Similar to Chip-Loy deposit (MG032).
Location: The Robert's PGM prospect is located on the crest of a steep, north-sloping ridgeline near the head of an unnamed tributary of the Middle Fork of the Kuskokwim River. The prospect is approximately 5 kilometers south-southwest of Middle Fork airstrip (Bob Fithian's guide camp) at an elevation of 4,350 (1,326 m) feet in the SE1/4 sec. 5, T. 24 N., R. 28 W., of the Seward Meridian. The reporter investigated the site in 1982 and 1998.
Geology: 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).
Workings: 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).
Age: Unknown; thought to be correlative with Late Triassic feeders in Tatina River Volcanics (Bundtzen, Harris, and Gilbert, 1997).
Alteration: Serpentinization of ultramafic phases of sill.
Reserves: None.

Commodities (Major) - Cu, Ni, Pd, Pt; (Minor) - Ag, Au, Bi, Co, Cr
Development Status: No
Deposit Model: Disseminated Ni-Cu-PGE in gabbro (Cox and Singer, 1986; model 5b).

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Standard Detailed Gallery Strunz Chemical Elements

Commodity List

This is a list of exploitable or exploited mineral commodities recorded at this locality.

Mineral List

7 valid minerals.