IMPLICATION OF SPHALERITE COMPOSITION AT A CENTRAL TENNESSEE MISSISSIPPI VALLEY TYPE DEPOSIT

The Central Tennessee Zinc District (CTZD) hosts Mississippi Valley Type (MVT) mineralization. The origin of the CTZD deposits remains ambiguous due to the complexities of ore fluid genesis, movement, evolution, and interaction with country rock. The CTZD is hosted within Lower Ordovician, nearly horizontal platform carbonates located along the paleo-structural highs of the Nashville Dome and Cincinnati Arch. Samples of sphalerite were collected from the Cumberland Mine in the CTZD where sphalerite ore occurs with barite, calcite, fluorite, and pyrite gangue. Sphalerite mineralization is present in two forms: open space filling in dolomite collapse breccia and replacement ore hosted in limestone. This study uses inductively coupled plasma optical emission spectrometry to investigate element compositional heterogeneity between nine breccia ore samples and six replacement ore samples. Analysis detected Zn up to 99 wt%, Fe up to 0.3 wt%, Mg up to 1,600 ppm, Mn up to 560 ppm, Mo up to 14 ppm, Cu up to 1,500 ppm, Pb up to 190 ppm, Cd up to 4,400 ppm, Co up to 22 ppm, and Ti up to 14 ppm. As, Sr, Ba, Cr, Sn, and Ag were below detection limit. Concentrations of Fe, Cd, and Co in sphalerite are similar to other MVT deposits in the U.S. Cu concentrations commonly exceed concentrations reported in MVT districts globally. Data show that sphalerite in replacement ores is depleted in Cu, Cd, and Pb, but enriched in Mg relative to the breccia-hosted ore counterpart. A weak correlation exists between Fe and commonly substituting Mg and Mn, and a strong positive correlation (R²=0.97) exists between Cd and Pb. The data indicate that the ores either precipitated from different compositional fluids or precipitated during different mineralizing events from a single evolved fluid. These results have important implications in understanding the nature of MVT mineralization and act as a reference for pathfinder elements when searching for such deposits.