Explained by Sandra H.B. Clark
On the choice of deposit models
Although no sedimentary-exhalative Zn-Pb deposits are known in the east-central region, the probability for undiscovered deposits of this type was evaluated because conditions in some parts of the Appalachian basin are geologically analogous to the settings of known deposits elsewhere (Goodfellow and others, 1993). The analogous setting was recognized and resulted in some exploration interest in the late 1970s and early 1980s. (Wedow, 1983; Nuelle and Shelton, 1988). Wedow (1983) redesigned an empirical model for sedimentary-exhalative deposits that had been developed by Large (1980) to apply to a computer-assisted search for exploration targets in Appalachian Devonian rocks. Occurrences of barite in the Appalachian basin were studied to determine resource potential because of their similarity in geologic setting to large, economically important sedimentary-exhalative Zn-Pb and barite deposits in other areas such as Meggen, Germany (Nuelle and Shelton, 1986, 1988; Clark, 1988; Clark and Mosier, 1989). An assessment of the mineral resources of West Virginia (Cannon and others, 1994) suggests potential for sedimentary-exhalative Zn-Pb deposits based on geological characteristics and stream-sediment geochemistry. These studies were reevaluated in the light of more recent work about the nature and characteristics of sedimentary-exhalative Zn-Pb deposits to determine if parts of the Appalachian basin sedimentary sequence are permissive for the occurrence of undiscovered deposits.
On the delineation of permissive tracts
Examination of the metallogenic map for zinc, lead, and barite in the east-central United States (Clark, 1987) shows some occurrences in shale of Ordovician through Pennsylvanian age in Appalachian basin rocks. However, most occurrences are in shale of Middle to Late Devonian age. The Devonian black shale sequence was considered by Wedow (1983) to be the most likely sequence for hosting sedimentary-exhalative Pb-Zn deposits because major known deposits (Meggen and Rammelsburg, Germany and deposits in the Selwyn basin in Canada) are Devonian. Wedow's analysis (1983) consisted of maps showing the occurrence of nine attributes or parameters that were observed to be associated with known deposits and the areas of coincidence of more than one attribute within a 7.5-minute cell. The attributes were: (1) epicenter density of modern seismic activity; (2) extent of Silurian salt; (3) zone of Devonian volcanic activity, based on the distribution of the Tioga Ash Bed and heavy development of chert in Middle Devonian (Onesquethaw Group) rocks; (4) high-angle faults; (5) possible geosuture (the Alabama-New York lineament); (6) possible Silurian to Devonian hingeline based on isopach maps of the Silurian to Devonian carbonate sequence; (7) possible Middle to Late Devonian hingeline based on maps of the cumulative thickness of Devonian black shale; (8) occurrence of anomalously high amounts of supergene Mn-Fe; and (9) occurrence of the pteropod, Styliolina fissurella, which indicates favorable sedimentary environments. Maps showing the coincidence of attributes (both unweighted and weighted) led to identification of the following four areas as possible exploration targets needing further study: (1) lowermost Upper Devonian rocks straddling the Clarendon-Linden fault in western and central New York; (2) an area in western Virginia and eastern West Virginia, generally coincident with the
"Oriskany" Mn-Fe ores; (3) an area in West Virginia, Maryland, and Virginia along and near the trend of the Alabama-New York lineament between 38° and 40° N. latitude; and (4) an area in northeastern Ohio overlying and coincident with a significant thickness of Silurian salt and modern seismic activity.
Studies of barite occurrences in the Appalachians suggest that they can be interpreted as indicating the introduction of submarine-hydrothermal fluids (Nuelle and Shelton, 1986, 1988). However, because the barite nodules in the Appalachians are sparse, they can also be interpreted as resulting from biogenic processes at redox boundaries without the involvement of submarine-exhalative fluids that could have produced Zn-Pb deposits (Clark, 1988; Clark and Mosier, 1989).
The lack of evidence for strong Devonian tectonic activity was discussed by the assessment team and considered to be a basis for doubting the existence of undiscovered sedex deposits in the Appalachian basin. However, the thick sequences of black shale in the basin and indications that later tectonic activity may represent reactivation of pre-existing structures leaves open the possibility for Devonian synsedimentary-tectonic activity. Because black shale normally contains anomalously high base-metal contents, base-metal anomalies in themselves were not considered sufficient evidence for mineralization, but base-metal or barite anomalies and occurrences in combination with possible synsedimentary tectonism led to the conclusion that the presence of undiscovered sedimentary-exhalative Pb-Zn deposits could not be ruled out. The team concluded that the thick sequences of Middle Devonian to Lower Mississippian black shale and clastic sedimentary rocks are permissive for the occurrence of sedimentary-exhalative Pb-Zn deposits. The lower contact for the tract is derived from the map of generalized geologic setting for metal deposits in the eastern United States (J.D. Peper, written commun.; see Gair and others, 1987). The extension of the tract to depths of one kilometer or less is based on analysis of stratigraphic cross sections, structure contours, drilling depths, and lithofacies maps (deWitt and others, 1993; Oliver and others, 1971). Shale units less than 100 ft thick, such as the Chattanooga Shale, are excluded from the tract.
