Roberts (near Middle Fork of Kuskokwim River)

Prospect, Active

Commodities and mineralogy

Main commodities Cu; Ni; Pd; Pt
Other commodities Ag; Au; Bi; Co; Cr
Ore minerals bravoite; chalcopyrite; galena; pentlandite; pyrite; pyrrhotite; te-bi sulfosalts; unidentified pge minerals
Gangue minerals hematite; magnetite

Geographic location

Quadrangle map, 1:250,000-scale MG
Quadrangle map, 1:63,360-scale A-3
Latitude 62.196
Longitude -154.4487
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Roberts prospect is on the crest of a steep, north-sloping ridge near the head of an unnamed tributary of the Middle Fork of the Kuskokwim River. It is at an elevation of about 4,170 feet about 0.2 mile southeast of the center of section 5, T. 24 N., R. 28 W., of the Seward Meridian. The location is accurate.

Geologic setting

Geologic description

The Roberts prospect is in a differentiated, mafic-ultramafic sill 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 N 55 E and dips about 35SE parallel to bedding; it varies in composition from picrite to diorite but mostly is gabbro. Surface sampling and mapping indicate that the sill is differentiated with picrite-gabbro near the base and diorite near the top (T.K. Bundtzen and G.M. Laird, written communication, 1998). Another mafic sill mapped by Gilbert and others (1988) about 3 kilometers southwest is on strike with the Roberts occurrence.
The mineralization at the Roberts prospect is mainly disseminated and network-style sulfides with a notable content of platinum group elements (PGE) in the lower and middle part of the sill. The ore minerals 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 identify the PGE minerals at the prospect. Based on limited chip-channel sampling, the thickness of the mineralized zone varies from 2 to 4 meters and extends for a strike length of at least 25 meters.
Five samples taken perpendicular to the strike of the sill contained up to 0.49 parts per million (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 Bureau of Mines 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; Bundtzen, Harris, and Gilbert, 1997).
Brozdowski and Taylor (2009) reported on recent work by Nycon Resources Inc. They drilled the prospect and flew two airborne geophysical surveys over it covering about 195 square kilometers. Brozdowski and Taylor describe the Roberts prospect as an anastomosing dike- to sill-like body of olivine gabbro and peridotite that extends for about 430 meters along strike and is about 50 feet thick. Its magnetic expression and drill holes indicate it dips steeply west. Several rock chip samples contained 2.27 to 2.32 percent nickel, 1.31 to 2.30 percent copper and 13.22 to 16.88 ppm platinum group elements. A drill intercept 8.4 meters long contained 0.94 ppm platinum and 1.07 ppm palladium. The geophysical data suggests the Roberts prospect is part of a belt of similar deposits that includes the nearby Chip-Loy prospect (MG032).
Geologic map unit (-154.450939623452, 62.1953656136374)
Mineral deposit model Disseminated Ni-Cu-PGE in gabbro (Cox and Singer, 1986; model 5b).
Mineral deposit model number 5b
Age of mineralization Probably correlative with Late Triassic feeders in Tatina River Volcanics (Bundtzen, Harris, and Gilbert, 1997).
Alteration of deposit Serpentinization of ultramafic phases of sill.

Production and reserves

Workings or exploration The Robert's prospect was discovered in 1981 by W.S. Roberts of the U.S. Bureau of Mines while working with the Alaska Division of Geological and Geophysical Surveys in the McGrath quadrangle. The prospect was sampled by the Alaska Division of Geological and Geophysical Surveys and the U.S. Bureau of Mines several times in the 80s and 90s. In 2009, Nycon Resources Inc. was active on the property; they drilled the prospect and flew two airborne geophysical surveys over it that covered about 195 square kilometers.
Indication of production None
Reserve estimates None.
Production notes None.


MRDS Number 10307843


Reporters T.K. Bundtzen (Pacific Rim Geological Consulting); D.J. Grybeck (Contractor, U.S. Geological Survey)
Last report date 4/2/2010