Unnamed (at head of Coco Harbor) (ARDF #CR140) Ag, Au, Cu, Mo

Unnamed (at head of Coco Harbor)

Occurrences, Undetermined

Commodities and mineralogy

Main commodities	Ag; Au; Cu; Mo
Ore minerals	chalcopyrite; molybdenite; pyrite; pyrrhotite
Gangue minerals	quartz

Geographic location

Quadrangle map, 1:250,000-scale	CR
Quadrangle map, 1:63,360-scale	A-4
Latitude	55.04602
Longitude	-133.08517
Nearby scientific data	Find additional scientific data near this location
Location and accuracy	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.

Geologic setting

Geologic description 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).
Geologic map unit	(-133.086815705374, 55.0456692705291)
Mineral deposit model	Ag-Cu-Mo skarn (Cox and Singer, 1986; model 18b).
Mineral deposit model number	18b
Age of mineralization	Silurian or Ordovician or younger.
Alteration of deposit	Mineralization associated with skarn and hornfels.

Production and reserves

Workings or exploration	Only surface sampling by government and industry geologists.
Indication of production	None
Additional comments These occurrences are on land that has been conveyed to the Sealaska Corporation, who hold the surface and subsurface rights, or the land is under application for transfer to them.

References

References 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: U.S. Bureau of Mines Open-File Report 33-91, 139 p., 12 sheets, scale 1:63,360.
Reporters	D.J. Grybeck (Applied Geology)
Last report date	5/1/2004