COMPOSITION, TEMPERATURE, AND BEHAVIOR OF MINERALIZING BRINES IN THE TRI-STATE ZN-PB DISTRICT: CONTRIBUTIONS FROM FLUID INCLUSION ANALYSIS AND REACTIVE TRANSPORT MODELING

Laser ablation ICP-MS and microthermometry analysis of fluid inclusions and numerical reactive transport modeling were used to seek insights into the geochemical, thermal, and hydrologic controls on the formation of Zn-Pb ore deposits in the Tri-State Mississippi Valley-type district of Oklahoma, Missouri, and Kansas.  Fluid inclusions were studied in sphalerite and the gangue phases, quartz and sparry pink dolomite, which at least partially overlap the period of sulfide deposition.  Last-ice melting and homogenization temperatures ranged primarily from about -12 to -26° C and 80 to 140° C, respectively, and showed a strong correlation of increasing homogenization temperature with decreasing last-ice melting temperature.  None of the fluid inclusions was found to contain anomalously high concentrations of ore metals compared to typical sedimentary brines.  Fluid inclusions in quartz and pink dolomite had zinc concentrations primarily on the order of 1's of ppm and lead concentrations on the order of tenths to 1's of ppm, while copper concentrations were all below the detection limit, typically on the order of 1's of ppm.  Barium concentrations from all three minerals ranged from about 10-60 ppm.  Numerical reactive transport modeling was also carried out to test the behavior of the fluids characterized from the fluid inclusion analysis within the context of the local hydrogeology in the Blue Goose mine in the western part of the Tri-State district.  The results showed that mineralizing fluids could have ascended the principal faults transecting the ore body provided overpressures on the order of at least 15% above hydrostatic had existed in the lower stratigraphic units.  Fluids ascending the faults would have cooled by 10-20° C over a 400-500 meter distance and begun migrating laterally upon encountering highly permeable carbonate units in the Mississippian Springfield Plateau aquifer, producing MVT mineralization that decreases in intensity with distance away from the faults.  The results of the study thus far indicate that relatively metal-poor fluids were circulating through the Tri-State district over much of the time of MVT mineralization, but that nonetheless could have produced enrichments of metals in spatial patterns similar to those observed in the field.