A petrographic and geochemical model for the origin of calcite cap rock at Damon Mound salt dome, Texas, U.S.A

Calcite cap rock at Damon Mound, Texas, consists of five calcite generations recognizable through standard petrography and cathodoluminescence. Stage-IA calcite formed by dissolution of anhydrite followed by calcite precipitation and was partly replaced by stage-IB calcite. Stage II formed above stage-IA and -IB substrates either filling open voids or cementing breccia fragments. Stage-IIIA and -IIIB calcites fill late fractures that crosscut all previously formed generations. Microprobe analysis indicates that each calcite generation is geochemically distinct. Sr/Mg ratios indicate that calcite precipitated from variable proportions of formation water, meteoric water, and/or seawater. Wide variation in calcite fluid-inclusion compositions (1.0–9.4 eq.wt.% NaCl) indicates mixing of high-salinity water and low-salinity meteoric water. The single-phase nature of most fluid inclusions and the homogenization temperatures indicate that calcite precipitated at temperatures of