Dust Mountain Prospect, Nelchina Mining District, Valdez-Cordova Census Area, Alaska, USAi

Regional Level Types
Dust Mountain Prospect	Prospect
Nelchina Mining District	Mining District
Valdez-Cordova Census Area	Census Area
Alaska	State
USA	Country

Location: 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]).
Geology: 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 malchite- 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).
Workings: Considerable reconnaissance surface oberservations and sampling have been completed.
Age: 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: Sepentinization.
Reserves: 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).

Commodities (Major) - Cr; (Minor) - Ni, PGE
Development Status: None
Deposit Model: Podifrom chromite (Cox and Singer, 1986; model 8a)

Other Databases

Link to USGS - Alaska:	VA016

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References