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