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Abstract

The Middle Cambrian through Lower Ordovician carbonate rocks of North America host some of the largest economic Mississippi Valley-type (MVT) base-metal sulfide deposits in the world. These rocks also host numerous subeconomic MVT deposits, minor and trace occurrences of mineralization, and hydrocarbon fields. Mississippi Valley-type deposits commonly contain bitumen, pyrobitumen, and/or liquid petroleum, suggesting that MVT mineralization is associated with the generation and migration of hydrocarbons and thus is a normal part of basin evolution.

In addition to sulfide and sulfate mineralization, common characteristics of MVT deposits are large-scale dissolution and brecciation of carbonate rocks, precipitation of large volumes of dolomite and calcite cements, epigenetic (hydrothermal) dolomitization, and recrys-tallization of preexisting dolomite. Mineralizing fluids have the effects both of increasing the original porosity by dissolution and brecciation and of occluding porosity because of precipitation of cements.

Mississippi Valley-type fluids are not localized but affect sedimentary rocks across large regions. It is likely that most, if not all, Cambrian–Ordovician carbonate rocks in North America have undergone at least some diagenetic alteration because of exposure to these fluids. This conclusion is supported by the observation that subeconomic MVT mineralization has been observed in Cambrian and Lower Ordovician carbonates throughout much of North America. These fluids commonly have affected carbonate petroleum reservoir rocks in regions distal from known ore deposits.

Mississippi Valley-type mineralization is believed to result from a complex mixing and/or cooling of saline fluids expelled from sedimentary basins. These fluids have temperatures ranging from 60 to 250°C. Most of the fluids originate from evaporated seawater or water that has dissolved halite and that has interacted with sedimentary rocks and, possibly, basement rocks. Several geochemical and hydrogeological mechanisms have been proposed for MVT deposits. However, the precise mechanisms driving fluid flow and deposition are not yet completely understood.

Major tectonic events associated with MVT mineralization of the great American carbonate bank strata include the Acadian orogeny (Late Devonian–Early Mississippian) for early mineralization in the Appalachian Mountain region, the Alleghanian-Ouachita orogeny (Pennsylvanian–Permian) for mineralization in the Appalachian and midcontinent regions, and the Laramide orogeny (Late Cretaceous–early Tertiary) for the Cordilleran region.

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