Explained by Marti L. Miller, Jeanine M. Schmidt, and Bruce M. Gamble
On the choice of deposit models
Paleozoic platform carbonate rocks in southwest Alaska are permissive for Kipushi-type deposits as defined by Cox and Bernstein (1986), because they were formed in an appropriate geologic setting (a continental platform or shelf with continental or passive margin rifting) and because they contain appropriate host rocks (dolostone, shale, and shallow-water or platform carbonate rocks) for the deposit model. Although no occurrences are known in this tract, correlative rocks in west-central Alaska contain several carbonate-hosted base-metal prospects.
On the delineation of permissive tracts
This tract consists of three areas that form a northeast-trending belt. The northeast area (in the McGrath quadrangle) and central area (in the Sleetmute, Taylor Mountains, and Lime Hills quadrangles) are both underlain by Cambrian to Devonian shallow-water facies platform carbonate rocks of the Farewell terrane (Decker and others, 1994). No geochemical data are available to assist in evaluating the Kipushi Cu-Pb-Zn potential of these rocks, but the depositional environment is appropriate, and correlative rocks to the northeast host several Zn prospects. The southwestern area (in Goodnews Bay quadrangle) contains blocks of Ordovician, Devonian, and Permian limestone, which constitute less than 5 percent of a structurally disrupted unit of mixed marine sedimentary and volcanic rock that forms the Nukluk subterrane of the Goodnews terrane (Decker and others, 1994). The limestone blocks show algal reef and reef breccia textures (Hoare and Coonrad, 1978), indicating a depositional environment permissive for Kipushi-type deposits.
Important examples of this type of deposit
Examples of carbonate-hosted base-metal deposits occur in equivalent rocks in the Reef Ridge district in the Medfra quadrangle of west-central Alaska (Schmidt, 1997).
On the numerical estimates made
There are no tonnage and grade curves for Kipushi-type deposits; no quantitative estimates of the number of undiscovered deposits were attempted.
References
Cox, D.P., and Bernstein, L.R., 1986, Descriptive model of Kipushi Cu-Pb-Zn, in Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 227.
Decker, John, Bergman, S.C., Blodgett, R.B., Box S.E., Bundtzen, T.K., Clough, J.G., Coonrad, W.L., Gilbert. W.G., Miller, M.L., Murphy, J.M., Robinson, M.S., and Wallace, W.K., 1994, Geology of southwestern Alaska, in Plafker, George, and Berg, H.C., eds., The geology of Alaska: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G1, p. 285-310.
Hoare, J.M., and Coonrad, W.L., 1978, Geologic map of the Goodnews and Hagemeister Island quadrangles region, southwestern Alaska: U.S. Geological Survey Open-file Report 78-9-B, 2 sheets, scale 1:250,000.
Schmidt, J.M., 1997, Strata-bound carbonate-hosted Zn-Pb and Cu deposits of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 90-119.
