VARIATION IN SPHALERITES FROM ELMWOOD AND GORDONSVILLE MINES, TN

Mississippi Valley-Type (MVT) ore deposits are found throughout the world but the most extensive deposits are found in North America. These ores are commonly stratabound, epigenetic, and some cases are locally stratiform. These deposits typically occur in platform carbonate sequences along foreland thrust belts or sedimentary basins. MVT mineralization is commonly controlled by faults, factures, lithological transitions, and collapse breccias. The MVT deposits studied in this project are hosted in Ordovician Knox Dolostone and are located along the tectonic crest of the brecciated Cincinnati Arch dividing the Illinois Basin to the northwest and the Appalachian Basin to the east. Two competing theories concerning the origin of the heavy metal fluids state that the source of the heavy metals is oil brines, but disagree as to the origin of the brines. Anderson’s theory states that the Appalachian Basin is responsible for the brines while Bonnet’s theory states that the brines originated from the Illinois Basin.

MVT deposit sphalerites were collected from the Elmwood and Gordonsville mines in Smith County, Tennessee. Color variations in sphalerite and the origin of the two fluids are the subjects of this research. Doubly-polished thick sections were made of samples containing coarsely crystalline sphalerite of different colors to characterize compositional differences to account for the color variations and zoning using reflected light microscopy, SEM-EDS, Raman Microscopy, and powder XRD. Sphalerites were split into specific colors (black, brown, reddish, yellow, etc.) and representative crystals were analyzed using single-crystal XRD. Splits of these were then powdered and analyzed using powder XRD and ICP-MS. Color in black, brown, and yellow sphalerites is largely dependent on the total iron content, while more exotic colors (reds, oranges, etc.) are associated with Cu and Ag. Fluid inclusions in the polished thin sections were mapped and identified as primary, secondary, and pseudo-secondary. Temperatures of crystallization were established using a Linkam fluid inclusion stage. Matrix compositions of the sphalerites are being analyzed and will be used to correlate with the other instruments.