GREGG, JAY M., Department of Geology and Geophysics, University of Missouri-Rolla, Rolla, MO

ABSTRACT: Porosity Reduction by Basinal Fluids During Mississippi Valley-Type Mineralization, Bonneterre Dolomite (Cambrian) Southeastern Missouri

Dolomites may undergo porosity reduction due to cementation during basinal brine migrations associated with Mississippi Valley-type (MVT) mineralization. The Bonneterre Dolomite (Cambrian) southeastern Missouri hosts the large MVT deposits of that region. The Bonneterre consists of dolomitized algal bioherms, grainstone banks, and associated facies that were deposited on a carbonate platform surrounding the Precambrian age St. Francois Mts.

Samples were collected at (approx.)3 m intervals from an unmineralized core that penetrated the entire Bonneterre (84 m). Samples containing sphalerite, galena, and chalcopyrite were collected from several active mines in the area. Porosity was determined by point counting thin sections. Paleoporosity was estimated by counting open space filling cement using cathodoluminescence. Cement volumes were subtracted from porosity to estimate paleoporosity during various stages of mineralization. Prior to the onset of mineralization, porosity in the Bonneterre averaged approximately 20%. Precipitation of early dolomite cement (roughly concurrent with cuboctahedral galena) reduced average porosity to 7% and closed off much of the intercrystalline pore space. Later dolomite cementation (occurri g prior to cubic galena) reduced porosity to a 5% average and quartz and calcite cementation further reduced porosity to the present value of (approx.) 4%.

Platform dolomites commonly are affected by MVT mineralization, including some important dolomite reservoirs in petroleum producing regions. Cementation during mineralizing events may significantly have altered porosity in many of these rocks.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.