Dust Mountain

Prospect, Undetermined

Commodities and mineralogy

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

Geographic location

Quadrangle map, 1:250,000-scale VA
Quadrangle map, 1:63,360-scale C-3
Latitude 61.6025
Longitude -144.9436
Nearby scientific data Find additional scientific data near this location
Location and accuracy This prospect is one-half mile northwest hill 5,210 (which is informally but commonly referred to as 'Dust Mountain') and 1.2 mile due east of Dust Creek. It is at an elevation of about 4,700 feet near the southwest corner of section 18, T. 3 S., R. 3 E., of the Copper River Meridian. This prospect is approximately located, perhaps to wtithin one-ha;f mile. It is locality 52 of Cobb and Matson (1972) and 43A of Winkler and others (1981 [OFR 80-892-B]).

Geologic setting

Geologic description

Dunite, pyroxenite, and peridotite of the Tonsina ultramafic complex (Winkler and others, 1981 [OFR 80-892-A]; Burns, 1985; Foley and others, 1988) contain disseminated chromite, some podiform chromite, and local anomalous concentrations of nickel and platinum-group metals. At Dust Mountain, a 190-foot by 3,600-foot area was estimated to contain 2.6 to 5.3 million tons of chromic oxide (Berg and Cobb, 1967; Foley and Barker, 1985; Foley and others, 1986; Dahlin and others, 1985). High-grade samples contained 36.0 to 57.7 percent chromite, with a chrome to iron ratio of 1.20 to 3.06. Samples of chromatite contained as much as 11,936 ppb palladium (Pd), 8,918 ppb platinum (Pt), 330 ppb iridium, 87 ppb osmium, 550 ppb rhenium, and 140 ppb ruthenium (Foley and others, 1988, 1989). Pt/Pt+Pd ratios ranged from 0.44 to 0.87. Identified PGE minerals include PGE-amalgams, arsenides, alloys, and sulfides (Foley and others, 1988). The PGE-bearing minerals are concentrated along fractures and grain boundaries in serpentinized, metasomatically altered, high chromium spinel. Maximum PGE concentrations are in malachite- and native copper-bearing magnetic chromatites associated with dunite and coarse-grained clinopyroxenite. The Tonsina ultramafic complex may be deep parts 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 may part be older than Jurassic mafic and ultramafic intrusives of the region (Burns, 1985).
Geologic map unit (-144.945705349989, 61.6020186983822)
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 deep parts 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 may part be older than Jurassic mafic and ultramafic intrusives of the region (Burns, 1985).
Alteration of deposit Sepentinization.

Production and reserves

Workings or exploration Considerable reconnaissance surface observations and sampling have been completed.
Indication of production None
Reserve estimates At Dust Mountain, a 190-foot by 3,600-foot area was estimated to contain 2.6 to 5.3 million tons of chromic oxide (Berg and Cobb, 1967; Foley and Barker, 1985; Foley and others, 1986; Dahlin and others, 1985).

References

MRDS Number A011493

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.
Reporters Travis L. Hudson
Last report date 12/14/2001