Explained by Jeanine M. Schmidt, Thomas D. Light, Bruce M. Gamble, and Marti L. Miller
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.
This tract is permissive for Mississippi Valley type deposits because it contains platform carbonate rocks hosting deposits thought to be Mississippi Valley type, and local Pb-Zn geochemical anomalies.
On the delineation of permissive tracts
This tract consists of lower Paleozoic (Ordovician through Devonian) platform carbonate rocks of the Nixon Fork terrane (Patton and others, 1994) in the Medfra and Ruby quadrangles. Pb and Zn locally occur in stream-sediment and panned-concentrate samples. Sphalerite, chalcopyrite, cinnabar, pyrite and malachite are present in some concentrates.
Important examples of this type of deposit
The Reef Ridge deposit in the Medfra quadrangle is an example of a Mississippi Valley type deposit (Andrews and Rishel, 1982).
On the numerical estimates made
No estimate of the number of undiscovered deposits was attempted.
Andrews, T., and Rishel, John, 1982, 1981 Annual Report-Reef Ridge I-XIII project areas, Kuskokwim Block, Volume 1 of 3: unpublished report, Patino Inc., Anchorage, Alaska, 49 p.
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.
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., 1994, Geology of west-central 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. G-1, p. 241-269.