Unnamed (east side of Dundas Bay)

Occurrence, Inactive

Commodities and mineralogy

Main commodities Cu
Other commodities Ag; Au; Mo; Pb; Zn
Ore minerals chalcopyrite; galena; malachite; pyrite; sphalerite
Gangue minerals muscovite (sericite); quartz

Geographic location

Quadrangle map, 1:250,000-scale MF
Quadrangle map, 1:63,360-scale B-1
Latitude 58.375
Longitude -136.288
Nearby scientific data Find additional scientific data near this location
Location and accuracy This occurrence is east of the mouth of the Dundas River and is mostly in the west half of section 26, T. 40 S., R. 55 E., of the Copper River Meridian. It ranges from 100 feet wide to more than 1000 feet wide and extends from the shoreline northerly for about one mile. The coordinates are at the approximate center of the occurrence, which is the same as number 31 of MacKevett and others (1971), number 57 of Cobb (1972), and within figure C73 of Kimball and others (1978).

Geologic setting

Geologic description

Pyritic quartz-mica semischist, that possibly is a metarhyolite, and metabasalt appear to occur within a unit that elsewhere consists mainly of graywacke and argillite of Silurian and Devonian age (Brew and others, 1978). Foliated granitic rocks crop out west of the pyritic schists. The granitic rocks are inferred to be extensive, but are covered by water in Dundas Bay.
The pyritic semischist, which is the mineralized unit, forms a steep, north trending body from 100 feet to more than 1000 feet thick. It in sharp contact with adjacent metabasalt. Pyrite is more abundant than chalcopyrite in the semischist unit, but chalcopyrite is generally present. The semischist is cut locally by veinlike zones that contain relatively abundant chalcopyrite.
The occurrence is probably syngenetic and volcanogenic. Chalcopyrite may have been remobilized into the vein-like zones during Cretaceous metamorphism.
Selected samples of the semischist unit contained as much as 1 ppm silver and 2000 ppm copper (MacKevett and others, 1971, table 9). Kimball and others (1978) reported 2 ppm silver, 320 ppm copper, and 230 ppm zinc in a 50 foot long chip sample of the semischist. Molybdenum is commonly present and assayed as much as 100 ppm in a 12 foot chip sample. Moerlein (1968) collected 35- and 20-ft. chip samples from a northwest-striking shear zone within the semischist; his samples contained, respectively, 0.51 and 0.34 percent copper.
Geologic map unit (-136.289838826787, 58.3746426639499)
Mineral deposit model Volcanogenic, cupriferous metarhyolite - similar to Kuroko massive sulfide (Cox and Singer, 1986; model 28a).
Mineral deposit model number 28a?
Age of mineralization Devonian-Silurian with possible remobilization during the Cretaceous.
Alteration of deposit Local oxidation of copper minerals.

Production and reserves

Workings or exploration Claims reportedly staked for gold, lead, zinc and copper were reported in the east Dundas Bay area by Wright and Wright (1937). In the 1966 examination by the U.S. Geological Survey, only surface exposures were examined and sampled. A chip sample collected across 40-feet of semischist contained 1000 ppm copper; a malachite stained zone contained 2000 ppm copper (MacKevett and others, 1971). Moerlein (1968) found as much as 0.51 percent copper in a northwest trending shear zone within the pyritic semischist unit. Samples of semischist collected by the U.S. Bureau of Mines (Kimball and others, 1978) showed anomalous silver, molybdenum, and zinc as well as copper. Maximum reported molybdenum content was 100 ppm.
Indication of production None

Additional comments

Although the reported grade of the deposit is subeconomic, its size and possible volcanogenic nature encourages further exploration and the mapping of the Devonian-Silurian volcanic stratigraphy.
The site is in Glacier Bay National Park and Preserve.

References