Geochemistry of Mississippi Valley-type mineralizing fluids of the Ozark Plateau: A regional synthesis

Abstract

The compositions of fluid inclusions hosted in ore and gangue minerals from Mississippi Valley--type (MVT) Pb-Zn-Ba deposits of the Ozark Plateau region were measured to develop a regional hydro-geochemical conceptual model for ore emplacement. This model may explain the diverse compositions of fluids involved in mineral precipitation, the ore precipitation mechanism, and the temporal change in composition of fluids invading the ore districts. The conceptual model additionally provides evidence for what factors may have controlled deposit size, stratigraphic location, and Zn/Pb ratio. High Pb concentrations up to 1,0009s of ppm were identified in sphalerite-hosted fluid inclusions from all of the region9s districts. If these high Pb concentrations were transported in the same fluid with sulfide, then total sulfur concentration in the fluid must have been low. Mass balance calculations demonstrate that the Arkoma Basin, a presumed source basin for the mineralizing fluids, is too small to have contained enough fluid to precipitate the observed masses of sulfur in the larger MVT districts, given the low sulfide concentrations that could coexist in the fluid with such high concentrations of Pb. High methane concentrations in sphalerite-hosted fluid inclusions from all of the region9s districts, in dolomite-hosted fluid inclusions from the Tri-State and Northern Arkansas districts, and quartz-hosted fluid inclusions from the Northern Arkansas district suggest that the prevailing redox conditions during MVT mineralization were reducing, making it unlikely that sulfate was transported in large concentrations in the fluids. During sphalerite precipitation, assuming saturation with respect to carbon dioxide, measured methane concentrations in sphalerite-hosted fluid inclusions would have required oxygen fugacity to have been at least two log units below the sulfate predominance field boundary. Fluid inclusion methane concentrations can also be used to estimate minimum burial depths of mineralization of about 0.08 to 1.2 km. Available evidence indicates that sulfide mineral precipitation in the Ozark Plateau MVT districts most likely occurred primarily as a result of the introduction of sulfide into a Pb- and Zn-rich ore fluid. In the two larger MVT districts, sulfide may have been supplied by local organic- and sulfur-rich carbonate facies. An apparent mixing line between high Ca/Na ratio and low Ca/Na ratio fluids hosted predominantly in main stage sulfide minerals and paragenetically late minerals, respectively, indicates the ore fluid was relatively Ca enriched. The lack of continuity in high Pb concentrations in fluid inclusions in sulfide and nonsulfide minerals from across the mineral parageneses suggests that the ore fluids entered the districts intermittently and/or had variable metal contents over time.