Pumicestone Bay

Prospect, Inactive

Commodities and mineralogy

Main commodities Ag; Au
Other commodities Ba; Pb; Zn
Ore minerals chalcopyrite; galena; pyrite; sphalerite
Gangue minerals chlorite; epidote; magnetite; specularite

Geographic location

Quadrangle map, 1:250,000-scale UN
Quadrangle map, 1:63,360-scale B-3
Latitude 53.5195
Longitude -166.9579
Nearby scientific data Find additional scientific data near this location
Location and accuracy South of the head of Pumicestone Bay, on south side of river entering bay from the southwest. Anomaly 41 of Christie (1974).

Geologic setting

Geologic description

Christie (1974) reported a single 3,000 ft (900 m) by 1,500 ft (450 m) color anomaly whereas later evaluations by Randolph and Ellis (1989) and Randolph (1991) described three color anomalies aligned along a steep valley wall. Country rock is the Unalaska Formation (see Drewes and others, 1961), near a sheared contact with the Shaler batholith. Granodiorite of the pluton occurs below the color anomalies at elevations below about 600 feet (200 m). Prospect lies at contact with metamorphosed impure sandstone, breccia, and quartz pebble conglomerate. Stockwork veining and silicification is locally developed within an exposed volume of 1600 by 160 feet, having a 75 foot thickness (80 x 50 x 20 m). Semi-parallel northeast-trending shear zones cut off the quartz stockwork and silicification to the northwest. Quartz veining occurs parallel and/or marginal to these shear zones. Largest of these quartz veins is about 1 foot (30 cm) wide, gossanous, and contains up to 20 percent pyrite (Randolph, 1991) and rare chalcopyrite, sphalerite, and/or galena (Christie, 1974; Randolph, 1991). Randolph's (1991) interpretation was that there were two episodes of veining, where the sulfide-bearing shear-hosted veins cut the earlier quartz stockwork.
Geologic map unit (-166.959770847014, 53.5186364812922)
Mineral deposit model Polymetallic vein, epithermal gold vein
Mineral deposit model number 22c, 25
Age of mineralization Miocene or younger.
Alteration of deposit Rocks are regionally propylitically altered and also hornfelsed as a result of proximity to the Shaler batholith (Simpson, 1986). Highest gold grades are associated with zones of 'pyrite-silica' alteration in 'spotted andesite' (Simpson, 1986).

Production and reserves

Workings or exploration Battle Mountain Exploration Co. (BMEC) established a sampling grid over one exposure of hornfelsed volcaniclastic rocks and/or graywacke that has been strongly stockworked and silicified. Quartz stringer densities were as high as 2 per inch (2.54 cm) and only rarely contain visible sulfides. The quartz stockwork contained gold to 272 ppb and slightly anomalous lead and barium. Quartz veins hosted in the northeast-trending shear zones contain up to 275 ppb gold, 31.5 ppm silver, 0.24 percent lead, 0.14 percent zinc, elevated barium and insignificant mercury and arsenic (Randolph, 1991).
Indication of production None

Additional comments

Randolph (1991) gave the prospect a poor potential for economic gold mineralization due to remoteness, ruggedness of the environment, and minimal evidence for economic mineralization.

References

MRDS Number A106001

References

Christie, J.S., 1974, Aleut-Quintana-Duval 1974 joint venture, final report: Unpublished Quintana Minerals Corporation report, 24 p., 3 appendices, 2 maps. (Report held by the Aleut Corporation, Anchorage, Alaska.)
Randolph, D.B., 1991, Unalaska project, 1990 final report: Battle Mountain Exploration Company, Alaska District, 62 p., 5 appendices, 15 plates, various scales. (Report held by the Aleut Corporation, Anchorage, Alaska.)
Randolph, D.B., and Ellis, W.T., 1989, Unalaska project, 1989 final report: Battle Mountain Exploration Company, Alaska District, 41 p., 5 appendices, 11 plates, various scales. (Report held by the Aleut Corporation, Anchorage, Alaska.)
Simpson, D.F., 1986, Aleutian Islands project, 1985 final report: Kennecott Alaska Exploration Company, 54 p. (Report held by the Aleut Native Corporation, Anchorage, Alaska.)
Reporters F.H. Wilson
Last report date 5/6/1996