Explained by T.L. Klein and K.J. Schulz
On the choice of deposit models
The southern part of the Early Proterozoic Penokean orogen (the Wisconsin magmatic terranes of Sims and others, 1989), exposed in northern Wisconsin, consists of volcano-sedimentary rocks and related granitoid intrusions that are similar in many respects to older Archean greenstone belts. Transpressive shear zones, some of which represent collisional suture zones, are also present. Au-quartz vein deposits are know within similar Early Proterozoic terranes of the Trans-Hudson orogen in Canada (Ansdell and Kyser, 1992).
The Proterozoic greenstone-dominated terranes of the Lake Superior region are thought to contain gold deposits that fit the descriptive model for low-sulfide Au-quartz veins (Berger, 1986) by analogy with the setting of deposits in other areas of Proterozoic age. These deposits have lower grades and tonnages than similar deposits of Archean age.
On the delineation of permissive tracts
The permissive tract encloses the Early Proterozoic magmatic terranes of the Penokean orogen. These terranes consist mainly of tholeiitic and calc-alkaline volcanic rocks, lesser volcanogenic sedimentary rocks, and a variety of granitoid intrusions. These terranes contain northeast to eastward-trending shear zones and faults. The tract is bounded on the north by the Niagara fault zone, a presumed suture zone, and the Archean rocks of the Superior Province. It is bounded on the south by Archean gneisses and Paleozoic sedimentary rocks, and is truncated on the east by the Middle Proterozoic Wolf River Batholith and on the west by rocks of the Middle Proterozoic Midcontinent Rift System. This tract is thought to be permissive for low-sulfide Au-quartz vein deposits based on lithologic and structural similarity to other Early Proterozoic terranes in Canada and elsewhere that host such gold deposits (e.g., Flin Flon domain of the Trans-Hudson orogen, Canada). Also, a number of occurrences of gold in quartz veins are known, and a small structurally controlled deposit, the Reef prospect, is reported to contain about 408,000 metric tons of mineralized rock that grades about 9 g/metric ton Au and 0.28 percent Cu (DeMatties, 1994).
On the numerical estimates made
For the 90th, 50th, and 10th percentiles, the team estimated, respectively, 1, 3, and 6 or more low-sulfide gold quartz vein deposits consistent with the grade and tonnage model for this deposit type. Small numbers of low-sulfide Au-quartz vein deposits were estimated for this tract because of the limited exploration success in the exposed parts of the tract. However, given the large proportion of covered, relatively unexplored bedrock in the Lake Superior region, we feel that there is a large of degree of uncertainty in the numerical deposit estimate for this region.
Ansdell, K.M., and Kyser, T.K., 1992, Mesothermal gold mineralization in a Proterozoic greenstone belt: western Flin Flon domain, Saskatchewan, Canada: Economic Geology, v. 87, p. 1496-1524.
Berger, B.R., 1986, Descriptive model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239.
DeMatties, T.A., 1994, Early Proterozoic volcanogenic massive sulfide deposits in Wisconsin—an overview: Economic Geology, v. 89, p. 1122–1151.