Explained by K.J. Schulz, W.F. Cannon, P.K. Sims, and G.L. LaBerge
On the choice of deposit models
Exploration has been extensive and is continuing in the Late Archean greenstone-granite terrane, particularly in northern Minnesota, but so far as known, no potentially minable deposits have been discovered. Massive sulfide deposits of the kuroko type are known in the Canadian part of the Late Archean terrane (Franklin and others, 1981).
On the delineation of permissive tracts
The permissive tract is delineated based on: (1) its 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; and (3) the known occurrence of massive sulfide deposits in extensions of the area in Canada. The tract encompasses the Late Archean volcanic belts in the Lake Superior region where the rocks are within one km of the surface. Excluded from the permissive area is a region of biotite schist, gneiss and granite, termed the Quetico subprovince (Card, 1990), that lacks both known volcanogenic massive sulfide deposits and suitable host rocks.
On the numerical estimates made
No estimate of undiscovered deposits was made for this tract because: (1) the lack of known exploration success at finding minable deposits in exposed parts of this terrane, and (2) the apparent paucity of known volcanogenic massive sulfide deposits in the western part of the Superior province (Fyon and others, 1992).
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.
Fyon, J.A., Breaks, F.W., Heather, K.B., Jackson, S.L., Muir, G.M., Stott, G.M., and Thurston, P.C., 1992, Metallogeny of metallic mineral deposits in the Superior Province of Ontario, in Geology of Ontario, Ontario Geological Survey, Special Volume 4, pt. 2, p. 1091-1174.