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 is 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). It should be noted that most of the Late Archean greenstone-granite terrane in the Lake Superior region is covered by Pleistocene glacial deposits to depths locally exceeding 200 m. This widespread glacial cover has restricted the effectiveness of the geophysical exploration methods used in the region.
Franklin and others (1981) have shown that Precambrian massive sulfide deposits typically contain mainly copper and zinc and little lead. This conclusion is supported by the grade information available for the Early Proterozoic massive sulfide deposits in the northern part of the Wisconsin magmatic terranes, where only the recently discovered Lynne deposit contains significant lead values. In contrast, Paleozoic massive sulfide deposits typically contain higher lead contents. The kuroko grade and tonnage model given in Singer and Mosier (1986) contains data for massive sulfide deposits of all ages and, as a result, has a higher mean lead grade then would be true for Precambrian deposits only. A modified grade and tonnage model with Phanerozoic deposits deleted (Mark3 index 103) is believed to better represent undiscovered deposits in the permissive tract.
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; (3) the known occurrence of massive sulfide deposits in extensions of the area in Canada; and (4) past exploration activity for this deposit type in the region. The tract includes the Rainy Lake area of Archean metavolcanic rocks in northern Minnesota and is considered favorable for volcanogenic massive sulfide deposits based on: (1) the presence of massive sulfide occurrences in the area and deposits directly to the east of the area in Canada (Klein, 1989) and (2) the occurrence of felsic volcanic rocks similar compositionally to the distinctive tholeiitic rhyolites recognized to host many Archean massive sulfide deposits in the Superior province (Lesher and others, 1986).
On the numerical estimates made
The estimate of undiscovered deposits for this tract reflects: (1) the lack of known exploration success at finding deposits in 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). For the 90th, 50th, and 10th percentiles, the team estimated 1, 2, and 3, or more kuroko massive sulfide deposits consistent with the grade and tonnage model of Singer and Mosier (1991) with Phanerozoic deposits deleted.
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.
Klein, T.L., 1989, Mineral occurrence and drill-hole location map of the International Falls 1o x 2o quadrangle, Minnesota and Ontario: U.S. Geological Survey Miscellaneous Field Studies Map MF-2082, 1:250,000.
Lesher, C.M., Goodwin, A.M., Campbell, I.H., and Gorton, M.P., 1986, Trace-element geochemistry of ore-associated and barren, felsic metavolcanic rocks in the Superior Province, Canada: Canadian Journal of Earth Sciences, v. 23, p. 222-237.
Sangster, D.F., 1980, Quantitative characteristics of volcanogenic massive sulfide deposits: CIM Bulletin, v. 73, p. 74-81.
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.