National mineral assessment tract AK-SW01 (Basaltic Cu/Kennecott Cu)

Tract AK-SW01
Geographic region Southwestern, AK
Tract area 1,347sq km
Deposit type Basaltic Cu/Kennecott Cu
Deposit age Triassic


Explained by Marti L. Miller and Thomas K. Bundtzen
On the choice of deposit models
The well known Kennecott copper deposits are associated with carbonate and greenstone units of the Wrangellia terrane (Jones and others, 1987). Similar carbonate and greenstone lithologies, which may correlate with those of the Wrangellia terrane (Detterman and Reed, 1980), occur locally in the Iliamna quadrangle. The Kennecott copper deposits were included in the basaltic copper descriptive model (Cox, 1986), and there has been considerable debate over this classification. However, whether the Kennecott deposits are classified as basaltic Cu, replacement, or sabkha, the controlling factor for the delineation of a permissive tract is the distribution of the greenstones. With this consideration, we have avoided the controversy by lumping the models together.
On the delineation of permissive tracts
This tract includes the Cottonwood Bay Greenstone as mapped and described by Detterman and Reed (1980), and the overlying Upper Triassic carbonate, shale, siltstone, and chert of the Kamishak Formation.
Important examples of this type of deposit
No basaltic copper or Kennecott-type mineral occurrences are known in southwest Alaska. Most copper occurrences in the Iliamna quadrangle are contact metamorphic skarns (Detterman and Reed, 1980). One locality described only as a “bedrock occurrence of copper sulfide” reportedly is in carbonate rock in the vicinity of greenstone (Cobb and Reed, 1981).
On the numerical estimates made
Because the geologic host rock associations are inferred, and there are no known deposits, this tract is considered to be speculative. No estimate of undiscovered basaltic copper or Kennecott-type deposits was attempted.
Cobb, E.H., and Reed, B.L., 1981, Summaries of data on and lists of references to metallic and selected nonmetallic mineral occurrences in the Iliamna, Lake Clark, Lime Hills, and McGrath quadrangles, Alaska—supplement to Open-file Report 76-485, Part A: U.S. Geological Survey Open-file Report 81-1343A, 25 p.
Cox, D.P., 1986, Descriptive model of basaltic Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 130.
Detterman, R.L., and Reed, B.L., 1980, Stratigraphy, structure, and economic geology of the Iliamna quadrangle, Alaska: U.S. Geological Survey Bulletin 1368-B, 86 p., scale 1:250,000.
Jones, D.L., Silberling, N.J., Coney, P.J., and Plafker, George, 1987, Lithotectonic terrane map of Alaska (west of the 141st meridian): U.S. Geological Survey, Miscellaneous Field Studies Map MF-1874-A, scale 1:2,500,000.

Geographic coverage

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