HYDROTHERMAL CALCITES FROM THE MISSISSIPPI VALLEY-TYPE ELMWOOD-GORDONSVILLE ZINC DEPOSITS, CENTRAL TENNESSEE, USA - FLUID INCLUSION AND STABLE ISOTOPE DATA

Mississippi Valley-type mineralization in the Elmwood-Gordonsville deposits of the Central Tennessee zinc district is hosted in a Lower Ordovician carbonate sequence. The mineralization, localized in collapse breccia bodies below a regionally extensive unconformity, is predominantly of the cavity-filling type and has the following generalized paragenetic sequence: Early-stage calcite (white); sphalerite; galena (minor); fluorite; main-stage calcite (white to lavender); barite; and late-stage calcite (clear to amber). An integrated fluid inclusion and stable (oxygen and carbon) isotopic study of the three stages of calcite, which span the entire paragenetic sequence, indicates the involvement of at least two different types of hydrothermal fluids in the mineralization process. Fluids for the early-stage and main-stage calcites were formation waters characterized by moderate to high temperatures (mostly 100-150-degrees-C), high salinities (mostly 18 to 23 wt.% NaCl equiv.), and delta O-18(H2O) values of 3 to 10 per mil. The relatively large range of salinity for the early-stage calcite (approximately 10 to 22 wt.% NaCl equiv.), without corresponding variation in temperature, probably reflects mixing of formation fluids with pore fluids at the sites of deposition. The calculated delta O-18(H2O) and delta C-13(CO2) values of main-stage calcite fluids (approximately 7 to 10 per mil and -3 per mil, respectively) are consistent with extensive interaction of formation fluids with carbonate rocks of the host sequence. Late-stage calcite crystals are zoned in terms of both fluid inclusion characteristics and isotopic composition. These crystals precipitated from fluids that evolved from moderately hot (approximately 100-degrees-C), high-salinity (approximately 18 to 22 wt.% NaCl equiv.) formation waters to fluids of lower temperature (as low as approximately 60-degrees-C) and salinity (as low as 7 to 8 wt.% NaCl equiv.) by progressive dilution with meteoric water. The concurrent decrease in delta C-13(CO2) values from -7 to -9 per mil in the "cores" to -10 to -11 per mil in the "rims" of late-calcite crystals indicates an increasing contribution of organic carbon during the fluid mixing process.