FLUID CHEMISTRY OF SPHALERITE-HOSTED FLUID INCLUSIONS FROM THE CENTRAL MISSOURI MISSISSIPPI VALLEY-TYPE (MVT) DISTRICT

The Central Missouri district hosts numerous small occurrences of barite-dominant MVT mineralization that are temporally related to the sulfide-dominant Southeast Missouri, Tri-State, and Northern Arkansas MVT districts in the Ozark Plateau. The presence of minor sphalerite in the Central Missouri district provides an opportunity to compare fluid inclusion compositions of sulfate- and sulfide-dominant MVT districts and to test for genetic relationships and precipitation mechanisms. Sphalerite-hosted fluid inclusions from the Brown, Ford and Tiff mines in the Central Missouri district were analyzed by microthermometry, LA-ICP-MS and Raman spectroscopy. Of 72 fluid inclusions analyzed, the average homogenization temperature was 98° C and the average salinity was 24 equivalent weight percent NaCl. The mean atomic Mg/Na, K/Na, Ca/Na, Sr/Na, and Ca/Mg ratios were 0.028, 0.024, 0.165, 0.0028, and 6.2, respectively. Barium concentrations ranged from 11-34 ppm. Fluid inclusion Pb concentrations were difficult to determine due to interferences from Pb in the host mineral matrix. However, matrix Pb interference in two fluid inclusion analyses was low enough to allow determination of aqueous Pb concentrations of 66 and 96 ppm. Raman analysis of fluid inclusion vapor bubbles showed methane to be the only detectable gas. The compositions of fluid inclusions reported here are similar to the compositions of sphalerite-hosted fluid inclusions from the Southeast Missouri, Tri-State, and Northern Arkansas districts. These similarities indicate that a chemically similar fluid, likely enriched in Pb and other metals, was involved in sphalerite mineralization throughout the Ozark Plateau. The predominance of methane over CO₂ in the fluid inclusions indicates that this fluid was reducing and thus poor in sulfur if it was also metal-rich. If the sphalerite-hosted inclusion fluid was also responsible for precipitating barite, then an oxidized sulfur source must have been present where barite deposition occurred. This implies that the local chemical environment encountered by the invading metal-bearing fluid is the controlling factor for ore deposition. It is also possible that barite and sphalerite were deposited from two completely different fluids.