Tongass

Prospect, Undetermined

Commodities and mineralogy

Main commodities Au
Ore minerals gold; pyrite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale B-6
Latitude 55.409
Longitude -131.809
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Tongass prospect is at tidewater on northeast Gravina Island, about 0.3 mile southwest of Rock Point. The site is in section 36, T. 74 S., R. 89 E., of the Copper River Meridian. It corresponds to loc. 56 in Elliott and others (1978). The location is accurate within 0.1 mile.

Geologic setting

Geologic description

The country rocks on northern Gravina Island are chiefly Upper Jurassic to mid-Cretaceous andesitic and basaltic volcanic rocks that gradationally intertongue with flyschlike sedimentary rocks. The rocks are intruded by at least one pluglike body of Tertiary gabbro (Berg, 1973; Berg and others, 1988). The bedded rocks were regionally metamorphosed to low-greenschist grade in Late Cretaceous time, and subsequently contact metamorphosed to hornblende hornfels near the contact of the Tertiary gabbro. The country rocks are cut by a high-angle fault along Tongass Narrows that displays about 4 miles of right-lateral offset.
The deposit is in Jurassic or Cretaceous slate and phyllite and consists of a reportedly auriferous, pyrite-bearing quartz fissure vein about one foot thick (Brooks, 1902, p. 61). The vein strikes NE and dips steeply NW, at an acute angle to the layering in the slate and phyllite.
Geologic map unit (-131.810682833699, 55.4086452779934)
Mineral deposit model Low-sulfide Au-quartz vein (Cox and Singer, 1986; model 36a)
Mineral deposit model number 36a
Age of mineralization Late Cretaceous or younger.

Production and reserves

Indication of production None

References