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.

A substantial section of Precambrian rock is exposed over an area of approximately 19,400 km 2 (7,500 mi 2 ) in the western part of the Northern Peninsula of Michigan. This province is a portion of the exposed southern terminus of the Canadian Precambrian Shield and contains a large variety of igneous, sedimentary, and metamorphic rocks. Significant amounts of iron and copper from Precambrian rocks of Michigan have provided important contributions to the growth of the state and national economy for nearly 150 years. Archean rocks consist of volcanics, sediments, and younger felsic and mafic intrusives, some of batholitic dimensions. Volcanic and associated sedimentary rocks occur as greenstone belts included in the Ramsay Formation, Gogebic County; Dickinson Group, Dickinson and Iron Counties; and Marquette Greenstone Belt, Marquette County. Volcanic rocks consist of mafic to felsic lava flows and pyroclastics and sediments derived from volcanic rocks. Volcanic flows include amygdaloidal and ellipsoidal varieties. Pyroclastics consist of agglomerate, conglomerate, breccia, and tuff. Sediments are described as graywacke, argillite, siltstone, conglomerate, quartzite, iron formation, and chert. Granite and granitic gneiss, principally tonalite and granodiorite, intrude the periphery and interiors of the greenstone belts. Mafic intrusives, including peridotite, are subordinate. Shearing is prominent in some areas, and metamorphic grade ranges from lower-greenschist to upper-amphibolite facies. Minor amounts of gold and silver have been produced from the Marquette Greenstone Belt. Early Proterozoic strata are subdivided into four groups, in ascending order: the Chocolay, Menominee, Baraga, and Paint River Groups. Copper mineralization occurs in the Kona Dolomite of the Chocolay Group. The Menominee Group contains three major iron formations of equivalent age, the Negaunee Iron Formation of the Marquette Iron Range; the Vulcan Iron Formation of the Menominee Iron and Felch Mountain Districts and the Ironwood Iron Formation of the Gogebic Iron Range. In the Baraga Group, the Goodrich Quartzite contains concentrations of monazite, and the Michigamme Formation has vast amounts of graphitic carbon. The Paint River Group includes the highly productive Riverton Iron Formation. Iron was discovered in 1844 on the Marquette Iron Range, and an early pig-iron industry flourished. The east-west–trending Marquette syncline, containing the Negaunee Iron Formation, is more than 65 km (40 mi) long. The Negaunee has a maximum thickness of 1,060 m (3,500 ft), and iron-formation resources have been estimated at 205 billion long tons. There are four iron formations on this range, three of which have been productive. However, 97 percent of the 588 million tons mined came from the Negaunee. The east-west–trending Menominee Iron-bearing District, in southern Dickinson County, consists of a north and south range segmented by longitudinal faulting. The Vulcan Iron Formation is exposed over a strike length of 28 km (16 mi) and has a maximum thickness of 180 m (600 ft). Production amounted to nearly 82 million long tons. In the Felch Mountain District of central Dickinson County, only eroded remnants of the Vulcan Iron Formation remain. Production of 36 million tons was principally from the Groveland low-grade iron mine. The Gogebic Iron Range, in Gogebic County, is an essentially east-west–trending, northward-dipping sequence of sediments containing the Ironwood Iron Formation. In Michigan, the Ironwood has a strike length of about 40 km (25 mi) and a maximum thickness of about 490 m (1,600 ft). Iron ore production totals 255 million long tons. The Negaunee, Vulcan, and Ironwood iron formations are considered to be stratigraphically equivalent. The Iron River–Crystal Falls District in Iron County is primarily composed of the Paint River Group containing the Riverton Iron Formation. The Paint River Group is outlined in a triangular-shaped basin approximately 260 km 2 (100 mi 2 ) in area. The Riverton has a maximum thickness of 240 m (800 ft) and has been intensely and complexly folded. A high phosphorous and manganese content characterizes the Riverton and its naturally derived iron ores. Production amounted to 207 million long tons. Middle Proterozoic rocks in Michigan consist of a very thick sequence of volcanics and sediments. For the most part, strata dip uninterrupted toward Lake Superior at varying degrees. Native copper was the exclusive mineral produced from the Portage Lake Volcanics in Michigan’s Keweenaw Peninsula. Stratabound native copper mineralization forms ore bodies in amygdaloidal and brecciated tops of lava flows, and in interflow conglomerates. Minor amounts were produced from transverse fissures. Production of refined copper through 1976 amounted to 4,769,465 metric tons (5,257,438 short tons). Sulfide copper (chalcocite) with some native metal is mined from the Nonesuch Formation several thousand feet about the Portage Lake Volcanics in the Porcupine Mountain area. Copper mineralization is confined to siltstone and shale of the basal portion of the Nonesuch. Small amounts of disseminated native copper are produced from the uppermost sandstone of the underlying Copper Harbor Conglomerate. Through 1987, 1,364,800 metric tons (1,504,433 short tons) of refined copper has been produced.

Abstract Approximately 3.4 billion tons (Gt) of iron ores containing >50 percent Fe were produced from U.S. mines in the Lake Superior region from 1848 until they were exhausted 20 to 30 years ago. The Vermilion Range in Minnesota produced nearly 100 million tons (Mt) of this ore from Archean greenstone belt-hosted iron formation. The remaining production has come from Proterozoic strata including 2.3 Gt from the Mesabi and 100 Mt from the Cuyuna Ranges in Minnesota while Michigan and Wisconsin contributed 230 Mt from the Marquette Range, 290 Mt from the Menominee Range, and 325 Mt from the Gogebic Range. The protore of these direct-shipping ores are carbonate- or oxide-facies banded iron formations that contained 25 to 35 percent Fe prior to undergoing leaching (desilicification), oxidation, and volume loss. The conventional model ascribing these changes to supergene processes has recently been challenged by research showing that hypogene fluids, channeled by faults into structurally favorable horizons and settings, have played a dominant role in producing some of the high-grade (>60% Fe) ores that are presently providing much of the world's iron ore. Descriptions of the North American iron ores, generally starting with the U.S. Geological Survey monographs published at the beginning of the 20 th century provide many tantalizing clues, suggesting that hypogene fluids have indeed played an important role in the evolution of some of these districts. Application of modern geophysical techniques and structural and geochemical analyses may well guide the discovery of new high-grade ores either below or adjacent to the historic mining areas. The time seems to be ripe for exploration to return to the area that can claim to have begun geologists' understanding of this most important ore deposit type.