Sources of basinal fluids that precipitated the dolomite-hosted Mississippi Valley-type orebodies of southeast Missouri historically have been a subject of debate. This study presents microthermometric data for fluid inclusions in the regionally extensive epigenetic dolomite at the base of the ore-hosting Bonneterre Dolomite and in gangue dolomite of the Viburnum Trend Pb-Zn district. Samples of epigenetic dolomite cover an area of more than 25,000 km2 west of the St. Francois Mountains and permit determination of regional variations of temperature and composition of the mineralizing fluids and possible fluid interactions.
Homogenization temperature-ice-melting relationships (temperature-salinity) among these inclusions document at least two end-member fluid components: a warmer, less saline fluid (120 to 187 °C; 5 wt% equiv. NaCl) and a cooler, more saline fluid (60 to 80 °C; >30 wt% equiv. NaCl). Intermediate temperatures and fluid compositions indicate that the end-member fluids likely mixed as they flowed through the region. Mixing was not confined to a stationary front but occurred throughout the study area. Comparison of homogenization temperatures to distance from possible basinal fluid sources indicates no discernible temperature gradient over the >25,000 km2 study area. The data are interpreted to indicate multiple basinal-fluid interactions coeval with dolomitization and associated Pb-Zn ore formation.