Explained by Sandra H.B. Clark, Joseph A. Briskey, Jr., and Dennis P. Cox
On the choice of deposit models
Large resources of zinc occur in the Central Tennessee district in deposits like Elmwood and Gordonsville that have many similarities to major stratabound Appalachian Zn or Mississippi Valley (MVT) districts that extend from Tennessee to Newfoundland. Because of the distinctive geological features and importance of the Eastern United States deposits, they are the basis for definition of the world-wide Appalachian Zn model (Briskey, 1986a). This differentiation of Appalachian Zn as a subtype of MVT is supported by recent studies of lead isotopes, which indicate that the Appalachian sulfides have unusually homogeneous compositions, distinctly different from the arrays typical of MVT deposits in the mid-continent of the United States (Kesler and others, 1994; Carlson, 1994). Known Appalachian Zn districts in east-central United States are stratabound within Lower Cambrian to Lower Ordovician dolostones and limestones that formed as platform carbonate deposits in the Appalachian basin sedimentary sequence. Host rocks for known deposits are the Upper Cambrian and Lower Ordovician Knox Group and the Lower and Middle Ordovician Beekmantown Group (East and Central Tennessee, Shenandoah Valley, and Friedensville districts) and the Lower and Middle Cambrian Shady Dolomite (Austinville-Ivanhoe district).
The grade and tonnage model currently in use combines Appalachian Zn (Briskey, 1986a) with southeast Missouri Pb-Zn (Briskey, 1986b). Although MVT deposits have heterogeneous characteristics, the southeast Missouri Pb-Zn district differs from other major MVT districts in several significant ways (Sangster, 1983). The southeast Missouri deposits formed in a stable interior platform sequence separated from the underlying Middle Proterozoic craton by a major erosional surface and lie closer to cratonic basement than any other major MVT district. The proximity to granitic basement rocks may account for the Pb-dominant ores in southeast and central Missouri where Zn/( Zn+Pb) ratios are less than 0.3 (Sangster, 1983). Sorby Hills, Australia, has both geology and Zn/(Zn+Pb) ratio that are similar to southeast and central Missouri, but other major MVT deposits have Zn/(Zn+Pb) ratios between 0.5 and 1.0. Sangster (1983) suggested that a progression from Pb-rich to Zn-rich MVT deposits might reflect decreasing "communication" with metal sources within the craton. The silver content of southeast Missouri ores is higher than other MVT ores, and also may be related to proximity to the craton. Although no further work has been published to test Sangster's (1983) suggested classification, the anomalous nature of southeast and central Missouri ores relative to those formed in platform carbonate rocks of the Appalachian basin is clear.
Because of the anomalous nature of the southeast and central Missouri districts, we have deleted them from the grade and tonnage model (Mark3 index 109) to better represent undiscovered districts in this tract. The results for zinc are similar using both modified and unmodified versions and are considered to be realistic estimates based on the known deposits in the tract. However, the estimates for lead and silver endowment are much lower when southeast and central Missouri are deleted and are considered to be a more realistic estimate of the expected lead endowment for the platform deposits not close to cratonic basement.
On the delineation of permissive tracts
The tract is defined by the distribution of Cambrian and Ordovician dolostones and limestones that are: (1) below the Middle Ordovician unconformity; (2) west and north of the Appalachian basin; and (3) less than one kilometer below the surface. These rocks contain the Central Tennessee district. The tract is bounded to the west by the Great Plains Region, and to the north by the Adirondack Region, both of which contain tracts permissive for MVT districts. On the east are other MVT tracts in the East Central Region. MVT mineralization may be localized near basement highs, facies changes, karst features, broad crestal areas of regional domes and local structural highs, joints, and faults—features that concentrated porosity and permeability, focused the flow of regional hydrothermal brines, and permitted introduced brines to mix with local fluids of different compositions. The most important districts in Tennessee are hosted in breccias and other structures resulting from dissolution and collapse of limestones and dolostones below the Middle Ordovician unconformity. Mixing of fluids with different chemical composition, hydrocarbon contents, and redox potential are among the possible causes of mineral precipitation.
