Unnamed

Occurrence, Inactive

Commodities and mineralogy

Main commodities Cu; Fe; Ni
Other commodities Bi; Cr
Ore minerals chalcopyrite; pyrite; pyrrhotite
Gangue minerals magnetite

Geographic location

Quadrangle map, 1:250,000-scale MG
Quadrangle map, 1:63,360-scale B-2
Latitude 62.389
Longitude -153.96
Nearby scientific data Find additional scientific data near this location
Location and accuracy The unnamed occurrence is located on a sharp northwest-trending ridge immediately north of a 5,550 foot (1,692 m) knob on the USGS McGrath B-2 1:63,360 quadrangle at an elevation of 5,540 feet (1,689 m) in the NW1/4 sec. 32, T. 27 N., R. 25 W., of the Seward Meridian. The reporter found the site in 1981 and 1989; at station no. 81BT549.

Geologic setting

Geologic description

This unnamed occurrence consists of disseminated sulfides in a differentiated picrite-diorite sill that intrudes highly altered chert, sandstone, and siltstone of the Middle Devonian to Permian Sheep Creek Formation, a unit of the Mystic subterrane (Bundtzen, Harris, and Gilbert, 1997). The 15- meter-thick, mafic sill trends roughly EW and dips about 35N. Exposures continue along strike for about 3 kilometers before dipping below talus cover on both ends. Locally, the sill has produced a large 150-meter-wide alteration area in the sedimentary rocks, which is composed of chalcedonic-carbonate replacement of both country rock and the sill itself. The differentiated sill is interpreted by Bundtzen, Harris, and Gilbert (1997) to be a feeder for mafic volcanism in the overlying Late Triassic to Lower Jurassic Tatina River Volcanics.
Mineralization appears at the base of the sill, and consists of abundant magnetite, pyrite, pyrrhotite, and traces of chalcopyrite. The size of the mineralized area was not determined. Samples of the sill are composed of up to 50 percent coarse grained magnetite interlocked with silicate melt minerals of magmatic origin. Some network style pyrite-pyrrhotite-magnetite zones contain up to 200 ppm copper, 600 ppm nickel, 100 ppm bismuth, and 1,000 ppm chromium (Bundtzen, Kline, and Clough, 1982; Bundtzen, Harris, and Gilbert, 1997). Anomalous copper and chromium values were found in stream sediment samples taken in the area (Reed and Elliott, 1968, OFR).
Geologic map unit (-153.962221361078, 62.3883858985167)
Mineral deposit model Disseminated copper-nickel-cobalt (Cox and Singer, 1986; model 5b).
Mineral deposit model number 5b
Age of mineralization Sill assigned a Late Triassic age by Bundtzen, Harris, and Gilbert (1997).
Alteration of deposit Chalcedonic, carbonate.

Production and reserves

Workings or exploration Surface samples contain up to 200 ppm copper, 600 ppm nickel, 100 ppm bismuth, and 1,000 ppm chromium (Bundtzen, Kline, and Clough, 1982). Anomalous copper and chromium values were found in stream sediment samples taken in the occurrence area (Reed and Elliott, 1968, OFR).
Indication of production None
Reserve estimates None.

Additional comments

Similar to Roberts PGM (MG030) and Chip Loy (MG032) prospects in McGrath A-3 quadrangle.

References