Explained by K.J. Schulz, W.F. Cannon, P.K. Sims, and G.L. LaBerge
On the delineation of permissive tracts
The permissive tract is delineated based on: (1) general geologic similarity to other Precambrian terranes that are known to host volcanogenic massive sulfide deposits; (2) the known presence of suitable host rocks and deposit environments; (3) the known occurrence of massive sulfide deposits within the designated area; and (4) past exploration activity for this deposit type in the region. The tract includes the Pembine-Wausau terrane which is considered as favorable for undiscovered volcanogenic massive sulfide deposits. The area encompasses the known massive sulfide deposits and most known occurrences, and also encloses the area of compositionally distinct bimodal volcanic rocks that host at least some of the deposits in northern Wisconsin (Sims and others, 1989).
On the numerical estimates made
In contrast to the Archean terrane, the estimated massive sulfide resources of this tract are large and, if verified by future work, would establish this area as one of the principal base-metal massive sulfide regions of the world (Spooner and Barrie, 1993). This large resource estimate is based on the following factors: (1) Several massive sulfide deposits, including the world-class Crandon deposit, have already been discovered in the area. (2) Much of the area designated as favorable is blanketed by glacial deposits that have restricted the effectiveness of geophysical exploration methods to areas of relatively shallow cover, thus effectively leaving much of the area still unexplored. (3) Studies of other massive sulfide districts have shown that deposits tend to be clustered, with each cluster containing as many as about 20 geologically distinct orebodies and an "average" cluster containing about 12 (Sangster, 1980). Although the number of possible massive sulfide districts present within the area is not known, DeMatties (1989, 1994) has suggested the existence of at least three based on the observed clustering of known deposits. For the 90th, 50th, and 10th percentiles, the team estimated 30, 65, and 85, or more kuroko massive sulfide deposits consistent with the grade and tonnage model of Singer and Mosier (1991) with Phanerozoic deposits deleted.
DeMatties, T.A., 1989, A proposed geologic framework for massive sulfide deposits in the Wisconsin Penokean volcanic belt: Economic Geology, v. 84, no. 4, p. 946-952.
DeMatties, T.A., 1994, Early Proterozoic volcanogenic sulfide deposits in Wisconsin—An overview: Economic Geology, v. 89, no. 5, p. 1122–1151.
Franklin, J.M., Lyndon, 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.
Lambe, R.N., and Rowe, R.G., 1987, Volcanic history, mineralization, and alteration of the Crandon massive sulfide deposit, Wisconsin: Economic Geology, v. 82, no. 5, p. 1204-1238.
Sangster, D.F., 1980, Quantitative characteristics of volcanogenic massive sulfide deposits: CIM Bulletin, v. 73, p. 74-81.
Sims, P.K., VanSchmus, W.R., Schulz, K.J., and Peterman, Z.E., 1989, Tectono-stratigraphic evolution of the Early Proterozoic Wisconsin magmatic terranes of the Penokean Orogen: Canadian Journal of Earth Sciences, v. 26, p. 2145-2158.
Singer, D.A., 1986, Descriptive model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 189–190.
Singer, D.A., and Mosier,D.L., 1986, Grade and tonnage model of kuroko massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 190–197.
Spooner, E.T.C., and Barrie, C.T., 1993, A special issue devoted to Abitibi ore deposits in a modern context: preface: Economic Geology, v. 88, no. 6, p. 1307-1322.