Explained by Sandra H.B. Clark, Joseph A. Briskey, Jr., and Dennis P. Cox
On the choice of deposit models
Carbonate sequences permissive for the occurrence of Appalachian Zn deposits occur in the Lower and Middle Devonian Onondaga Limestone and stratigraphically equivalent formations below Middle to Upper Devonian shale sequences, and in Middle Ordovician to Lower Devonian dolostones and limestones. Some small occurrences of zinc or lead have been reported in these rocks, but no deposits clearly representative of the Appalachian Zn type are known (Clark, 1987).
On the delineation of permissive tracts
The tract includes Middle Ordovician to Middle Devonian dolostone and limestone exposed or covered by less than 1 km of younger rocks. This tract is permissive for the occurrence of Appalachian Zn deposits but has a low probability for the occurrence of undiscovered districts.
On the numerical estimates made
Most Appalachian Zn districts are in, or spatially associated with, dissolution collapse breccias formed in the paleoaquifer below the Middle Ordovician section. However, in the Burkesville, Kentucky, deposit in the Central Tennessee district, large areas of porous and permeable primary carbonate depositional textures and structures in the Knox Group also are mineralized (Briskey and others, 1986). Although the relationship between these features and associated mineralized collapse breccias is debatable, the existence of this deposit suggests that porous and permeable zones other than those associated with the paleoaquifer might host Appalachian Zn districts. Consequently, carbonate rocks in this tract might be potential host rocks providing that the age of mineralization is Acadian or younger. Nonetheless, because the carbonate rocks that comprise the tract are above the paleoaquifer, and contain no known districts or deposits of this type, the study team estimated the probability for the occurrence of an undiscovered district to be less than one percent.
Briskey, J.A., Dingess, P.R., Smith, Fred, Gilbert, R.C., Armstrong, A.K., and Cole G.P., 1986, Localization and source of Mississippi Valley-type zinc deposits in Tennessee, USA, and comparisons with Lower Carboniferous rocks of Ireland, in Andrew, C.J., Crowe, S.F., Pennell, W.M., and Pyne, J.F., eds., Geology and genesis of mineral deposits in Ireland: Dublin, Irish Association for Economic Geology, p. 635-661.
Clark, S.H.B., 1987, Metallogenic map of zinc, lead, and barium deposits and occurrences in Paleozoic sedimentary rocks, east-central United States: U.S. Geological Survey Miscellaneous Investigations Series Map I-1773, scale 1:2,500,000 and pamphlet, 77 p.