FORMATION OF BASE METAL AND SILVER SULFIDE OCCURRENCES IN GULF COAST SALT DOME CAP ROCKS FROM FORMATION FLUIDS

Zn-Pb sulfide (±barite, celestite, strontianite) occurrences in Gulf Coast salt dome cap rocks are generally considered to be analogs to MVT ores, although the former occur within a basin as opposed to the latter that formed outside on carbonate shelves adjacent to basins. Thus, salt dome cap rock mineralization occupies a hybrid position between MVT and some types of SEDEX systems. Our studies of the genesis of mineralization in cap rocks at Hockley and Davis Hill (TX), Tatum and Hazlehurst (MS), and Winnfield (LA) salt domes indicate: 1) low temperature of formation (~100 to 40^oC); 2) that temperature range, in conjunction with extensive sulfur-isotope data, indicate biogenic sulfate reduction led to Fe-Pb-Zn-Ag sulfide precipitation at circum-neutral pH; 3) silver content of these occurrences is higher than typical MVT ores; and 4) metal-rich formation brines were the source of metals (and Ba, Sr) and also organic compounds were required to fuel biogenic sulfate reduction. Because the composition of present-day oil field brines are well characterized and similar to the interpreted cap-rock mineralizing fluids, we have used them as a starting fluid composition to numerically model various geochemical scenarios involved in the formation of these deposits. Modeling was performed using SOLVEQ/CHILLER, and results support biogenic sulfate reduction in cap rock as the most important geochemical process leading to sulfide mineral precipitation, but inorganic processes (mixing, anhydrite dissolution, and the common ion effect) lead to celestite, strontianite, and barite deposition. Silver has been detected in a few present-day oil field brines (up to 21 ppb), and occurs as aqueous chloride complexes, as do Fe, Zn, and Pb. In this cap rock setting, silver typically occurs as a trace element in sphalerite and galena and as acanthite (Ag₂S). However, silver also occurs as discrete grains of acanthite not associated with the major Zn-Pb concentrations. Modeling suggests that H₂S-rich solutions can transport higher concentrations of Ag than Cl-rich brines, where Ag is transported by aqueous complexes such as Ag(HS)₂^-. Such H₂S-rich solutions cannot also transport appreciable Zn and Pb, and perhaps other sediment-hosted Ag occurrences exist and have not been previously recognized in the Gulf Coast or elsewhere.