Bernard Mountain (Red Mountain)

Prospect, Undetermined

Commodities and mineralogy

Main commodities Cr
Other commodities Ni; PGE
Ore minerals chromite; pge minerals
Gangue minerals olivine; pyroxene

Geographic location

Quadrangle map, 1:250,000-scale VA
Quadrangle map, 1:63,360-scale C-4
Latitude 61.5725
Longitude -145.1411
Nearby scientific data Find additional scientific data near this location
Location and accuracy This prospect is located near VABM Bernard on the northern-most high divide between Bernard Creek and the Little Tonsina River. It is about 3 miles east of the Richardson Highway. The prospect is near the middle of the south boundary of section 25, T. 3 S., R. 1 E., of the Copper River Meridian. This prospect is approximately located, perhaps to within a mile. It is locality 42 of Winkler and others (1981 [OFR 80-892-B]). Cobb and Matson (1972, locality 51) erroneously show this prospect to be located about 3 miles to the southwest of the location used by Winkler and others (1981 [OFR 80-892-B]).

Geologic setting

Geologic description

Dunite, pyroxenite, and peridotite in the Tonsina ultramafic complex (Winkler and others, 1981 [OFR 80-892-B]; Burns, 1985; Foley and others, 1988) contain disseminated chromite, some podiform chromite, and local anomalous concentrations of nickel and platinum-group metals. At Bernard Mountain, seven low-grade deposits of banded and disseminated chromite in dunite are estimated to contain 343,000 tons of Cr2O3 (Berg and Cobb, 1967; Hoffman, 1972; Foley and Barker, 1985; Foley and others, 1986; Dahlin and others, 1985). High-grade samples contain 36.0 to 57.7 percent chromite with a chrome to iron ratio of 1.20 to 3.06. The maximum nickel concentration is 5,000 ppm (Hoffman, 1972). Two chromite-rich table concentrate samples contained 823 and 1,749 ppb palladium, but PGEs were not detected in other samples from Bernard Mountain (Foley and others, 1988; 1989). The Tonsina ultramafic complex may be a deep part of large gabbroic plutons in the root zones of the Jurassic Talkeetna arc (Winkler and others, 1981 [OFR 80-892-A]; Burns, 1985). Because the Tonsina ultramafic complex contains granulites and tectonized harzburgite and dunite, it may have some mantle components and in part be older than Jurassic mafic and ultramafic intrusives of the region (Burns, 1985).
Geologic map unit (-145.143204003768, 61.5720134842138)
Mineral deposit model Podiform chromite (Cox and Singer, 1986; model 8a)
Mineral deposit model number 8a
Age of mineralization The Tonsina ultramafic complex may be a deep part of large gabbroic plutons in the root zones of the Jurassic Talkeetna arc (Winkler and others, 1981 [OFR 80-892-A]; Burns, 1985). Because the Tonsina ultramafic complex contains granulites and tectonized harzburgite and dunite, it may have some mantle components and in part be older than Jurassic mafic and ultramafic intrusives of the region (Burns, 1985).
Alteration of deposit Serpentinization is widespread in the ultramafic rocks.

Production and reserves

Workings or exploration Considerable reconnaissance surface observations and sampling have been carried out.
Indication of production None
Reserve estimates At Bernard Mountain, seven low-grade deposits of banded and disseminated chromite in dunite are estimated to contain 343,000 tons of Cr2O3 (Berg and Cobb, 1967; Hoffman, 1972; Foley and Barker, 1985; Foley and others, 1986; Dahlin and others, 1985).

References

MRDS Number A011492; M045413

References

Burns, L.E., 1985, The Border Ranges ultramafic and mafic complex, south-central Alaska; Cumlative fractionates of island-arc volcanics: Canadian Journal of Earth Science, v. 22, p.1020-1038.
Foley, J.Y., Mardock, C.L., and Dahlin, D.C., 1988, Platinum-group metals in the Tonsina ultramafic complex, southern Alaska, in Vassiliou, A.H., Hausen, D.M., and Carson, D.J.T., Process Mineralogy VII: The Metallurgical Society, p. 165-195.
Hoffman, B., 1972, Geology of the Bernard Mountain area: Fairbanks, University of Alaska, M.Sc. thesis, 68 p.
Reporters Travis L. Hudson
Last report date 12/14/2001