Unnamed (east of North Fork Rainy Creek)

Occurrences, Inactive

Commodities and mineralogy

Main commodities Co; Cr; Cu
Other commodities Pt
Ore minerals chalcocite; chromite; copper; digenite; magnetite
Gangue minerals olivine

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale B-4
Latitude 63.3254
Longitude -145.9039
Nearby scientific data Find additional scientific data near this location
Location and accuracy This location is at the approximate center of a half-mile-long area of three similar occurrences (Nokleberg and others, 1991, localities S150, S151, S152). It is also within one-quarter mile of a native copper occurrence reported by Foley (1992). For this record the site is at about 5,600 feet elevation at the center of section 30, T. 18 S., R. 10E., Fairbanks Meridian.

Geologic setting

Geologic description

The occurrences consist of chromite and minor to trace amounts of native copper and copper sulfide(s) (?) in olivine cumulate (dunite) in the middle part of the Rainy ultramafic-mafic complex of Late Triassic age (Nokleberg and others, 1991). U.S. Geological Survey grab samples of dunite contained more than 5,000 parts per million (ppm) chrome and 500 ppm cobalt (Nokleberg and others, 1991).
Very fine grained disseminated native copper blebs in dunite were observed by U.S. Bureau of Mines geologists (Foley, 1992). U.S. Bureau of Mines microprobe analyses of several dunite samples identified platinian copper (platinian copper alloy with 19.5 to 28.6 percent platinum) that occurs as 5- to 15-micron grains in the chromite, along with chromian-magnetite and copper sulfide (chalcocite or digenite).
The Rainy ultramafic-mafic complex is a steeply north dipping tabular intrusion of dunite, peridotite, and gabbro that varies from less than 100 feet thick to more than 6,000 feet thick and extends for more than 12 miles in length (W.T. Ellis, oral communication, 2001). Discontinuous marginal gabbro extends along most of the southern (lower) contact and is less continuous along the northern (upper) contact. The complex intrudes the Slana Spur Formation of Pennsylvanian age (Nokleberg and others, 1991). Magmatic mineralization is synchronous with emplacement of the Rainy complex, which is part of a 120-mile-long belt of mafic-ultramafic and associated rocks in the east-central Alaska Range.
Geologic map unit (-145.906159003608, 63.3249835765372)
Mineral deposit model Disseminated chromite in a layered mafic-ultramafic complex.
Age of mineralization Late Triassic.

Production and reserves

Workings or exploration
Exploration in this area has been by American Copper and Nickel Company working with Fort Knox Gold Resources, Inc. Exploration from 1995 through 1998 included rock sampling, airborne and ground geophysical surveys, and completion of one diamond drill hole (W.T. Ellis, oral communication, 2001).
U.S. Bureau of Mines microprobe analyses of several dunite samples identified platinian copper (platinian copper alloy with 19.5 to 28.6 percent platinum) that occurs as 5- to 15-micron grains in the chromite, along with chromian-magnetite and copper sulfide (chalcocite or digenite).
The occurrences are on active claims of Fort Knox Gold Resources.
Indication of production None

References