Explained by T.L. Klein and K.J. Schulz
On the choice of deposit models
Greenstone-dominated subprovinces of the Archean Superior Province 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 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 motion 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 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
This tract includes three areas within northern Minnesota and northern Michigan that are thought to be favorable for the occurrence of undiscovered Archean low-sulfide Au-quartz vein deposits.
The westernmost of the three areas, composed of low- to medium-grade metavolcanic and plutonic rocks of the Wabigoon subprovince, is a triangular area in northern Minnesota bounded by the U.S.-Canadian border, the Vermilion fault, the Rainy Lake-Seine River fault and a segment of the Quetico fault. Gold deposits are present in this tract near International Falls (Little America mine) and in the extension of these rocks in Canada. Geochemical anomalies are present in soil, rock, and glacial drift, and hydrothermally-altered rocks and carbonate alteration typical of low-sulfide Au-quartz vein deposits are found along major and minor faults and shear zones located at the boundaries of this tract, as well as within it. Exploration in the central and western part of the tract is complicated by glacial drift up to 60 m thick. Some parts of the area have been moderately well explored, whereas others are virtually untested.
The central of the three areas contains the Vermilion district which contains metamorphosed volcanic and sedimentary rocks of the Wawa subprovince. The Vermilion district is bounded on the north by subprovince boundary faults, the Vermilion and Burntside Lake faults, and is bounded on the south by the Giants Range batholith. Several east-west trending dextral fault segments of the Vermilion fault and northwest-trending sinistral faults are located internal to the district. Several gold prospects, gold-bearing shear zones, and geochemical anomalies are present along both the bounding and internal faults (Sims and Day, 1992).
The mafic metavolcanic-dominated Ishpeming greenstone belt of the Wawa subprovince in northern Michigan defines the easternmost of the three areas. This greenstone belt is bounded on the north and south by high-grade gneissic rocks and is in part overlain by rocks of the Early Proterozoic Marquette Range Supergroup. Several internal dextral shear zones affect the Archean rocks in the belt (Johnson and Bornhorst, 1991). The Ishpeming greenstone belt hosts the Ropes gold mine, as well as several low-sulfide Au-quartz vein prospects.
Important examples of this type of deposit
The Wabigoon subprovince extends into northern Minnesota and ranks third in gold production when compared with other Superior subprovinces. Total known production of gold in the Wabigoon subprovince acounts for about 3 percent of the total gold production in the Superior Province (data from Card and others 1989). In the U.S. part of the southern Wabigoon subprovince, the only gold production has come from quartz veins in the Little America mine located along the U.S.-Canadian border near International Falls, Minnesota (Klein, 1989). This deposit and several nearby prospects in the U.S. and many other small deposits found nearby in Canada are located along the dextral Rainy Lake-Seine River fault. Small deposits also occur further north in Canada along the Quetico fault. Both of these major transcurrent faults can be traced into northern Minnesota beneath thick glacial cover using airborne magnetic data.
In the Ishpeming greenstone belt, in northern Michigan, gold has been produced only at the Ropes mine, a typical low-sulfide Au-quartz vein deposit (see Bornhorst and others, 1986; Brozdowski and others, 1986). Gold in this deposit is associated with finely disseminated pyrite in altered felsic volcanic rocks and in quartz-tetrahedrite veins in a ductile shear zone. In the Vermilion district in northeastern Minnesota, greenstones contain several gold prospects in hydrothermally altered rocks along the dextral Vermilion fault and related secondary shear zones (Sims and Day, 1992). Several areas along the Vermilion fault system contain soil geochemical anomalies typical of anomalies associated with low-sulfide Au-quartz vein deposits.
On the numerical estimates made
For the 90th, 50th, and 10th percentiles, the team estimated, respectively, 2, 3, and 6 or more Archean low-sulfide gold quartz vein deposits consistent with the grade and tonnage model of T.L. Klein and W.C. 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.
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, scale 1:250,000.
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.
Sims, P.K., and Day, W.C., 1992, A regional structural model for gold mineralization in the southern part of the Archean Superior province, United States: U.S. Geological Survey Bulletin 1904-M, 19 p.