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 that form mainly by submarine hydrothermal processes within intercalated clastic metasedimentary rocks and metabasalt (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, as well as minor zinc and pyrrhotite, the latter for manufacture of sulfuric acid. In southwestern Virginia, the Gossan Lead district contains several separate Besshi massive sulfide deposits that together accounted for significant production of copper, zinc, and sulfur (Stose and Stose, 1957; Gair and Slack, 1984). 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. In contrast with the model 24b of Singer (1986), which only included data for Besshi deposits in Japan, this model 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 outlines the Late Proterozoic Ashe Formation (Rankin and others, 1973) in southwestern Virginia and northwestern North Carolina, which consists of metamorphosed clastic metasedimentary rocks and variable amounts of associated metabasalt that host the Besshi deposits of the Gossan Lead district and the Ore Knob mine. This tract also includes rocks of similar age and lithology along strike to the southwest, in western and southwestern North Carolina, and in northeastern Georgia.
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.
Espenshade, G.H., 1963, Geology of some copper deposits in North Carolina, Virginia, and Alabama: U.S. Geological Survey Bulletin 1142-I, 50 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.
Gair, J.E., and Slack, J.F., 1984, Deformation, geochemistry, and origin of massive sulfide deposits, Gossan Lead district, Virginia: Economic Geology, v. 79, no. 7, p. 1483–1520.
Kinkel, A.R., Jr., 1967, The Ore Knob copper deposit, North Carolina, and other massive sulfide deposits of the Appalachians: U.S. Geological Survey Professional Paper 558, 58 p.
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.
Rankin, E.W., Espenshade, G.H., and Shaw, K.W., 1973, Stratigraphy and structure of the metamorphic belt in northwestern North Carolina and southwestern Virginia—A study from the Blue Ridge across the Brevard fault zone to the Sauratown Mountains anticlinorium: American Journal of Science, v. 273A, p. 1–40.
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.
Wittington, D., 1982, Geology of the Stone Hill massive sulfide copper mine, Cleburne and Randolph Counties, Alabama: Tuscaloosa, University of Alabama, Unpublished M.Sc. Thesis, 143 p.