Mississippi Valley-type mineralization in the Jurassic Smackover Formation, Gulf Coast Basin, USA: Controls on the origin of Zn-Pb-Ag systems in deep sedimentary basins

Abstract

The northern margin of the central Gulf Coast Basin provides an opportunity to examine Mississippi Valley-type (MVT) Zn-Pb-Ag sulfide mineralization at depths of 2.5+ km and under interpreted formative conditions. This geologically young province is a significant petroleum producer, including from the Upper Jurassic Smackover Formation. The Smackover in this area contains circa 100 °C Na-Ca-Cl brines with elevated H₂S, Br, and Li concentrations. The Smackover intercepted in the No. 1 Teague well contains Zn-Pb-Ag sulfides throughout the entire cored interval from 2549 to 2586 m with the highest concentrations in ooid grainstones in the upper 10 m. Stylolites are locally dilated and contain euhedral sulfide crystals, indicating that at least some sulfide mineralization post-dates maximum burial and compaction. Ag content of the high Zn-Pb zones generally is low (<1 to 10 ppm), but a 1.5-m acanthite-bearing interval averages 545 ppm.

Fluid inclusions (FI) were studied in late-stage calcite and sphalerite in the No. 1 Teague core. Homogenization temperatures (T_h) for aqueous fluid inclusions in sphalerite range between 93 and 113 °C, whereas T_h for FI in calcite are between 72 and 126 °C. Oil inclusions in late-stage calcite homogenize between 82° and 105 °C. Final ice melting temperatures of FI in sphalerite range from -24.9° and -26.5 °C, corresponding to salinities of 25.5 to 26.5 wt% NaCl equivalent. Final melting temperature of FI in calcite range from -26.0 to -23.2 °C for water ice and -5.8 to -7.8 °C for hydrohalite, corresponding to NaCl and CaCl₂ of concentrations ranging from 15.2 to 20.3 wt% NaCl and 5.6 to 11.7 wt% CaCl₂, respectively. The first melting temperatures for FI ranging ∼ -50 to -60 °C are comparable to the eutectic (-52 °C) for the NaCl-CaCl₂-H₂O system, and fluid salinities ∼26 wt% TDS are compatible with current Smackover formation waters. δ³⁴S_CDT of Smackover sulfides ranges from 8.9 to 16.6 ‰, and Smackover H₂S has a similar δ³⁴S_CDT range (18 ± 2 ‰) as Late Jurassic seawater sulfate, suggesting that thermochemical reduction of reservoir sulfate was the source of H₂S for sulfide precipitation. A complex history of local crustal Pb evolution is indicated by the radiogenic Pb isotope characteristics of galena. These relationships support the introduction of a metal-bearing brine into a H₂S-rich Smackover reservoir resulted in local Zn-Pb-Ag sulfide precipitation. Burial history plots for the Smackover in southern Arkansas indicate that maximum burial occurred during the early Paleogene. Smackover MVT sulfides not only occur at depths >2.5 km, but sulfide minerals within dilated stylolites indicate that mineralization also took place at these depths. Smackover Zn-Pb-Ag sulfide concentrations represent one of the youngest and deepest known examples of MVT mineralization.