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

Prospect in Alaska, United States with commodities Copper, Nickel, Silver, Gold, Bismuth, Cobalt, Chromium
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Sections on this page
  1. Identification information
  2. Geographic coordinates
  3. Site location context
  4. Geographic areas
  5. Commodities
  6. Materials information
  7. Alteration
  8. Mineral occurrence model information
  9. Nearby scientific data
  10. Economic information about the deposit and operations
  11. Mining district
  12. Links to other databases
  13. Bibliographic references
  14. General comments
  15. Reporter information

Geologic information

Identification information

Deposit ID 10307843
Record type Site
Current site name Robert's PGM (near Middle Fork of Kuskokwim River)

Geographic coordinates

Geographic coordinates: -154.44884, 62.19546 (WGS84)
Relative position 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.

Site location context

Political divisions (FIPS codes)

Yukon-Koyukuk(Census area)

Alaska(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

McGrath A-3(quadrangle 1:63,360 scale)

McGrath SE(quadrangle 1:100,000 scale)

McGrath C(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

Upper Kuskokwim River(hydrologic accounting unit)

Southwest(hydrologic subregion)

Alaska(hydrologic region)

Geographic areas

Country State
United States Alaska

Commodities

Commodity Importance
Copper Primary
Nickel Critical Primary
Silver Secondary
Gold Secondary
Bismuth Critical Secondary
Cobalt Critical Secondary
Chromium Critical Secondary

Materials information

Materials Type of material
Bravoite Ore
Chalcopyrite Ore
Galena Ore
Pentlandite Ore
Pyrite Ore
Pyrrhotite Ore
Hematite Gangue
Magnetite Gangue

Alteration

  • (Local) Serpentinization of ultramafic phases of sill.

Mineral occurrence model information

Model code 10
USGS model code 5b
Deposit model name Noril'sk Cu-Ni-PGE

Nearby scientific data

(1) -154.44884, 62.19546
ListMap

Economic information

Comments on the geologic information

  • 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).
  • Ore Material = Te-Bi sulfosalts
  • Age = Unknown; thought to be correlative with Late Triassic feeders in Tatina River Volcanics (Bundtzen, Harris, and Gilbert, 1997).

Economic information about the deposit and operations

Development status Prospect

Comments on exploration

  • Status = Inactive

Mining district

District name McGrath

Comments on the workings information

  • 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).

Reference information

Bibliographic references

Comments on the references

  • Primary Reference = Roberts and O'Connor, 1985

General comments

Subject category Comment text
Deposit Model Name = Disseminated Ni-Cu-PGE in gabbro (Cox and Singer, 1986; model 5b).
Deposit Other Comments = Similar to Chip-Loy deposit (MG032).

Reporter information

Type Date Name Affiliation Comment
Reporter 25-NOV-98 T.K. Bundtzen Pacific Rim Geological Consulting

Beyond USGS

Supplemental information added by qvyshift.com. Not part of the original USGS MRDS record.

Maps centered on this site

Authoritative Alaska resources

These are landing pages for further research — the state agencies don't currently expose per-mine deep links.