Mississippi Valley-Type Ore Deposits in the Cambrian-Ordovician Great American Carbonate Bank

Gregg, Jay M.; Shelton, Kevin L.

Middle Cambrian through Lower Ordovician carbonate rocks of the Great American Carbonate Bank (GACB) contain some of the largest carbonate-hosted Mississippi Valley-type (MVT) base-metal sulfide deposits in the world. These include the southeast Missouri Pb-Zn districts, the large Zn districts of the Appalachian Basin region, and several important deposits in the Cordilleran region. MVT fluids were not localized, but affected sedimentary rocks over large regions. It is likely that many of the Cambrian-Ordovician carbonate rocks in North America have undergone diagenetic alteration due to exposure to these fluids. This is indicated by numerous minor and trace occurrences of sulfide minerals throughout the GACB including regions distal to known ore deposits.

Carbonate-hosted MVT deposits are characterized by precipitation of pyrite, sphalerite, and galena as well as other sulfide and sulfate minerals. They always are associated with alteration of host carbonate lithologies, which may include: epigenetic (hydrothermal) dolomitization; precipitation of large volumes of dolomite and calcite cements; and large-scale brecciation. Mineralizing fluids have the effect of both increasing original carbonate porosity by dissolution and brecciation and of occluding porosity by textural change and precipitation of cements. MVT deposits commonly contain bitumen, pyrobitumen, and/or liquid petroleum, indicating that MVT mineralization is associated with the generation and migration of hydrocarbons. Many "hydrothermal" dolomite petroleum reservoirs in the GACB and other strata share the characteristics of MVT ore deposits.

We believe that MVT mineralization results from a complex mixing and/or cooling of saline fluids expelled from sedimentary basins. Most of the fluids originate from evaporated seawater and/or water that dissolved halite and have circulated deeply in sedimentary basins. These fluids have temperatures ranging from 60° to 250°C.

Major tectonic events associated with MVT mineralization in GACB strata include the Acadian orogeny (Late Devonian-Early Mississippian) for early mineralization in the Appalachian Mountain region, the Alleghenian-Ouachita orogeny (Pennsylvanian-Permian) for mineralization in the Appalachian and Midcontinent regions, and the Laramide orogeny (Late Cretaceous-Paleogene) for the Cordilleran region.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013