A SULFATE CONUNDRUM: DISSOLVED SULFATES OF DEEP-SALINE BRINES AND CARBONATE ASSOCIATED SULFATES

Sulfates in deep-circulating brines and carbonate associated sulfates (CAS) within sedimentary units of Cambrian strata in the Illinois Basin record a complex groundwater history. Dissolved sulfate in the Mt. Simon Sandstone brines appears to be related to Cambrian seawater sulfate, either original seawater or sourced from evaporite deposits. Dissolved sulfate from the Ironton-Galesville Sandstone shows influences from younger groundwater. CAS and evaporites from carbonate units between the Mt. Simon and Ironton-Galesville Sandstones show distinctly different isotope values that are related to the Late Pennsylvanian-Early Permian Mississippi Valley-type (MVT) ore pulses which deposited large sulfide minerals in the Viburnum Trend/Old Lead Belt ore districts. The isotopic disparity between consecutive Cambrian sedimentary lithologies is not well understood. Greater fluid densities of the Mt. Simon brines may have prevented significant interaction with the less dense MVT fluids. The Mt. Simon brines appear to be a mixture of evaporated Cambrian seawater, recirculating deep-seated crystalline basement brines, and meteoric water.