Abstract Precambrian rocks in the Lake Superior region underlie all or parts of Minnesota, Wisconsin, and Michigan, an area along the southern margin of the Superior province of the Canadian Shield (Fig. 1). Except on the north, adjacent to Canada, the Precambrian rocks are overlapped by sedimentary strata of Paleozoic and Mesozoic age, which constitute a thin platform cover of relatively undisturbed rocks that thicken to the west, south, and east. Inliers of Precambrian rocks are exposed locally in southern Minnesota and Wisconsin, mainly in the flat valleys of major rivers, where erosion has cut below the Phanerozoic strata. The present landscape is subdued, and is inherited largely from Pleistocene continental glaciations, which produced a variety of erosional and depositional landforms. The glacier ice scoured the bedrock in the northern parts of the region, in much the same way as throughout most of Canada, and deposited materials of diverse lithology and provenance, as much as 200 m thick, over much of the remainder of the region. The Precambrian rocks in the region record an extended interval of crustal development and evolution that spans nearly 3 b.y. of earth history. This interval of geologic time is not continuously recorded in layered and intrusive units, but instead is punctuated by specific rock-forming and tectonic events that can be deduced from geologic relations and placed in a chronometric framework by isotopic dating. (Fig. 2, also see correlation chart for Precambrian rocks of the Lake Superior region, Morey and Van Schmus, 1986; and Bergstrom and Morey, 1985.)

The Penokean foldbelt is a northeast-trending zone of deformed and metamorphosed Archean and early Proterozoic rocks, as much as 250 km wide in the Lake Superior region, along the southern margin of the Superior province. The rocks within the foldbelt were deformed 1,880–1,770 m.y. ago during the Penokean tectonothermal event. Evolution of the foldbelt began with rift-faulting that was localized along a preexisting zone of weakness in Archean rocks, the boundary between the two crustal segments previously recognized in the region: a greenstone-granite terrane (~2,700 m.y. old) to the north (Superior province) and a mostly older (in part 3,500 m.y. old) gneiss terrane to the south. The faulting and later broad foundering provided sites for deposition of detritus shed mainly from the craton to the north and for chemical sediments, including the vast iron-formations for which the region is famed. The central part of the basin also received mafic volcanic rocks intimately intercalated with the sedimentary rocks, and similar rocks dominate in the southern part. The terminal, compressional stage ended deposition. It involved folding of the supracrustal rocks, penetrative deformation of the basement rocks, and emplacement of diapiric gneiss domes with accompanying appression of intervening supracrustal rocks. Deformation was complex and prolonged, and because of basement participation it differed from place to place in style and orientation of folds and in intensity and nature of metamorphism. Granite-tonalite plutons locally were emplaced in the southern part of the foldbelt near and after the end of deformation. The rifting and terminal compression occurred in an intracratonic or continental-margin environment; possibly the compression was caused by forces transmitted from a remote distance to the southeast, in an area undergoing rifting of a continental margin and subsequent continent-continent collision.

New geologic and geochronologic data clearly establish that the two Archean basement-rock terranes recognized previously in Minnesota also occur in northern Wisconsin and Michigan, on the east side of the midcontinent gravity high. Greenstone-granite complexes (about 2,700 m.y. old) that are remarkably similar to those in the greenstone terrane of northern Minnesota are present on the south side of the Gogebic Range in northern Wisconsin and adjacent Michigan and in the northern complex of the Marquette district. Migmatitic gneisses and amphibolite, which are similar to the high-grade rocks in the gneiss terrane in southern Minnesota, are sporadically exposed south of the greenstone terrane and appear to compose the basement in most of northern and central Wisconsin and northern Michigan. The gneisses in Wisconsin and Michigan have been dated at two widely separated localities; they have radiometric ages of more than 3,000 m.y. and, for one rock type, an age of approximately 3,500 m.y. The boundary between the greenstone and gneiss terranes is covered by younger supracrustal rocks, but is interpreted to trend approximately eastward across northern Wisconsin to the vicinity of Marquette, Michigan, and beyond. Subsequent to its welding, probably 2,600 to 2,700 m.y. ago, abundant mafic dikes of Precambrian X and Y ages were emplaced in a wide zone parallel to the boundary, which indicates that it was predominantly a zone of extensional tectonics throughout much of post-Archean–pre-Phanerozoic time.

