Formation waters from Cambrian-age strata, Illinois Basin, USA: Constraints on their origin and evolution

The most plausible interpretation of the halide and halide/cation ratio data is that the brines of the Cambrian-age strata were introduced to the Illinois Basin from outside of the basin, perhaps when the Illinois Basin was connected to the Arkoma (Oklahoma and Arkansas) and Black Warrior Basins (Alabama and Mississippi) via the Reelfoot Rift during Cambrian and early Ordovician time. In addition, the presence of some percentage of high NaCl, low Cl/Br brines from the crystalline basement is suggested given the geochemical relationships of the halide and cation/halide ratios and the tectonic history of the Illinois Basin. Finally, halide and cation/halide ratios determined by this investigation, and regional geochemical evidence and hydrogeologic modeling (by others) suggest that the brines of these strata probably were affected by regional hydrothermal activity during Permian time that was responsible for the MVT ore deposits of the Midwestern U.S. Thus, the brines of the deepest strata of the Illinois Basin constitute a different, more complex type of fluid than those found elsewhere in the basin. Halide and halide-cation ratios suggest that these deep brines are dominated by residual evaporitic brine (possibly originating as ore-forming brines) with dilution by seawater and dilute groundwater. Other components may include subaerially evaporated seawater and crystalline basement brines.