On the numerical estimates made
Although a large permissive tract was defined, the team estimated less than a one percent chance for an undiscovered deposit. This was because of the lack of known deposits and the lack of strong evidence of syndepositional tectonic activity or other indicators of the existence of undiscovered sedimentary-exhalative deposits.
Cannon, W.F., Clark, S.H.B., Lesure, F.G., Hinkle, M.E., Paylor, R.L., King, H.M., Simard, C.M., Ashton, K.C., and Kite, J.S., 1994, Mineral resources of West Virginia: U.S. Geological Survey Miscellaneous Investigations Series Map I-2364-A, scale 1:500,000, with pamphlet, 14 p.
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, with pamphlet, 77 p.
Clark, S.H.B., 1988, Origin of some shale-hosted barite nodules of the Appalachian basin of the eastern United States, in Zachrisson, Ebbe, ed., Proceedings of the seventh Quadrennial International Association on the Genesis of Ore Deposits Symposium, Luleå, Sweden, 1986: Stuttgart, Germany, E. Schweizerbart'sche Verlagsbuchhandlung, p. 259-268.
Clark, S.H.B., and Mosier, E.L., 1989, Barite nodules in Devonian shale and mudstone of western Virginia: U.S. Geological Survey Bulletin 1880, 30 p.
deWitt, Wallace, Jr., Roen, J.B., and Wallace, L.G., 1993, Stratigraphy of Devonian black shale and associated rocks in the Appalachian basin, in Roen, J.B., and Kepferle, R.C., eds., Stratigraphy of
Devonian black shales and associated rocks in the Appalachian basin: U.S. Geological Survey Bulletin 1909-B, 57 p.
Gair, J.F., Cannon, W.F., Peper, J.D., Cannon, S.S., and Martin, B., 1987, Appalachian metallogenic map (abs.), in Sachs, J.S., ed., USGS research on mineral resources—1987, Third Annual V.E. McKelvey Forum on Mineral and Energy Resources: U.S. Geological Survey Circular 995, p. 22–23.
Goodfellow, W.D., Lydon, J.W., and Turner, R.J.W., 1993, Geology and genesis of stratiform sediment-hosted (SEDEX) Zn-Pb-Ag sulphide deposits, in Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and Duke, J.M., eds., Mineral deposit modeling: Geological Association of Canada Special Paper 40, p. 201–251.
Large, Duncan, 1980, Geological parameters associated with sediment-hosted, submarine exhalative Pb-Zn deposits: Geologisches Jahrbuch, Riehe D, Heft 40, p. 59-129.
Nuelle, L.M., and Shelton, K.L., 1986, Geologic and geochemical evidence of possible bedded barite deposits in Devonian rocks of the Valley and Ridge province, Appalachian Mountains: Economic Geology, v. 81, no. 6, p. 1408-1430.
Nuelle, L.M., and Shelton, K.L., 1988, Sediment-hosted barite of the Appalachian-basin Valley and Ridge Province: A product of tectonic influence on sedimentation, in Kisvarsanyi, Geza and Grant, S.K., eds., North American Conference on Tectonic Control of Ore Deposits and the Vertical and Horizontal Extent of Ore Systems, p. 501-511.
Oliver, W.A., Jr., deWitt, Wallace, Jr., Dennison, J.M., Hoskins, D.M., and Huddle, J.W., 1971, Isopach and lithofacies maps of the Devonian in the Appalachian basin: Pennsylvania Topographic and Geologic Survey, Progress Report 182, 6 sheets.
Peper, J.D., in prep., Map showing the U.S. generalized geologic setting for metal deposits in the eastern USA: U.S. Geological Survey Investigations Series Map, 2 sheets, scale 1,100,000.
Wedow, Helmuth, Jr., 1983, A search for stratiform massive-sulfide exploration targets in Appalachian Devonian rocks—A case study using computer assisted attribute-coincidence mapping: U.S. Geological Survey Open-File Report 83-352, 38 p.