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 major stratabound Appalachian Zn or Mississippi Valley (MVT) districts from Tennessee to Newfoundland including the deposits at Friedensville, Austinville-Ivanhoe, Mascot-Jefferson City, Copper Ridge, and Newfoundland zinc. Because of the distinctive geological features and importance of the eastern United States deposits, they are the basis for definition of the worldwide Appalachian Zn model (Briskey, 1986a). This differentiation of Appalachian Zn from MTV is supported by recent studies showing that the Appalachian sulfides have unusually homogeneous lead-isotope compositions, distinctly different from the typical leads of MVT deposits in the mid-continent of the United States (Kesler and others, 1994; Carlson, 1994). Known Appalachian Zn districts in the 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 Lower and Middle Cambrian Shady Dolomite (Austinville-Ivanhoe district), the Upper Cambrian and Lower Ordovician Knox Group and the Lower and Middle Ordovician Beekmantown Group, and stratigraphically equivalent carbonate units (East and Central Tennessee, Shenandoah Valley, and Friedensville districts).
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 geology and a Zn/(Zn+Pb) ratio that is 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 the 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 other tracts. 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
This tract is the concealed counterpart of Tract EC05. It is defined by the presence, in the subsurface, of Cambrian to Ordovician dolostones and limestones beneath the Middle Ordovician unconformity on the east side of the Appalachian basin, and includes the host rocks for the major known Appalachian Zn districts. These rocks do not crop out within the tract but are covered by no more than one kilometer of younger Paleozoic sedimentary rocks, commonly in the troughs of synclines. 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 the Appalachian area is uncertain. Some investigators suggest that mineralization may be as young as late Paleozoic and be associated with the Alleghenian orogeny (Hearn and others, 1987), which took place from about 330 Ma to 230 Ma in the central and southern Appalachians (Glover and others, 1983). Other investigators have summarized evidence for a pre-Alleghenian age for mineralization, which may have been associated with the Taconic or Acadian orogenies, or perhaps the earliest part of the Alleghenian event (e.g., Briskey and others, 1986; Kesler and others, 1990, 1994).
On the numerical estimates made
Because the rocks in this tract are the concealed equivalents of exposed, well-explored rocks in the adjacent tract, the study team decided that the best way to estimate the number of undiscovered districts was to assume this tract also has the same aereal density of Appalachian Zn districts as does the exposed tract. Although there are no known districts in this concealed tract, there also are no known geologic reasons why these two tracts would have different densities of districts. The exposed tract contains four known and one expected (50th percentile) undiscovered districts in an area of 42,300 km2. Consequently, this concealed tract, which has a somewhat larger area of 46,200 km2 is expected (at the 50th percentile) proportionally to contain about 6 undiscovered districts. For the 90th, 50th, 10th, 5th and 1st percentiles, the team estimated 3, 6, 8, 11 and 14 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 (Mark3 index 109).
The density of known districts relative to surface area in this tract, like the exposed tract, is higher than in the largely subsurface tracts to the west. Among the probable causes for a higher density are: (1) the presence of at least two, rather than one, major stratigraphic horizons containing MVT districts; (2) aerial concentration of these horizons in steeply dipping, tectonically repeated sections in narrow, elongate bands of rock; and (3) their exposure in mountains and ridges, which prevents accumulations of thick covering rocks and sediments. It also is possible that the Appalachian tracts have a naturally higher density of MVT districts for geologic reasons unknown today.
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., 1994, Geologic, fluid, inclusion, and isotopic studies of the Findlay Arch district, northwestern Ohio: Economic Geology, v. 89, no. 1, p. 67-90.
Glover, L., Speer, J.A., Russell, G.S., and Farrar, S.S., 1983, Ages of regional metamorphism and ductile deformation in the central and southern Appalachians: Lithos, v. 16, p. 223-245.
Hearn, P.P., Jr., Sutter, J.F., and Belkin, H.E., 1987, Evidence for Late-Paleozoic brine migration in Cambrian carbonate rocks of the central and southern Appalachians—Implications for Mississippi Valley-type sulfide mineralization: Geochimica et Cosmochimica Acta, v. 51, no. 5, p. 1323-1334.
Kesler, S.E., and van der Pluijm, D.A., 1990, Timing of Mississippi Valley-type mineralization—Relations to Appalachian orogenic events: Geology, v. 18, p. 1115-1118.
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.