Nunatak Occurrence (south of Snowcap Mountain), McGrath Mining District, Bethel Census Area, Alaska, USAi

Regional Level Types
Nunatak Occurrence (south of Snowcap Mountain)	Occurrence
McGrath Mining District	Mining District
Bethel Census Area	Census Area
Alaska	State
USA	Country

Location: This occurrence is at an elevation of 5,500 feet on the south end of a small nunatak, 3.25 miles south of Snowcap Mountain. It is in the NW1/4 section 8, T 15 N, R 24 W, of the Seward Meridian. This location is accurate; other vein occurrences are within a quarter of a mile of this location (Gamble and others, 1989).
Geology: Barren quartz veins, quartz-amphibole veins, and sulfide-bearing veins occupy east-west trending, sub-vertical fractures in biotite-hornblende monzodiorite to quartz monzodiorite in this area (Gamble and others, 1989). Two sulfide-bearing veins at an elevation of 5,500 feet on the south end of a small nunatak are about 1 to 2 feet wide and contain 5 to 70 percent chalcopyrite, arsenopyrite, and magnetite, and lesser amounts of galena, sphalerite, pyrite, and pyrrhotite. Erythite (cobalt bloom) is locally present and gangue minerals in the sulfide-bearing veins include amphibole, apatite, clinopyroxene, chlorite, epidote, quartz, and plagioclase. Composite chip samples of these veins contain as much as 1.5 ppm gold, 150 ppm silver, and greater than 20,000 ppm copper, 2,000 ppm arsenic, and 2,000 ppm copper. Apatite is locally abundant in the veins, and some samples contain as much as 1 percent phosphorus. Other sulfide-rich samples from the area contain up to 12 ppm gold, 700 ppm silver, and 640 ppm bismuth, and greater than 20,000 ppm copper, 2,000 ppm arsenic, and 2,000 ppm cobalt. A sample that contains greater than 20,000 ppm arsenic, 5,000 ppm cobalt, and 10,000 ppm nickel and only 7 percent iron may be glaucodot- or alloclasite-bearing (Gamble and others, 1989). The gangue mineralogy of the sulfide-bearing veins may indicate that they are in part mineralized mafic dikes.
Workings: Reconnaissance sampling has been done in this area (Gamble and others, 1989).
Age: Cretaceous or Tertiary? The veins crosscut granitic rocks that may be Late Cretaceous or Tertiary parts of the Alaska-Aleutian Range batholith.
Alteration: The sulfide-bearing veins may be variably altered and replaced mafic dikes.

Commodities (Major) - Ag, Au, Cu; (Minor) - Co, Ni, Pb, Zn
Development Status: None
Deposit Model: Polymetallic veins? (Cox and Singer, 1986, model 22c)

Other Databases

Link to USGS - Alaska:	LH025

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References