Unnamed (near Black Lake)

Occurrences, Undetermined

Commodities and mineralogy

Main commodities Ag; Au; Cu; Mo
Other commodities W
Ore minerals chalcopyrite; molybdenite; pyrite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale CR
Quadrangle map, 1:63,360-scale C-3
Latitude 55.5654
Longitude -132.88268
Nearby scientific data Find additional scientific data near this location
Location and accuracy This occurrence is at an elevation of about 500 feet, about 0.7 mile north-northwest of the outlet of Black Bear Lake. It is near the center of the SW1/4 section 1, T. 73 S., R. 82 E. This site includes another occurrence about 0.2 mile to the south in a quarry shown by symbol on the USGS 1:63,360-scale topographic map; it is about 0.5 mile south-southwest of the center of section 1, T. 73 S., R. 82 E.

Geologic setting

Geologic description

This occurrence, discovered in 1988, is what was originally interpreted as a vertical, roughly elliptical, breccia pipe about 600 feet long and 300 feet wide (Hedderly-Smith and Glavinovich, 1991; Maas and others, 1991; Hedderly-Smith, 1999 [Inventory]). Later work by Hedderly-Smith indicates that the occurrence may not be a vertical pipe but instead is a northeast-trending breccia zone along a fault. The occurrence is at or near the contact between a Cretaceous granitic pluton and metamorphosed volcanic and sedimentary rocks of the Luck Creek Breccia of Silurian and Ordovician age (Eberlein and others, 1983; Brew, 1996). The breccia is composed of intensely altered clasts of diorite in a quartz-pyrite matrix; the breccia contains minor chalcopyrite and molybdenite. Three samples collected by Hedderly-Smith (1991) contained up to 0.07 percent copper, 0.064 percent molybdenum, 0.06 percent tungsten, and 0.03 ounce of gold per ton. Samples taken by Maas and others (1991) of material with masses of pyrite up to 6 inches across contained up to 23 parts per billion gold, 2.5 parts per million (ppm) silver, 0.53 percent copper, and 101 ppm molybdenum. Aerial photographs indicate that several arcuate structures surround the breccia. Ground geophysical surveys show that the electric response of the breccia is weak, but show an anomalous zone downslope from the breccia.
Geologic map unit (-132.884330445915, 55.5650389422862)
Mineral deposit model Breccia zone with chalcopyrite and molybdenite.
Age of mineralization The breccia is probably related to the contact of a Cretaceous granitic pluton.
Alteration of deposit The diorite clasts in the breccia are intensely altered.

Production and reserves

Workings or exploration Only surface sampling by geologists from government and private industry.
Indication of production None

References

References

Hedderly-Smith, D.A., 1989, Report of the 1988 field season - Sealaska Mineral Reconnaissance Project: Sealaska Corporation, 66 p. (Unpublished report held by the Sealaska Corporation, Juneau, Alaska.)
Hedderly-Smith, D.A., 1991, Report of the 1990 work in the T75S, R83E, and T75S, R84E-Sealaska minerals reconnaissance project: Sealaska Corportion (Unpublished report held by the Sealaska Corporation, Juneau, Alaska.)
Hedderly-Smith, D.A., 1992, Report on 1991 work in the Upper Harris River area-Sealaska minerals reconnaissance project: Sealaska Corporation, 12 p. plus plates. (Unpublished report held by the Sealaska Corporation, Juneau, Alaska.)
Hedderly-Smith, D.A., 1994, Report on the 1993 work in the Black Lake breccia area-Sealaska minerals reconnaissance project: Sealaska Corporation, 15 p. plus plate. (Unpublished report held by the Sealaska Corporation, Juneau, Alaska.)
Hedderly-Smith, D.A., 1999, Inventory of metallic mineral prospects, showings and anomalies on Sealaska lands, 1988 through 1998: Sealaska Corporation, Juneau, Alaska, 217 p. (internal report held by Sealaska Corporation, Juneau, Alaska).
Hedderly-Smith, D.A., and Glavinovich, P.S., 1991; Moss-mat stream sediment sampling in the Threemile Creek-Black Lake area, southeastern Alaska: Explore, no. 71, p. 14-17.
Reporters D.J. Grybeck (Applied Geology)
Last report date 5/1/2004