The age of MVT mineralization in central Tennessee is uncertain, but a Mississippian or younger age has been proposed by Gaylord and Briskey (1983), based on similarities between stratabound deposits in the Knox Group and veins that cut overlying Middle Ordovician to Mississippian beds.
On the numerical estimates made
The area occupied by the Central Tennessee district was considered, and estimates in part were guided by how may undiscovered districts of approximately the same size as this district would fit into the permissive tract extended to a depth of 1 km. The team also considered vein deposits of the Central Kentucky district (Anderson, 1982), numerous exposures of non-economic MVT mineralized areas in northwestern Ohio and parts of adjacent States, especially the occurrences in Cambrian to Lower Ordovician host rocks (Botoman and Stieglitz, 1978; Carlson, 1983, 1994; Clark, 1987). The identification of brines with compositions of MVT fluids in Rossie vein deposits (Ayuso and others, 1987) also was considered of importance as a possible indication of undiscovered MVT districts in the carbonate sequences of western New York. Because MVT ore-forming processes in this region appear to have been concentrated in the broad crestal areas of domes and arches (Gaylord and Briskey, 1983; Briskey and others, 1986), these areas are considered to be more favorable than elsewhere. From a consideration of the sizes of the tract and favorable crestal areas of domes and arches, the size of the Central Tennessee district, and the number and distribution of non-economic MVT mineralized areas, to a depth of one km, the assessment team estimated that the probability of more than five undiscovered MVT districts occurring in the tract is less than one percent. The team had a high degree of confidence that one undiscovered MVT district exists near or below the Central Kentucky fluorspar district. For the 90th, 50th, 10th, 5th and 1st percentiles, the team estimated, respectively, 1, 2, 3, 4, and 5 or more Appalachian Zn districts consistent with the grade and tonnage model of Mosier and Briskey (1986), but with the southeast and central Missouri districts deleted.
Anderson, W.H., 1982, Barite deposits of Kentucky: Kentucky Geologic Survey Bulletin, Series II, no. 1, 56 p.
Botoman, George, and Stieglitz, R.D., 1978, The occurrence of sulfide and associated minerals in Ohio: Ohio Department of Natural Resources, Division of Geological Survey, Report of Investigations No. 104, 11 p.
Ayuso, R.A., Foley, N.K., and Brown, C.E., 1987, Source of lead and mineralizing brines for Rossie-type Pb-Zn Veins in the Frontenac axis area, New York: Economic Geology, v. 82, no. 2, p. 489-496.
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.
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.
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.
Carlson, E.H., 1983, The occurrence of Mississippi Valley-type mineralization in northwestern Ohio, in Kisvarsanyi, Geza, Grant, S.K., Pratt, W.P., and Koening, J.W., ed., International conference on Mississippi Valley type lead-zinc deposits: Rolla, University of Missouri, p. 424-435.
Carlson, E.H., 1994, Geologic, fluid inclusion, and isotopic studies of the Findlay Arch district, northwestern Ohio: Economic Geology, v. 89, no. 1, p. 67-90.
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, scale 1:2,500,000 and pamphlet, 77 p.
Gaylord, W.B., and Briskey, J.A., 1983, Tennessee zinc deposits field trip guidebook: Blacksburg, Virginia Polytechnic and State University, Guidebook No. 9, p. 116-151.
Kesler, S.E., Cumming, G.L., Kristic, Dragan, and Appold, M.S., 1994, Lead isotopic geochemistry of Mississippi Valley-type deposits of the southern Appalachians: Economic Geology, v. 89, no. 2, p. 307-321.
Mosier, D.L., and Briskey, J.A., 1986, Grade and tonnage model of southeast Missouri lead-zinc and Appalachian Zinc, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 211.
Sangster, D.F., 1983, Mississippi Valley-type lead-zinc deposits—A geological melange, in Kisvarsanyi, Geza, Grant, S.K., Pratt, W.P., and Koening, J.W., ed., International conference on Mississippi Valley type lead-zinc deposits: Rolla, University of Missouri, p. 7-9.