Petrography and geochemistry of water-rock interactions in Richton Dome cap rock (southeastern Mississippi, U.S.A.)

Cap rock at Richton Dome consists of gypsum-veined anhydrite overlain by dark- and light-banded calcite. The anhydrite accumulated directly during dissolution of the salt stock without forming gypsum, indicating temperatures ≥60°C and NaCl salinity ≥4 m. Sr and ⁸⁷Sr ⁸⁶Sr were inherited directly from salt stock; the anhydrite is a simple dissolution residue. Textures and δ¹³C indicate that dark-colored calcite formed as a replacement of precursor anhydrite cap rock in the presence of methane-oxidizing bacteria. Coprecipitated light-colored calcite is more enriched in ¹³C, indicating influx of carbonate-bearing groundwater as well during calcite cap rock formation. All calcite appears to be enriched in trace metals relative to the anhydrite precursor. Sr is also enriched, but ⁸⁷Sr ⁸⁶Sr is similar to Jurassic seawater. The calcite cap rock geochemistry is most easily explained if meteoric groundwaters at the top of the salt diapir were mixed with Sr-rich metal-rich brines brought up the sides of the dome during calcite cap rock formation. Petrographic analysis suggests that gypsum veins in the anhydrite cap rock formed by simple hydration of the wall-rock anhydrite along fractures. δD and δ¹⁸O of the gypsum crystal water indicate that the incoming water was meteoric and that the water/rock ratio was very low. Modern hydrologic communication through the cap rock is suggested by dissolution enhancement of fractures in both anhydrite and calcite cap rock, particularly along the calcite-anhydrite interface.