Commonwealth

Prospect, Undetermined

Commodities and mineralogy

Main commodities Au; Cu; Mo; Zn
Ore minerals chalcopyrite; gold; molybdenite; pyrrhotite; sphalerite
Gangue minerals quartz; skarn minerals

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale D-1
Latitude 55.75
Longitude -130.22
Nearby scientific data Find additional scientific data near this location
Location and accuracy The presumed location of this prospect is based on Buddington's (1929, p. 111-112 [B 807]) description of the Commonwealth group of claims, which in turn refers to somewhat ambiguous landmarks that cannot be accurately located on the Ketchikan D-1 topographic map. The map site therefore is probably accurate only within about a mile. The site is in section 34, T. 70 S., R. 99 E., of the Copper River Meridian. It corresponds to loc. 14 in Elliott and others (1978).

Geologic setting

Geologic description

The country rocks in the area of the presumed site are porphyritic quartz monzonite and granodiorite of the Eocene Davis River Pluton (Smith, 1977; Berg and others, 1988). According to Buddington (1929, p. 111-112), the Commonwealth prospect(s) lie within a narrow belt of metamorphosed sedimentary rocks that form a roof pendant within the quartz monzonite. The metamorphosed strata are several hundred feet thick and include interbedded quartzite, schist, and marble. A lamprophyre dike cuts the quartz monzonite and metasedimentary rocks. The quartzite, schist, and marble have been contact metamorphosed by the intrusion of the quartz monzonite and locally contain disseminations, seams, and small masses of quartz, garnet, epidote, pyroxene, tremolite, and small amounts of sulfide minerals. The sulfide minerals include pyrrhotite, sphalerite, chalcopyrite, and molybdenite, and a quartz stringer was reported to yield gold. The age of this contact metamorphic (skarn) deposit probably is Eocene, roughly contemporaneous with the emplacement of the Davis River Pluton. The prospect was explored in the early 1900s by an 80-foot adit and an 11-foot adit.
Geologic map unit (-130.221701487997, 55.7496925581663)
Mineral deposit model Cu skarn (Cox and Singer, 1986; model 18b)
Mineral deposit model number 18b
Age of mineralization Eocene?
Alteration of deposit Weathered pyrrhotite gives a rusty color to the surface outcrop.

Production and reserves

Workings or exploration The prospect was explored in the early 1900s by opencuts, an 80-foot adit, and an 11-foot adit.
Indication of production None

Additional comments

Site is in Misty Fiords National Monument Wilderness.

References