Unnamed (western Gravina Island)

Prospect, Undetermined

Commodities and mineralogy

Main commodities Ag; Cu
Other commodities Mo
Ore minerals chalcopyrite; hematite; molybdenite; pyrite
Gangue minerals dolomite; quartz

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale B-6
Latitude 55.263
Longitude -131.834
Nearby scientific data Find additional scientific data near this location
Location and accuracy This prospect is at an elevation of about 150 feet on western Gravina Island, about 0.3 mile from tidewater, and about 1.0 mile south-southeast of triangulation point 'Pug.' The site is in section 23, T. 76 S., R. 89 E., of the Copper River Meridian. It corresponds to loc. 52.1 in Elliott and others (1978). The location is accurate within 0.1 mile.

Geologic setting

Geologic description

The country rocks at this prospect consist of Upper Triassic rhyolite and rhyolite tuff (Berg, 1973, Berg and others, 1988). This formation is depositionally overlain by, and locally in fault contact with, Upper Triassic carbonate, clastic, and basaltic volcanic rocks. The strata were regionally metamorphosed to prehnite-pumpellyite or to low-greenschist grade in Late Cretaceous time. Locally, the limestone is dolomitized, and the rhyolite contains conspicuous hydrothermal hematite.
The deposit is in iron-stained metarhyolite(?) and consists of a ten-foot-wide breccia zone containing quartz, hematite, dolomite, chalcopyrite, and pyrite (Berg, 1973, p. 37). An analysis of a sample of the mineralized breccia showed 1.5 ppm Ag and 2% Cu (Koch and Elliott, 1978; OFR 78-156A). In 1970, workings consisted of a water-filled adit that probably was driven in the early 1900s. Prospecting in this area in the 1960s and 70s reportedly revealed disseminated chalcopyrite and pyrite, and traces of molybdenite and secondary copper minerals in complexly sheared, hydrothermally altered metamorphic and intrusive country rocks (Berg, 1973, p. 37). Assuming that the mineralized breccia postdates the regional metamorphism, the deposit is Late Cretaceous or younger.
Geologic map unit (-131.835676802945, 55.2626498275953)
Mineral deposit model Polymetallic veins? (Cox and Singer, 1986; model 22c)
Mineral deposit model number 22c?
Age of mineralization Late Cretaceous or younger.
Alteration of deposit Limestone locally is dolomitized, and rhyolite locally contains conspicuous hydrothermal hematite. Local iron- and copper-staining.

Production and reserves

Workings or exploration In 1970, workings consisted of a water-filled adit that probably was driven in the early 1900s. An analysis of a sample of the mineralized breccia showed 1.5 ppm Ag and 2% Cu (Koch and Elliott, 1978 [OFR 78-156A]). Prospecting in this area in the 1960s and 70s reportedly revealed disseminated chalcopyrite and pyrite, and traces of molybdenite and secondary copper minerals in complexly sheared, hydrothermally altered metamorphic and intrusive country rocks (Berg, 1973, p. 37).
Indication of production None

References