GEOCHEMISTRY AND ISOTOPE CHEMISTRY OF CA-NA-CL BRINES IN SILURIAN STRATA, MICHIGAN BASIN, USA

Formation waters from Silurian-aged reefs in the northern and southern trends of lower Michigan were collected and analyzed for major, minor and isotope compositions. The results were combined with an analysis of an exceptionally concentrated (TDS 640 g/l) Silurian brine reported by Case in 1945 to determine the origin and possible-evolutionary pathways for the chemical and isotope components of the brines. The waters are extremely concentrated (TDS > 450 g/l) Ca-Na-Cl brines. Bromide values support that they originated from seawater concentrated into the MgSO4 and possibly the KCl salt facies. The brines have, however, evolved considerably from an expected seawater composition and now contain a dominant Ca-Cl composition. Dolomitization appears to have been very important in the brine evolution, but this process cannot explain all the Ca present in these brines. Four scenarios may explain the enrichment in Ca: (1) halite dissolution accompanied by the exchange of Na for Ca; (2) reactions involving aluminosilicate minerals, carbonates and halite; (3) an input of CaCl2 solutions derived from altered MgCl2 fluids released during the metamorphism of carnallite into sylvite; and (4) a pre-existing enrichment of Ca-Cl in the Early Paleozoic seawater that filled the basin. All four are possible, but the favored explanation involves the diagenesis of the Salina A-1 potash salts. The isotope composition of the waters is consistent with evaporated seawater, perhaps enriched by exchange with carbonates or by the input of hydration water from evaporite minerals. The isotopic evolution, however, is equivocal but the brine composition does not indicate they have been diluted with meteoric water. This implies the waters have remained isolated from surface-controlled hydrological systems.