Explained by Jeanine M. Schmidt, Karen D. Kelley, Richard I. Grauch, Byron R. Berger, John W. Cady, Gregory T. Spanski, James E. Kilburn.
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 geologic setting and host rocks in this tract are permissive for Mississippi Valley type deposits, and the Powdermilk Zn-Pb occurrence may be of this type.
On the delineation of permissive tracts
This tract outlines all pre-Mississippian carbonate rocks in the Brooks Range, including very shallow water, platform, slope and deeper water facies of late Proterozoic(?) through Devonian age. These units include the Katakturuk Dolostone, Nanook Limestone, Baird Group carbonates, Eli, Lougurourok, and Skajit Limestones, their metamorphosed equivalents, and various marbles of uncertain age and affinity.
Important examples of this type of deposit
Classification of the Powdermilk Zn-Pb-Ag deposit in carbonate rocks as an MVT is uncertain (Schmidt and Folger, 1986).
On the numerical estimates made
Geologic knowledge of this area is inadequate to attempt an estimate of the number of undiscovered deposits.
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.
Schmidt, J.M., and Folger, P.F., 1986, Pb-Zn-Ag mineralization in Paleozoic dolostones, Powdermilk prospect, Baird Mountains B-4 quadrangle, in Bartsch-Winkler, Susan, and Reed, K.M., eds., Geologic studies in Alaska by the U.S. Geological Survey during 1985: U.S. Geological Survey Circular 978, p. 19-21.