Explained by Martin B. Goldhaber, David L. Leach, Walden P. Pratt, Gregory T. Spanski, and John G. Viets
On the choice of deposit models
The midcontinent region of the U.S contains four known world-class Mississippi Valley districts and several smaller ones, and large areas of permissive host rocks—i.e., Paleozoic sedimentary carbonate rocks. General descriptions of the Mississippi Valley deposit type are in Heyl (1982); Pratt (1982); Sangster (1983); and Briskey (1986).
On the delineation of permissive tracts
The permissive tract is defined simply by the known or inferred presence of significant sections of sedimentary carbonate rocks of Cambrian through Mississippian age occurring within 1 km of the surface. It is bounded on the south by the boundary between the Great Plains and Lake Superior regions. The western boundary in the Dakotas is defined by burial of permissive carbonate rocks of the Ordovician Red River Formation beneath 1 km of cover. On the north, in the absence of definitive control on the northernmost extent of carbonate rock in the Cambrian interval, all Cambrian rocks are included, which results in the inclusion of a marginal zone where the rocks are predominantly clastic. Minor enclaves of non-permissive terrane occur in eastern Kansas and Nebraska overlying the Nemaha Ridge due to absence of carbonate rocks.
Important examples of this type of deposit
There are no known occurrences of Mississippi Valley-style mineralization in this tract. The nearest locality is the Upper Mississippi Valley zinc-lead district in Wisconsin, Iowa, and Illinois (Heyl and others, 1959) that produced 0.79 and 1.7 million metric tons of lead and zinc, respectively. In this assessment the potential for districts is considered rather than individual deposits, because the important Mississippi Valley deposits in the Central Region are the products of broad, pervasive hydrothermal mineralizing systems, and the mineralized areas are so interconnected that their designation as individual "deposits" may be only a matter of the location of property lines. The Mississippi Valley grade and tonnage curves given by Mosier and Briskey (1986) are themselves based on data for entire districts and not individual deposits.
On the numerical estimates made
For this part of the permissive terrane for Mississippi Valley deposits the assessors applied their combined general knowledge of the deposits and their processes of formation, both in the Mid-continent region and worldwide. They examined this permissive tract, and outlined areas believed to be favorable on the basis of four factors: (1) known presence of typical sulfide and gangue minerals, (2) known extent of a typical hydrothermal system, (3) proximity to foldbelts and arches, and (4) bleaching of underlying basal (originally red) sandstone. The tract is bordered on the west by the Williston and Salina Basins and on the south by the Forest City Basin. However, the abundance of carbonate host rocks in the stratigraphic interval is more highly restrictive. The assessment team specifically opted not to attempt the analogue type of estimate—i.e., how many districts of size x would fit into permissive tract—because of the insufficiency of data.
The group reached consensus for the 90th, 50th, 10th, 5th, and 1st percentiles, respectively, of 0, 0, 0, 1, and 2 undiscovered deposits consistent with the revised grade and tonnage model of Mosier and Briskey (1986) (Mark3 index 108). This estimate was anchored on the belief that there is a small probability (less than 10 percent) that an undiscovered district exists somewhere in the tract; and that the availability of favorable areas limits the range to a maximum of 2 districts at the 1st percentile.
Briskey, J.A., 1986, 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.
Heyl, A.V., Jr., Agnew, A.F., Lyons, E.J., and Behre, C.H., Jr., 1959, The geology of the Upper Mississippi Valley zinc-lead district: U.S. Geological Survey Professional Paper 309, 310 p.
Mosier, D.L., and Briskey, J.A., 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.
Pratt, W.P., 1982, A prospecting model for stratabound lead-zinc(-barite-fluorite) deposits ("Mississippi Valley-type" deposits), in Erickson, R.L., ed., Characteristics of mineral deposit occurrences: U.S. Geological Survey Open-File Report 82-795, p. 155-157.
Sangster, D.F., 1983, Mississippi Valley-type deposits—A geological mélange, in Kisvarsanyi, Geza, Grant, S.K., Pratt, W.P., and Koenig, J.W., eds., International conference on Mississippi Valley type lead-zinc deposits: Rolla, University of Missouri, Proceedings, p. 7-19.