Unnamed (west arm of Dundas Bay)

Occurrence, Inactive

Commodities and mineralogy

Main commodities Cu
Other commodities Ag; Co
Ore minerals chalcopyrite

Geographic location

Quadrangle map, 1:250,000-scale MF
Quadrangle map, 1:63,360-scale B-2
Latitude 58.356
Longitude -136.5
Nearby scientific data Find additional scientific data near this location
Location and accuracy This occurrence is on a small island in the south part of the west arm of Dundas Bay. The island is near the center of section 33, T.40 S., R. 54 E., of the Copper River Meridian. The location is accurate to 0.1 mile. It is the same site as number 67 of Kimball and others (1978) and number 58 of MacKevett and others (1971).

Geologic setting

Geologic description

The site is in a belt of quartz-feldspar-hornblende paragneiss and gneissic diorite. The protolith of the gneiss is Paleozoic in age; metamorphism occurred in the Cretaceous (Brew and others, 1978). At the site, hornblendite dikes as much as 10 feet thick contain disseminated chalcopyrite and lenses of nearly massive chalcopyrite as much as 2 inches thick and 6 inches long. The dikes cut garnetiferous gneissic diorite.
A grab sample of sulfide-bearing hornblendite collected during the U.S. Geological Survey's 1966 investigation of Glacier Bay contained 10,000 ppm copper by spectrographic analysis (MacKevett and others, 1971, table 9). Kimball and others (1978, p. C270) reported 1.8 percent copper, 3 ppm silver, and 700 ppm cobalt in selected samples of narrow chalcopyrite-rich lenses.
The dikes appear to be post-Cretaceous metamorphism. They are probably Late Cretaceous or younger in age.
Geologic map unit (-136.501851076139, 58.3556335080872)
Mineral deposit model Disseminated copper deposit in ultramafic (hornblendite) rock.
Age of mineralization Probably Cretaceous or younger. The copper minerals occur in hornblendite dikes which cut metamorphic strata of Paleozoic age.

Production and reserves

Workings or exploration MacKevett and others (1971) reported about 1 percent Cu in samples of hornblendite dikes. Kimball and others (1978) reported 1.8 percent copper in selected samples of copper-rich lenses; the selected samples contained as much as 3 ppm silver and 700 ppm cobalt. There are no mine workings.
Indication of production None

Additional comments

The copper and cobalt content of the occurrence is significant , but the size is limited because the dikes are narrow and the island small. The site is in Glacier Bay National Park and Preserve.