Keweenawan Copper Deposits of Western Upper Michigan
Nicholson. S. W., Cannon, W.F. and Schulz,'K.J., 1992. Metallogeny of the Midcontinent rift system of North America. In: G. Gaal and K. Schulz (Editors). Precambrian Metallogeny Related to Plate Tectonics. Precambrian Res., 58: 000-000.
The 1.1 Ga Mtidcontinent rift system of North America is one of the world's major continental rifts and hosts a variety of mineral deposits. The rocks and mineral deposits of this 2000 km long rift are exposed only in the Lake Superior region. In the Lake Superior region, the rift cuts across Precambrian basement terranes ranging in age from ~1850 Ma to more than 3500 Ma. Where exposed, the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks. Beneath the center of Lake Superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection proflles.
This region hosts two major classes of mineral deposits, magmatic and hydrothermal. All important mineral production in this region has come from hydrothermal deposits. Rift-related hydrothermal deposits include four main types: (1) native copper deposits in basalts and interflow sediments: (2) sediment-hosted copper sulfide and native copper: (3) copper sulfide veins and lode.s hosted by rift-related volcanic and sedimentary rocks: and (4) polymetallic (five-element) veins in the surrounding Archean country rocks. The scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals. Where hydrothermal sulfides occur (i.e., shale-hosted copper sulfides), the source of sulfur was local sedimentary rocks.
Mtagmatic deposits have locally supported exploration and minor production, but most are subeconomic presently. These deposits occur in intrusions exposed near the margins of the rift and include Cu-Ni-PGE and Ti-Fe (V) in the Duluth Complex. U-REE-Nb in small carbonatites, and breccia pipes resulting from local hydrothermal activity around small felsic intrusions. Mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization. Most of the sulfide deposits in intrusions, however, contain sulfur derived from country rocks: the interaction between magma and country rocks was important in generation of the magmatic Cu-Ni sulfide deposits.
A mantle plume origin has been proposed for the formation of the Midcontinent rift. Mtore than 1 million km3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift. providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines.
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