The Marenisco-Watersmeet area in the western part of the northern peninsula of Michigan contains a greenstone and granite terrane (Puritan Quartz Monzonite) of late Archean age on the north and a gneiss terrane (gneiss at Watersmeet) on the south. A granite and gneiss belt (collectively called the granite near Thayer) crops out between these contrasting terranes. Lower Proterozoic metasedimentary and metavolcanic rocks of the Marquette Range Supergroup are extensive. Radiometric dating of the tonalitic phase of the gneiss at Watersmeet establishes an early Archean age and a complex subsequent history. U-Th-Pb systematics provide a firm minimum age of 3,410 m.y. with the possibility of a much greater age—3,500 to 3,800 m.y. Cataclasis and recrystallization during the early Proterozoic Penokean orogeny are recorded on a regional scale by whole-rock and mineral Rb-Sr ages of 1,750 m.y. Intense cataclasis of granodioritic gneiss in the Watersmeet dome locally produced metamorphic zircon with concordant ages of 1,755 m.y. Zircons from a tonalitic phase of the granite near Thayer are dated at 2,750 m.y. Zircons from leucogranite dikes, which are abundant in the tonalitic phase of the gneiss at Watersmeet, are slightly younger at 2,600 m.y. These intrusive rocks are approximately contemporaneous with the development of the greenstone-granite terrane of late Archean age.

Granitic rocks ranging in composition from granite to tonalite and associated metavolcanic-metasedimentary rocks compose an east-trending belt as much as 180 km wide and 300 km long in northern Wisconsin. The granitic rocks have an initial 87 Sr/ 86 Sr of 0.7025 ± 0.0005 and a Rb-Sr whole-rock isochron age of 1,885 ± 65 m.y., which is interpreted as the time of crystallization of the granitic rocks. Rb-Sr whole-rock mineral secondary isochrons for two samples give ages of 1,655 ± 55 m.y. and 1,545 ± 55 m.y.; K-Ar ages of biotite from these samples are 1,615 ± 55 m.y. and 1,598 ± 54 m.y., respectively. These mineral ages are interpreted as resulting from isotopic resetting caused by a thermal event about 1,600 m.y. ago. The granitic rocks and associated metavolcanic-metasedimentary rocks constitute lower Proterozoic greenstone-granite complexes that are remarkably similar in pattern to the Archean greenstone-granite complexes in the Superior province of the Canadian Shield.

Abstract The gold-and silver-rich base-metal sulfide ore deposits of the Central City district, Colorado, are arranged in well-defined concentric zones. A core of pyritequartz veins is surrounded by an outer zone of veins containing dominant galena and sphalerite in a gangue of quartz or, less commonly, rhombohedral carbonates, barite, or fluorite. In an intermediate area transitional veins contain pyrite, copper minerals, galena, and sphalerite. The transition from the core outward is marked also by systematic changes in gold and silver content, metal ratios, and mineral textures. Temperatures of mineralization, as determined by sphalerite and fluid-inclusion thermometry, indicate that the veins in the inner parts of the district formed at temperatures on the order of 600° C, whereas those on the margins formed in the interval 150°–300° C. A steep geothermal gradient that shifted with in a relatively narrow zone separated the two areas of relatively uniform thermal regime. Heat-dissipating processes believed to be responsible for the thermal pattern are mainly expansion and cooling of the hot high-pressure magmatic fluids followed by mixing of the fluids with cooler meteoric solutions. Throttling (also known as Joule-Thomson expansion) probably was a major mechanism for cooli ng the hot magmatic solutions. The activity of S 2 decreased wi th temperature, but at a rate such that more sulfur-rich mineral assemblages could form toward the margins of the district. Most of the sulfide ore and the gold and silver were deposited in the intermediate and peripheral zones of the district, where changes in the ore-forming solutions were most rapid and drastic.