Epigenetic Dolomitization and Mississippi Valley-Type Mineralization in Cambro-Ordovician Carbonates of North America

Jay M. Gregg¹ and Kevin L. Shelton^2
1Boone Pickens School of Geology, Oklahoma State University, Stillwater, OK
²Department of Geological Sciences, University of Missouri-Columbia, Columbia, MO

Middle Cambrian through Lower Ordovician carbonate rocks of North America host some of the largest Mississippi Valley-type (MVT) base-metal sulfide deposits in the world. These include the Southeast Missouri Pb-Zn-Cu district and the East Tennessee Zn-Pb district, which place the United States as one of the world’s leading producers of lead and zinc. Other economic and sub-economic MVT deposits as well as minor occurrences of mineralization are found in carbonates of Cambro-Ordovician age throughout the continent.

In addition to sulfide and sulfate minerals, MVT deposits are associated with large-scale dissolution and brecciation of carbonate rocks, precipitation of large volumes of dolomite and calcite cements, epigenetic (hydrothermal) dolomitization, and recrystallization of pre-existing dolomite. Mineralizing fluids have the effect of both increasing original porosity via dissolution and brecciation as well as occluding porosity through precipitation of cements.

MVT mineralization is believed to result from saline fluids expelled from sedimentary basins, commonly associated with petroleum migration. The fluids have temperatures ranging from 60° - 250°C with most temperatures falling between 100° and 150°C. Some of the fluids originate from evaporated seawater that has interacted with sedimentary rocks and possibly, basement rocks. A number of geochemical and hydrogeological mechanisms have been proposed for MVT deposits. However, the precise mechanisms driving fluid flow and deposition are not yet completely understood. MVT fluids are not localized, but affect sedimentary rocks over large regions. It is likely that most if not all Cambro-Ordovician carbonate rocks in North America have undergone at least some diagenetic alteration due to exposure to these fluids, affecting the reservoir properties of carbonate rocks even at great distances from known ore deposits.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas