Explained by J.F. Slack
On the choice of deposit models
The southern Appalachians region contains several large Besshi massive sulfide deposits (Gair and Slack, 1980). These deposits are similar to other stratabound and commonly stratiform sulfide deposits within intercalated clastic metasedimentary rocks and metabasalt that form mainly by submarine hydrothermal processes (Franklin and others, 1981; Fox, 1984; Slack, 1993). The largest deposits of this type in the southern Appalachians were mined in the Ducktown district of southeastern Tennessee (Emmons and Laney, 1926; Magee, 1968; Slater, 1982), and produced major amounts of copper and pyrrhotite, the latter for manufacture of sulfuric acid, and minor amounts of zinc, gold, and silver. Smaller deposits of this type occur at Fontana and Ore Knob, North Carolina (Ross, 1935; Espenshade, 1963; Kinkel, 1967).
The Besshi grade-tonnage model of Slack (1993) was used in the calculation of undiscovered metal resources. This model (24b.1) is considered better than model 24b of Singer (1986), which only included data for Besshi deposits in Japan. The model of Slack (1993) incorporates grade and tonnage data for Besshi deposits throughout the world, including those in the United States Appalachians, and is therefore more comprehensive.
On the delineation of permissive tracts
The permissive tract contains the Ducktown district, Tennessee, within Late Proterozoic rift-facies metasedimentary rocks of the Great Smoky Group (Knoll and Keller, 1979), although in thistract, metabasalt is rare. This tract also contains the much smaller Fontana, North Carolina, deposit and several prospects.
On the numerical estimates made
Estimates of the number of undiscovered deposits were made by comparisons with the number of known deposits in other areas such as the Besshi district of Japan. The choices were influenced by the presence of mines, prospects, and mineral occurrences; by geochemical anomalies; and by the level of exploration interest. At 90th, 50th, and 10th percentiles, the estimated numbers of undiscovered deposits are 1, 3, and 5, respectively, consistent with the grade and tonnage model of Slack (1993).
Emmons, W.H., and Laney, F.B., 1926, Geology and ore deposits of the Ducktown mining district, Tennessee: U.S. Geological Survey Professional Paper 139, 114 p.
Fox, J.S., 1984, Besshi-type volcanogenic sulphide deposits—A review: Canadian Institute of Mining Bulletin, v. 77, no. 864, p. 57–68.
Franklin, J.M., Lydon, J.W., and Sangster, D.F., 1981, Volcanic-associated massive sulfide deposits, in Skinner, B.J., ed., Economic Geology Seventy-Fifth Anniversary Volume, 1905–1980: Lancaster, Pennsylvania, Economic Geology Publishing Company, p. 485–627.
Gair, J.E., and Slack, J.F., 1979, Map showing lithostratigraphic and structural setting of stratabound (massive) sulfide deposits of the U.S. Appalachians: U.S. Geological Survey Open-File Report 79–1517, scale 1:1,000,000 [four oversize sheets].
Gair, J.E., and Slack, J.F., 1980, Stratabound massive sulfide deposits of the U.S. Appalachians, in Vokes, F.M., and Zachrisson, Ebbe, eds., Review of Caledonian-Appalachian stratabound sulphides: Geological Survey of Ireland Special Paper No. 5, p. 68–81.
Knoll, A.H., and Keller, F.B., 1979, Late Precambrian microfossils from the Walden Creek Group, Ocoee Supergroup, Tennessee: Geologcial Society of America, Abstracts with Programs, v. 11, p. 185.
Magee, Maurice, 1968, Geology and ore deposits of the Ducktown district, Tennessee, in Ridge, J.D., ed., Ore deposits of the United States, 1933–1967 (Graton-Sales Volume): New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, v. I, p. 207–241.
Ross, C.S., 1935, Origin of the copper deposits of the Ducktown type in the southern Appalachian region: U.S. Geological Survey Professional Paper 179, 165 p.
Shearer, H.K., and Hull, J.P.D., 1918, A preliminary report on a part of the pyrite deposits of Georgia: Georgia Geological Survey Bulletin 33, 229 p.
Slack, J.F., 1993, Descriptive and grade-tonnage models for Besshi-type massive 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 No. 40, p. 343–371.
Slater, Randy, 1982, Massive sulfide deposits of the Ducktown mining district, Tennessee, in Allard, G.O., and Carpenter, R.H., eds., Exploration for metallic resources in the southeast: Athens, Georgia, University of Georgia, p. 91–99.
Stose, A.J., and Stose, G.W., 1957, Geology and mineral resources of the Gossan Lead district and adjacent areas in Virginia: Virginia Division of Mineral Resources Bulletin 72, 233 p.