Origin of ‘exotic’ minerals in Mississippi salt dome cap rocks: results of reaction-path modeling

Shallow salt domes in central Mississippi pierce Mesozoic formations hosting regionally extensive metal-rich brines. This association, coupled with the multiple fluid-migration pathways around salt domes, suggests that these brines were the source of Fe and base-metal sulfide, Sr, and Ba minerals in salt dome cap rocks. Fluid inclusions in barite and celestite indicate that the brines had a relatively constant average composition of approximately 200,000 mg/I total dissolved solids, which is similar to present-day oil field brines from central Mississippi. Geochemical reaction-path modeling indicates that the suite of observed cap rock minerals can be produced by alternating periods of biogenic SO4 reduction with inorganic anhydrite dissolution (or by mixing with seawater). Sulfide-rich portions of cap rock at Hazlehurst salt dome have molar Fe:Pb and Fe:Zn ratios similar to present-day oil field brines, indicating that mixing of brines with abundant amounts of H2S produced by biogenic SO4 reduction led to rapid precipitation of virtually all dissolved metals as relatively insoluble sulfide minerals.