Robert's PGM (near Middle Fork of Kuskokwim River)

Site Name

Primary: Robert's PGM (near Middle Fork of Kuskokwim River)

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Commodity

Primary: Nickel
Primary: Copper
Secondary: Bismuth
Secondary: Cobalt
Secondary: Chromium
Secondary: Gold
Secondary: Silver

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Location

State: Alaska
District: McGrath

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Land Status

Not available

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Prospect
Operation Type: Unknown
Years of Production:
Organization:
Significant:

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Physiography

Not available

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Mineral Deposit Model

Model Name: Noril'sk Cu-Ni-PGE

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Orebody

Not available

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Structure

Not available

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Alterations

Alteration Type: L
Alteration Text: Serpentinization of ultramafic phases of sill.

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Rocks

Not available

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Analytical Data

Not available

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Materials

Ore: Pyrrhotite
Ore: Pyrite
Ore: Pentlandite
Ore: Galena
Ore: Chalcopyrite
Ore: Bravoite
Gangue: Magnetite
Gangue: Hematite

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Comments

Comment (Deposit): Model Name = Disseminated Ni-Cu-PGE in gabbro (Cox and Singer, 1986; model 5b).

Comment (Deposit): Other Comments = Similar to Chip-Loy deposit (MG032).

Comment (Exploration): Status = Inactive

Comment (Geology): Geologic Description = The Robert's PGM prospect is hosted in a differentiated, mafic-ultramafic sill-form intrusion that cuts silty limestone and shale of the Late Cambrian to Early Ordovician Lyman Hills Formation, the oldest facies of the Dillinger subterrane (Bundtzen Harris, and Gilbert, 1997). The sill is undated, but believed to be correlative with Late Triassic feeders in the Tatina River Volcanics, a subdivision of the Middle Devonian to Lower Jurassic Mystic subterrane (Bundtzen, Harris, and Gilbert, 1997; T.K. Bundtzen and G.M. Laird, written communication,1998). The mafic-ultramafic sill trends N55E, dips about 35SE parallel to bedding, and ranges in composition from picrite to diorite but is generally a gabbro. Surface sampling and sampling indicates that the sill is differentiated with picrite-gabbro near the base and diorite occurring near the top (T.K. Bundtzen and G.M. Laird, written communication, 1998). Another mafic sill was mapped by Gilbert and others (1988) about 3 kilometers southwest and on strike with the Robert's PGM occurrence. Principle mineralization at the Robert's PGM occurrence consists of disseminated and network-style sulfides in the lower and middle part of the sill. These include megascopic chalcopyrite, pyrite, magnetite, and pyrrhotite, and microscopic bravoite, galena, Bi-Te sulfosalts, and pentlandite. According to W.S. Roberts (written communication, 1984) , the PGE mineral may be a palladium bismuth telluride. T.K. Bundtzen and G.M. Laird (written communication, 1998) could not firmly establish a mineralogical source of the PGE values at the prospect. The thickness of the mineralized zone, based on limited chip-channel sampling efforts, ranges from 2 to 4 meters thick; a minimum strikelength of about 25 meters was measured. Five samples taken perpendicular to the strike of the sill contained up to 0.49 ppm palladium, 0.35 ppm platinum, 0.8 ppm silver, 0.09 ppm gold, 1,315 ppm copper, 1,510 ppm nickel, and 1,530 ppm chromium (T.K. Bundtzen and G.M. Laird, written communication, 1998). Bench tests conducted by the U.S. Bureau of Mines (now U.S. Bureau of Land Management) show up to 4.71 percent nickel, 0.16 percent cobalt, 4.68 percent copper, 6.2 ppm platinum, 7.7 ppm palladium, and 4.7 ppm gold (W.S. Roberts, written communication, 1984; Foley and others, 1997; Bundtzen, Harris, and Gilbert, 1997).

Comment (Workings): Workings / Exploration = The Robert's PGM prospect was discovered in 1981 by W.S. Roberts, formerly of the U.S. Bureau of Mines, while conducting cooperative strategic mineral studies with the Alaska Division of Geological and Geophysical Surveys in the McGrath quadrangle (Roberts and O'Connor, 1985). Five samples taken perpendicular to the strike of the sill by T.K. Bundtzen and G.M. Laird (written communication, 1998) contained up to 0.49 ppm palladium, 0.35 ppm platinum, 0.8 ppm silver, 0.09 ppm gold, 1,315 ppm copper, 1,510 ppm nickel, and 1,530 ppm chromium. Bench tests conducted by the U.S. Bureau of Mines (now U.S. Bureau of Land Management) showed up to 4.71 percent nickel, 0.16 percent cobalt, 4.68 percent copper, 6.2 ppm platinum, 7.7 ppm palladium, and 4.7 ppm gold (W.S. Roberts, written communication, 1984; Foley and others, 1997).

Comment (Geology): Ore Material = Te-Bi sulfosalts

Comment (Geology): Age = Unknown; thought to be correlative with Late Triassic feeders in Tatina River Volcanics (Bundtzen, Harris, and Gilbert, 1997).

Comment (Reference): Primary Reference = Roberts and O'Connor, 1985

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References

Reference (Deposit): Bundtzen, T.K., Harris, E.E., and Gilbert, W.G., 1997, Geologic Map of the eastern McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 97-14, 34 pages, one sheet, scale 1:125,000.

Reference (Deposit): Foley, J.Y., Light, T.D., Nelson, S.W., and Harris, R.A., 1997, Mineral occurrences associated with mafic-ultramafic and related alkaline complexes in Alaska: Economic Geology, Monograph 9, p. 396-449.

Reference (Deposit): Gilbert, W.G., Solie, D.N., and Kline, J.T., 1988, Geologic map of the McGrath A-3 quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Professional Report 92, 2 sheets, 1:63,360 scale.

Reference (Deposit): Roberts, W.S., and O'Connor, W.K., 1985, Petrographic and geological summary, Robert's PGM occurrence, McGrath A-3 quadrangle, Alaska: U.S. Bureau of Mines unpublished prospect examination report, 9 pages.

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