Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips
On the choice of deposit models
Two Mississippi Valley type (MVT) deposits are described in Cox and Singer (1986). The differences between Southeast Missouri Pb-Zn (model 32a; Briskey 1986b) and Appalachian Zn (model 32b; Briskey, 1986a) are subtle enough that the grade and tonnage models for the two deposit types are plotted together with no differentiation between types (Mosier and Briskey, 1986). In Alaska, the lack of detailed geologic information prevents distinguishing between the two deposit types, and a generic Mississippi Valley type model has been used.
Paleozoic platform carbonate rocks in southwest Alaska are permissive for Mississippi Valley type deposits. Although no occurrences are known in this tract, correlative rocks in west-central Alaska host numerous strata-bound Zn prospects in carbonate rocks (informally called the Reef Ridge district in Medfra quadrangle), which are probably Mississippi Valley type deposits.
On the delineation of permissive tracts
This tract consists of three areas that form a north-east trending belt. The northeast area (in McGrath quadrangle) and central area (in 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 Mississippi Valley type potential of these rocks, but the depositional environment is appropriate, and correlative rocks to the northeast host several Zn prospects that are probably Mississippi Valley type. 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 appropriate for Mississippi Valley type host rocks.
Important examples of this type of deposit
Examples of this type of deposit are found in similar rocks in the Medfra quadrangle of west-central Alaska, and make up the informally named Reef Ridge district (Schmidt, 1997).
On the numerical estimates made
Although permissive host rocks are present, the limited areal extent of the tract led to a low numerical estimate. The estimate of the minimum number of undiscovered Mississippi Valley type deposits, consistent with the mean tonnage and grade values of Mosier and Briskey (1986), is:
Percentile 90 50 10 5 1
Estimated number of deposits 0 0 1 1 2
Briskey, J.A., 1986a, Descriptive model of Appalachian Zn, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 222-223.
Briskey, J.A., 1986b, Descriptive model of southeast Missouri Pb-Zn, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 220-221.
Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 379 p.
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.
Mosier, D.L., and Briskey, 1986, Grade and tonnage model of southeast Missouri Pb-Zn and Appalachian Zn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 224-226.
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.