Explained by Thomas D. Light and Jeanine M. Schmidt
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.
The carbonate units of east-central Alaska are permissive for Mississippi Valley type deposits, based on the geologic setting, appropriate host rocks, and known deposits that may fit the model.
On the delineation of permissive tracts
This tract is defined by Paleozoic platform carbonate rocks. Where the environment of deposition is not known or indeterminate, thick carbonate rocks of possible shelf, slope, or deeper-water-facies are included. Paleozoic metamorphic rocks that contain minor marble units of unknown origin, such as biotite gneiss and amphibolite (PzpCb of Foster, 1976), have not been included in this tract. In the Eagle and Charley River quadrangles geologically favorable units include the limestones of the Tindir Group, the Funnel Creek Limestone, the Hillard Limestone, and the Calico Bluff Formation. The western portion of this tract includes the Paleozoic limestones of the Quail, Amy Creek, Schwatka, and minor other units.
Important examples of this type of deposit
Step Mountain may be an example of a Mississippi Valley type deposit in this tract (Schmidt, 1997).
On the numerical estimates made
Geological information is inadequate to estimate the number of undiscovered Mississippi Valley type deposits in this tract.
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. 130.
Foster, H.L., 1976, Geologic map of the Eagle quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-922, 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.
Patton, W.W., Jr., Box, S.E., Moll-Stalcup, E.J., and Miller, T.P., 1989, Geology of west-central Alaska: U.S. Geological Survey Open-File Report 89-554, 41 p.
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.