdenum value was 0.24 percent across a foot-thick quartz vein. The maximum copper value was 601 parts per million (ppm); the highest gold value was 34 ppm, but most samples contained less than 5 parts per billion. Hedderly-Smith (1999 [Inventory]) sampled a piece of limestone float with an 8-inch-thick quartz vein containing chalcopyrite, pyrite, and an unidentified black mineral. The sample contained 2,375 ppm copper, 4.2 ppm silver, and 1.1 ppm gold. The rocks in the area are part of the Descon Formation of Silurian and Ordovician age (Eberlein and others, 1983; Brew, 1996). A Cretaceous(?) pluton is nearby (see CR139).
Workings: Only surface sampling by government and industry geologists.
Age: Silurian or Ordovician or younger.
Alteration: Mineralization associated with skarn and hornfels.
Commodities (Major) - Ag, Au, Cu, Mo
Development Status: None
Deposit Model: Ag-Cu-Mo skarn (Cox and Singer, 1986; model 18b).
5 entries listed. 5 valid minerals.
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Brew, D.A., 1996, Geologic map of the Craig, Dixon Entrance, and parts of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2319, 53 p., 1 sheet, scale 1:250,000. Eberlein, G.D., Churkin, Michael, Jr., Carter, Claire, Berg, H.C., and Ovenshine, A. T., 1983, Geology of the Craig quadrangle, Alaska: U.S. Geological Survey Open-File Report 83-91, 52 p. Hedderly-Smith, D.A., 1999, Inventory of metallic mineral prospects, showings and anomalies on Sealaska lands, 1988 through 1998: Sealaska Corporation, Juneau, Alaska, 217 p. (internal report held by Sealaska Corporation, Juneau, Alaska). Maas, K.M., Bittenbender, P E., and Still, J.C., 1995, Mineral investigations in the Ketchikan mining district, southeastern Alaska: U.S. Bureau of Mines Open-File Report 11-95, 606 p. Maas, K.M., Still, J. C., Clough, A. H., and Oliver, L. K., 1991, Mineral investigations in the Ketchikan mining district, Alaska, 1990: Southern Prince of Wales Island and vicinity--Preliminary sample location maps and descriptions: U.S. Bureau of Mines Open-File Report 33-91, 139 p.