Explained by John F. Slack, Sandra H.B. Clark, and John D. Peper
On the choice of deposit models
No major sandstone-hosted lead-zinc deposits are known in the United States, and deposits of this type are of relatively minor economic importance by world standards. The only significant deposit in the United States was developed at the Goose Creek mine in the Indian Creek district of southeast Missouri (Kyle and Gutierrez, 1988). However, the importance of some deposits, such as the large (80-million-metric-ton) Laisvall lead-zinc deposit in northern Sweden, demonstrates the importance of considering the occurrence of similar deposits in geologically analogous settings. Sandstone-hosted lead-zinc deposits are stratabound concentrations of galena and generally minor sphalerite that occur within quartz-rich sandstone and quartzite overlying feldspathic basement rocks (Rickard and others, 1979; Bjørlykke and Sangster, 1981). The origin of sandstone-hosted lead-zinc deposits by transport of metals through permeable channels to an environment with sufficiently high H2S content to precipitate sulfides from groundwater or basinal brines resembles that proposed for sediment-hosted copper and Mississippi Valley deposits.
Clastic sedimentary sequences that occur near crystalline basement rocks are potential hosts for lead-zinc deposits. Although no deposits are known in the United States Appalachians, several occurrences have been described from Quebec (Schrijver and Beaudoin, 1987), suggesting potential for mineralization of the Cheshire Quartzite and correlative units of Early Cambrian age that overlie feldspathic basement rocks. Galena samples from the Rossie veins of western N.Y. have lead isotopic compositions and fluid inclusions (Ayuso and others, 1987) that are compatible with an origin from a Paleozoic basinal brine, which migrated into fractures during tectonism. The proposed migration of base-metals in brines suggests the possibility of the formation of sandstone-hosted lead-zinc deposits in favorable host rocks such as the Upper Cambrian Potsdam Sandstone that forms a basal unit overlying the Grenvillian basement of the Adirondacks.
On the delineation of permissive tracts
The permissive tract includes the Cambrian Cheshire Quartzite in Vermont, Potsdam Sandstone in New York, and stratigraphically equivalent Cambrian basal clastic sequences which are potential host rocks for sandstone lead-zinc deposits.
On the numerical estimates made
No estimates of undiscovered deposits were made for the permissive tract because, while occurrences of galena are known in the region, large sandstone-hosted lead-zinc deposits are uncommon on a worldwide basis. The lack of any known large deposits in the eastern United States or other indications beyond appropriate geologic settings over broad areas argues against estimating undiscovered resources of this deposit type.
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.
Bjørlykke, A., and Sangster, D.F., 1981, An overview of sandstone lead deposits and their relation to red-bed copper and carbonate-hosted lead-zinc deposits, in Skinner, B.J., ed., Economic Geology Seventy-Fifth Anniversary Volume, 1905–1980: Lancaster, Pennsylvania, Economic Geology Publishing Company, p. 179-213.
Kyle, J.R., and Gutierrez, G.N., 1988, Origin of the Indian Creek sandstone-hosted lead deposits, southeast Missouri, USA, in Zachrisson, Ebbe, ed., Proceedings of the Seventh Quadrennial IAGOD Symposium, Luleå, Sweden, 1986: Stuttgart, E. Schweizerbart’sche Verlagsbuchhandlung, p. 669–684.
Rickard, D.T., Willdén, M.Y., Marinder, N.-E., and Donnelly, T.H., 1979, Studies on the genesis of the Laisvall sandstone lead-zinc deposit, Sweden: Economic Geology, v. 74, no. 5, p. 1255-1285.
Schrijver, K., and Beaudoin, G., 1987, Diverse occurrences of galena-cemented sandstones in the Paleozoic, northern Appalachians, Quebec: Canadian Smith, R.C., II, 1977, Zinc and lead occurrences in Pennsylvania: Pennsylvania Geological Survey, Mineral Resources Report 72, 318 p.