GEOCHEMISTRY OF FLUID INCLUSIONS FROM THE TRI-STATE ZN-PB DISTRICT

Mississippi Valley-type ore deposits in the Tri-State district have long been known to have formed from sedimentary brines on the basis of their major element compositions and bulk salinities. A key question remaining however is whether the mineralizing brines were anomalously rich in metals. To address this question, 35 fluid inclusions from quartz, pink sparry dolomite, and sphalerite were analyzed using laser ablation ICP-MS and microthermometry. Detection limits varied with respect to the inclusion size and salinity. About half of the inclusions in quartz and pink dolomite had detectable quantities of Zn and Pb on the order of 1's of ppm Zn and tenths to 1's of ppm Pb. Copper was detected in only one of the inclusions in quartz and pink dolomite and had a concentration of 6 ppm. In the remaining fluid inclusions in those minerals, the concentration of copper was below the detection limit, which was typically on the order of 1's of ppm. Barium was detected in nearly all of the fluid inclusions in quartz, pink dolomite, and sphalerite. Concentrations ranged from about 10-60 ppm. These concentrations of Zn, Pb, Cu, and Ba are low and not atypical of sedimentary brines, indicating that during the time of gangue mineral deposition, anomalously metal-rich fluids were not circulating through the Tri-State district.

Major element concentrations were also obtained from the ICP-MS analyses and compared with previously published data. The results show that fluids responsible for depositing quartz and pink dolomite were depleted in K relative to the fluids that deposited sphalerite. Also, fluids that deposited quartz had significantly higher Ca/Mg ratios than fluids that deposited sphalerite. These results leave open the possibility that a different, more metal-rich fluid may have deposited the zinc and lead sulfides compared to the fluid that deposited the gangue.