Cottonwood Creek

Occurrences, Inactive

Commodities and mineralogy

Main commodities Ag; Cu
Other commodities Pb; W; Zn
Ore minerals bornite; chalcocite; chalcopyrite; galena; malachite; pyrite; scheelite; sphalerite
Gangue minerals epidote; quartz

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale B-6
Latitude 63.2672
Longitude -146.6184
Nearby scientific data Find additional scientific data near this location
Location and accuracy This site represents several small copper mineral occurrences on the east side of Cottonwood Creek at elevations ranging from about 4,200 to 4,700 feet. The site is near the center of the west boundary of section 16, T. 19 S., R. 6 E., Fairbanks Meridian. The occurrences apparently are aligned about north-northwest. The site includes localities 20, 21, and 22 on figure 4 of Cobb (1979 [OFR 79-238]), multiple localities 15 of Kaufman (1964), and locality 29 in table 2 of Nokleberg and others (1991), and it corresponds closely to locality A16 of Kurtak and others (1992). It also corresponds approximately to copper occurrences discovered along Cottonwood Creek by prospectors E.O. Albertson and F.S. Pettyjohn, Jr., which were reported to Chapman and Saunders (1954). The location is accurate as a generalized one for copper occurrences along the east side of Cottonwood Creek.

Geologic setting

Geologic description

The rock in the area of these copper occurrences is metabasalt of the Nikolai Greenstone of Late Triassic age (Nokleberg and others, 1991). The host metabasalt is limonite-stained in a north-northwest zone subparallel to Cottonwood Creek; this zone contains copper-bearing veins and disseminations. The limonite is probably due to oxidation of disseminated pyrite and copper-iron sulfides; locally the metabasalt is stained with malachite. Kaufman (1964, locality 15, p. 8) reported the occurrence of chalcopyrite, bornite, chalcocite, and malachite in association with quartz-epidote veins; he also noted copper minerals as fillings in amygdules in the metabasalt.
The area may originally have been prospected by E.O. Albertson and F.S. Pettyjohn, Jr., in 1950 to 1952. These men reported that specimens from copper-bearing veins assayed 0.5 to 1.5 percent copper and that one piece of float material assayed 40.85 percent copper. Samples from a quartz vein about 4.3 feet thick assayed as much as 1.49 percent copper (Chapman and Saunders, 1954, p. 5). The area was also visited by Kaufman (1964). Sample 647 from a chalcopyrite-bearing vein contained as much as 8.58 percent copper (Kurtak and others, 1992, locality A16); this sample also contained 60 parts per billion gold, 3.7 ounces of silver per ton, 464 parts per million (ppm) lead, 450 ppm zinc, and 180 ppm tungsten. The values of lead, zinc, and tungsten are sufficiently high that the copper-rich vein filling probably contains small amounts of galena, sphalerite, and a tungsten mineral such as scheelite.
Geologic map unit (-146.62066624193, 63.2667670565443)
Mineral deposit model Cu-bearing disseminations and veins in cupriferous metabasalt. Basaltic Cu (Cox and Singer, 1986; model 23).
Mineral deposit model number 23
Age of mineralization Younger than Late Triassic; probably Cretaceous or early Tertiary (see MH060).
Alteration of deposit Oxidation of iron- and copper-bearing minerals.

Production and reserves

Workings or exploration No workings are reported. The area was prospected between about 1950 and 1952 (Chapman and Saunders, 1954) and later sampled by the U.S. Bureau of Mines (Kurtak and others, 1992).
Indication of production None

References