TRACE ELEMENT ANALYSIS OF SPHALERITE IN A MISSISSIPPI VALLEY TYPE DEPOSIT, CENTRAL TENNESSEE, USA

Zinc mineralization at the Cumberland Mine in central Tennessee, USA, is classified as Mississippi Valley Type (MVT). The deposit is situated on the flank of the Cincinnati Arch and is hosted in Lower Ordovician, undeformed carbonate platform rocks. Reddish-brown sphalerite is the dominant ore mineral with minor galena, pyrite, calcite, fluorite, and barite. Mineralization occurs as sulfide replacement of limestones and as open-space filling in collapse dolomite breccias. This study evaluates the geochemistry of sphalerite to identify trends in trace element distribution within the deposit. These data may provide important insights into temporal variations in the geochemistry of ore-bearing fluids. Samples of collapse breccia ore, limestone-hosted replacement ore, and calcite gangue were collected, coarsely crushed, and hand-picked using a binocular microscope to separate sphalerite and calcite. Samples were pulverized and analyzed using the mining and soil mode settings of a hand-held X-ray fluorescence analyzer. Analysis of standard material indicated that mining mode worked well for quantifying elements at high concentration and soil mode for elements at low concentrations. Detectable elements included Zn, Fe, As, Mo, Rb, Se, Hg, Cu, Cd, Cl, and S. Calcite samples contained minor (<10-20 ppm) concentrations of Mo, Se, Hg, and Zn along with Fe (200 ppm range), which may be associated with the presence of minor amounts of sphalerite in hand-picked samples. Sphalerite samples contained Zn up to 601,000 ppm and Fe up to 2900 ppm with elevated concentrations of As up to 70 ppm, Mo up to 44 ppm, Rb up to 61 ppm, Se up to 506 ppm, Cu up to 1210 ppm, Cd up to 3900 ppm, and S up to 308,000 ppm. Hg concentration were elevated (up to 4000 ppm), one or two orders of magnitude higher than levels reported for MVT deposits in central Tennessee, and may represent interference at the Hg wavelength analyzed or analytical error. A notable feature of the data is that Mo, Rb, Se, and Hg are more concentrated in replacement ores, and As, Cu, Cd, and Fe are higher in breccia ores. These trends suggest the possibility of two geochemically distinct mineralizing events. Further quantitative analysis is needed to more accurately characterize elemental concentrations.