Explained by Sandra H.B. Clark and John F. Slack
On the choice of deposit models
Sedimentary-exhalative or sedex zinc-lead deposits, a class of stratiform sulfide deposits, have served as major metal sources in many countries (Gustafson and Williams, 1981). This deposit type is believed to form by subaqueous hydrothermal processes related to exhalation of a metalliferous brine, precipitating lead and (or) zinc sulfides either as chemical sediments and (or) as shallow subsurface replacements (Carne and Cathro, 1982; Goodfellow and others, 1993).
In the northern United States Appalachians, studies by Clark (1990) and Foley (1990) have documented sedex features in a small (about 9,000 tonnes Pb+Zn) deposit in the Cambrian to Ordovician carbonate-siliciclastic shelf sequence at Lion Hill near Brandon, Vermont. The choice of a sedimentary-exhalative model (Briskey, 1986) is based on recognition of associated layered magnetite iron formation and on textural features that suggest a partly syngenetic to diagenetic origin for the mineralization at Lion Hill. This interpretation is supported by studies of the paleoenvironmental setting, the geochemical distribution of elements relative to lithologic units, and the lead and sulfur isotopic composition of the sulfide minerals (N.K. Foley, personal commun., 1994). In addition to Lion Hill, the rocks of the shelf sequence contain numerous other base-metal sulfide occurrences from eastern New York to northwestern Massachusetts and southern Quebec, suggesting potential for Irish-type sedimentary-exhalative mineralization in much of the shelf sequence. The estimated size of the known mineralized zones at the Lion Hill prospect is far below the smallest deposits included in the tonnage curve for sedimentary-exhalative Zn-Pb of Menzie and Mosier (1986).
On the delineation of permissive tracts
The entire Cambrian to Ordovician carbonate siliciclastic shelf sequence of Vermont, New York, Massachusetts, and Connecticut is considered to be permissive for the occurrence of undiscovered sedimentary-exhalative deposits. The most favorable zones are the Cambrian Dunham Dolomite and Monkton Quartzite, which host known deposits. Tract borders were not extended beyond areas of surface outcrop because, even though the early Paleozoic platform deposits are believed to extend beneath the Taconic thrust slices, the platform deposits are thought to be at depths of more than one kilometer beneath the surface in most parts of the Taconics (J.D. Peper, oral commun.). Two areas within the tract are considered favorable, one near Brandon, Vt., and another near Franklin, Vt. A limited amount of exploration, including core drilling, was done on base-metal sulfide prospects in both areas about 1980. The tract is also considered permissive for the occurrence of Appalachian Zn and sandstone-hosted Pb-Zn deposits because of the presence of shelf carbonate rocks and a basal sandstone unit, known occurrences of zinc and lead sulfides, and the tectonic position of the tract beneath the Taconic allochthons. However, if Appalachian Zn or sandstone-hosted Pb-Zn mineralization is present in the tract, it is related to a mineralizing event that occurred much later than the inferred syngenetic to diagenetic mineralization at Lion Hill.
On the numerical estimates made
The consensus method was used to estimate the numbers of undiscovered deposits. Factors considered were the presence of known occurrences, the generally low level of modern exploration activity, the recent recognition of potential for sedimentary-exhalative deposits, and the sizes of the tracts relative to each other. For the 90th, 50th, and 10th percentiles, the team estimated 0, 1, and 5 or more sedimentary-exhalative deposits consistent with the grade and tonnage model of Menzie and Mosier (1986).
Carne, R.C., and Cathro, R.J., 1982, Sedimentary exhalative (sedex) zinc-lead-silver deposits, northern Canadian Cordillera: Canadian Institute of Mining and Metallurgy Bulletin, v. 75, no. 875, p. 95-106.
Clark, S.H.B., 1990, Stratabound zinc-lead-copper deposits in the Cambrian carbonate-siliciclastic shelf sequence at Lion Hill, west-central Vermont, in Slack, J.F., ed., Summary results of the Glens Falls CUSMAP project, New York, Vermont, and New Hampshire: U.S. Geological Survey Bulletin 1887-K, 9 p.
Goodfellow, W.D., Lydon, J.W., and Turner, R.J.W., in press, 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.
Foley, N.K., 1990, The Lion Hill zinc-lead-copper deposit, Brandon, Vermont—Source of lead and fluid systematics, in Slack, J.F., ed., Summary results of the Glens Falls CUSMAP project, New York, Vermont, and New Hampshire: U.S. Geological Survey Bulletin 1887-L, 9 p.
Gustafson, L.B., and Williams, Neil, 1981, Sediment-hosted stratiform deposits of copper, lead, and zinc, in Skinner, B.J., ed., Economic Geology Seventy-Fifth Anniversary Volume, 1905–1980: Lancaster, Pennsylvania, Economic Geology Publishing Company, p. 139-178.
Menzie, W.D., and Mosier, D.L., 1986, Grade and tonnage model of sedimentary exhalative Zn-Pb, in Cox, D.P., and Singer, D.A., eds. 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 212-215.