Fluid-inclusion studies of regionally extensive epigenetic dolomites, Bonneterre Dolomite (Cambrian), southeast Missouri: Evidence of multiple fluids during dolomitization and lead-zinc mineralization

Abstract

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 km 2 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 km 2 study area. The data are interpreted to indicate multiple basinal-fluid interactions coeval with dolomitization and associated Pb-Zn ore formation.