Explained by T.L. Klein and K.J. Schulz
On the choice of deposit models
Greenstone-dominated subprovinces of the Archean Superior Province of the Canadian Shield are host to most of the major gold districts in Canada, with gold produced principally from quartz vein deposits or sulfide-mineral disseminations found near major shear zones that are tens to hundreds of kilometers long. Most deposits are immediately adjacent to major, generally east-trending, dextral, transpressive fault zones in greenstone belts or along their boundaries. The largest districts or deposits appear to have been formed during early episodes of dominantly high-angle reverse movement along these typically transpressive zones. Most current models favor an epigenetic origin for Archean gold deposits (Colvine and others 1988; Groves and others, 1989; Groves and Foster, 1991). The gold-bearing veins and sulfide mineral disseminations were emplaced in a zone of brittle-ductile deformation after peak deformation and metamorphism which, in most of the Superior Province, occurred between 2700 and 2680 Ma. Gold production does not correlate with any lithologic or geochemical characteristics or with the age of the major rock units within the subprovinces. Rather, production has been greatest in those subprovinces (i.e., the Abitibi and Uchi subprovinces) that show the highest degree of preservation of volcano-sedimentary rocks, reflected in thick sections with low-grade metamorphism and a high ratio of volcanic to plutonic rocks (Card and others, 1989).
Rocks of the southern Superior Province underlie the Lake Superior region and are the southwestward extensions of, from north to south, the Archean Wabigoon, Quetico, and Wawa subprovinces. The Wabigoon and Wawa are greenstone-granite dominated subprovinces that contain important gold districts in Canada. The metasedimentary rock-dominated Quetico subprovince has no history of gold production in the U.S. or Canada. The greenstone-granite dominated Wawa subprovince is the southwestward extension of the highly-productive Abitibi subprovince, which ranks first in production of gold in the Superior Province, and accounts for 84 percent of the total production from the province (data from Card and others 1989). However, the only major gold camp in the Wawa subprovince in Canada is at Wawa, Ontario. Rocks of the Wawa subprovince are exposed in the Vermilion district in northeastern Minnesota (Sims, 1976) and in the Ishpeming greenstone belt of northern Michigan (Johnson and Bornhorst, 1991).
The descriptive model for low-sulfide Au-quartz vein deposits of Archean age of Klein and Day (1994) is thought to best describe most of the potential Archean Au-quartz vein deposits in the Lake Superior region. Formerly, the only model for the occurrence of Archean gold deposits in greenstone belts was that for Homestake type deposits (model 36b of Mosier, 1986). Upon re-examination of the descriptive model and grade and tonnage information of model 36b, Klein and Day (1994) found that only 10 percent of the deposits used for the grade and tonnage curves fit the descriptive criteria, i.e. that the deposits were related spatially or genetically to iron-formation or chemical sediments. The new model for Archean low-sulfide Au-quartz vein deposits (Klein and Day, 1994) does not require iron-formation as a diagnostic criteria in defining permissive tracts and it serves to emphasize the importance of structural control on the formation of these deposits. Studies of Archean gold deposits elsewhere show that most deposits are epigenetic in character and not related genetically to chemical sediments. Several studies in the Superior Province of Canada and the Yilgarn Province of Western Australia
(Colvine and others, 1988; Groves and others, 1989) have shown that the Archean gold deposits do not show any consistent lithologic control and occur in most of the rock types that make up Archean greenstone belts. The Archean low-sulfide Au-quartz vein deposits share most geologic characteristics with Proterozoic and Phanerozoic low-sulfide Au-quartz vein deposits, but have distinct grade and tonnage.
The tracts outlined for the Archean low-sulfide Au-quartz deposits in the Lake Superior region could also contain Homestake gold deposits. However, because Homestake deposits are relatively rare in greenstone belts, quantitative resources estimates were not made for these deposits in the Lake Superior region.
On the delineation of permissive tracts
Deeply buried rocks of the greenstone-dominated Wabigoon and Wawa subprovinces and the metasedimentary Quetico subprovince in Minnesota and North and South Dakota are included in this tract. The tract is bounded on the south by the high-grade Middle Archean gneisses of the Minnesota River Valley and by Early Proterozoic rocks of the Animikie Basin and is bounded on the north by the border with Canada. The tract extends westward to where Archean rocks are buried by more than 1 kilometer of Paleozoic sedimentary rocks. Much of this area is covered by glacial deposits from 20-100 m thick and most of it has not been explored for metal deposits. We consider the tract permissive for the occurrence of undiscovered low-sulfide Au-quartz vein deposits because of the infered presence of Archean host rocks and structures and because such deposits occur in equivalent rocks elsewhere in the exposed parts of these subprovinces in Canada. The metasedimentary rocks and granites of the Quetico subprovince were included in this tract because, even though the terrain has not produced gold elsewhere, several gold-bearing shear zones are found along the subprovince boundaries or project into the subprovince. The rocks of this subprovince are in large part poorly explored.
On the numerical estimates made
For the 90th, 50th, and 10th percentiles, the team estimated, respectively, 2, 4, and 6 or more Archean low-sulfide gold quartz vein deposits consistent with the grade and tonnage model of Klein and Day (1994).
Card, K.D., Poulsen, K.H., and Robert, Francios, 1989, The Archean Superior Province of the Canadian Shield and its lode gold deposits, in Keays, R.R., and Skinner, B.J., eds., The geology of gold deposits-The perspective in 1988: Economic Geology Monograph 6, p. 19-36.
Colvine, A.C., Fyon, J.A., Heather, K.B., Marmont, S., Smith, P.M., and Troop, D.G., 1988, Archean lode gold deposits in Ontario: Ontario Geological Survey Miscellaneous Paper 139, 136 p.
Groves, D.I., and Foster, R.P., 1991, Archaean lode gold deposits, in Foster, R.P., ed., Gold Metallogeny and Exploration, Blackie, London, p. 63-103.
Groves, D.I., Barley, M.E., Ho, S.E., 1989, Nature, genesis, and tectonic setting of mesothermal gold mineralization in the Yilgarn Block, Western Australia, in Keays, R.R., Ramsay, W.R.H., and Groves, D.I., eds., The Geology of Gold Deposits-The Perspective in 1988: Economic Geology Monograph 6, p. 71-85.
Johnson, R.C., and Bornhorst, T.J., 1991, Archean gology of the northern block of the Ishpeming greenstone belt, Marquette County Michigan: U.S. Geological Survey Bulletin 1904-F, 20 p.
Klein, T.L., and Day, W.C., 1994, Desriptive and grade-tonnage models of Archean low-sulfide Au-quartz and a revised grade-tonnage model of Homestake Au: U.S. Geological Suvey Open-File Report 94-250.
Mosier, D.L., 1986, Grade-tonnage model of Homestake gold, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 245-247.
Sims, P.K., 1976, Early Precambrian tectonic-igneous evolution in the Vermilion district, northeastern Minnesota: Geological Society of America Bulletin, v. 87, p. 379-389.