Explained by David A. Brew, Cliff D. Taylor, Robert C. Jachens, Richard D. Koch, Donald Grybeck, James L. Drinkwater, David F. Barnes, and Larry J. Drew
On the choice of deposit models
In this tract Triassic subaerial basalts (greenstones) of the Wrangellia terrane contain local concentrations of Cu-sulfide minerals that are interpreted to be basaltic Cu deposits.
On the delineation of permissive tracts
This tract includes Paleozoic mafic volcanic and carbonate rocks, Triassic(?) and Cretaceous greenstones, and Triassic mafic volcanic rocks. Tract boundaries were delineated mainly by the distribution of mafic volcanic rocks and greenstones, occurrence of known deposits, and Ag, Cu, Pb, and Zn geochemical anomalies associated with mafic volcanic rocks and greenstones (Brew and others, 1991).
Important example of deposit type
Baker Peak on Chichagof Island contains massive to disseminated chalcopyrite, and pyrite in Triassic(?) basalt flows, and is possibly a basaltic Cu deposit (Brew and others, 1991).
On the numerical estimates made
Permissive rock types and associations, a few known deposits, and appropriate geochemical patterns were the main basis for the estimates. For assessment purposes, the team used an assumed mean grade and tonnage similar to the basaltic Cu deposits of the Upper Peninsula of Michigan, which have a median size of 15.7 million metric tons and a median grade of 1.1 percent Cu (White, 1968). This model may overestimate the number of deposits in this distant location, but it at least provides a preliminary and tentative estimate. The estimated number of undiscovered basaltic Cu deposits in this tract is:
Percentile 90 50 10 5 1
Estimated number of deposits 0 0 0 2 3
Brew, D.A., 1993, Regional geologic setting of mineral resources in southeastern Alaska, in Godwin, L.H., and Smith, B. D., eds., Economic mineral resources of the Annette Islands Reserve, Alaska: U.S. Dept. of the Interior, Bureau of Indian Affairs, Division of Energy and Mineral Resources Publication, p.13-20.
Brew, D.A., Drew, L.J., Schmidt, L.M., Root, D.H., and Huber, D.F, 1991, Undiscovered locatable mineral resources of the Tongass National Forest and adjacent areas, southeastern Alaska: U.S. Geological Survey Open-File Report 91-10, 370 p., 15 maps at 1:250,000, 1 map at 1:500,000, 11 figs.
White, W.S., 1968, The native-copper deposits of northern Michigan, in Ridge, J.D., editor, Ore Deposits of the United States 1933-1967; The Graton-Sales Volume, American Institute of Mining, Metallurigical, and Petroleum Engineers, Inc., New York, v. 1, p. 303-326.