Explained by T.L. Klein
On the choice of deposit models
The Talledaga terrane of the eastern Alabama Piedmont is host to several massive sulfide districts that occur within a Devonian(?) bimodal volcanic sequence, the Hillabee Greenstone. The felsic volcanic rocks are calc-alkaline in character, whereas the mafic volcanic rocks are tholeiitic, suggesting that the sequence may have developed in an arc setting over continental crust (Tull and Stow, 1982). In the Pyriton district, discontinuous pyrite-rich lenses ranging from 2–10 m thick extend 2.5 km along strike within the lower mafic part of the Hillabee Greenstone. At Pyriton, the upper felsic volcanic rocks, present elsewhere, have been removed by faulting. Production from the district was relatively small, probably about 100,000 metric tons of pyritic ore averaging 1.25 percent copper. Drilling in the district during exploration for pyrite has indicated that substantial low-grade copper resources (5.9 million metric tons at 0.3 percent copper and 0.06 percent zinc) may be present (see Stow and Tull, 1982).
Because the lead contents of Precambrian and Phanerozoic kuroko massive sulfide deposits are significantly different (Franklin and others, 1981), the grade and tonnage model of Singer and Mosier (1986), which combines deposits of all ages, did not seem appropriate for the Phanerozoic kuroko deposits in the southeastern United States. A new Phanerozoic kuroko model (Mark3 index 104) was used.
On the delineation of permissive tracts
Volcanic rocks of the Hillabee Greenstone in the Devonian Talladega terrane of east-central Alabama define the permissive tract. These dominantly mafic volcanic rocks occur in a narrow zone (up to 2.5 km wide) within a faulted nappe (Tull and Stow, 1982). Several massive sulfide districts and prospects occur along the 170-km-long belt; the largest deposits, those at Pyriton and in the Hatchet Creek-Millerville district, have produced pyrite.
On the numerical estimates made
The numbers of undiscovered deposits were estimated individually for favorable tract using the consensus method. The numbers of undiscovered deposits estimated at the 90th, 50th, and 10th percentiles respectively, are 0, 1, and 3 deposits consistent with the Phanerozoic kuroko model (Mark3 index 104). This estimate is low because of the small size of the tract and the apparent low base-metal content of the known deposits relative to the grade and tonnage model.
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.
Singer, D.A., and Mosier, D.L., 1986, Grade and tonnage model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p.190–197
Stow, S.H., and Tull, J.F., 1982, Geology and geochemistry of the strata-bound sulfide deposits of the Pyriton district, Alabama: Economic Geology, v. 77, no. 2, p. 322–334.
Tull, J.F., and Stow, S.H., 1982, Geologic setting of the Hillabee metavolcanic complex and associated strata-bound sulfide deposits in the Appalachian Piedmont of Alabama: Economic Geology, v. 77, no. 2, p. 312–321.