GEOCHEMICAL AND PETROGRAPHIC ANALYSIS OF THE SPARKS HILL DIATREME, HARDIN COUNTY ILLINOIS: EVIDENCE FOR A BURIED CARBONATITE COMPLEX BENEATH THE ILLINOIS-KENTUCKY FLUORSPAR DISTRICT

Geochemical analyses, energy dispersive X-ray fluorescence spectrometry, optical microscopy, scanning electron microprobe studies, and cathodoluminescence petrography of samples from the Sparks Hill Diatreme and other Permian age igneous rocks from the Illinois-Kentucky Fluorspar District (IKFD) suggest that the Sparks Hill Diatreme is related to a carbonatite complex. Rare earth minerals identified during this study in the Sparks Hill Diatreme with electron microprobe techniques are synchysite and florencite, with monazite previously being identified nearby at the Hicks Dome. The identification of rare earth element (REE) carbonate and REE-phosphate minerals in the Sparks Hill Diatreme along with the convex pattern from REE plots of fluorite ore from the IKFD suggest that the fluorite may be originally sourced from late stages of carbonatite activity. Fluorite mineralization was observed in a vein bisecting a well-rounded dolostone clast in the Sparks Hill Diatreme. This mineralization is assumed to be a late-stage event because the clast is well-rounded with a second generation of calcite along a rim through which the mineralized vein traverses. The REE plots of fluorite from the IKFD normalized with chondrite exhibit a light rare earth element depleted or convex-up pattern, which has been observed in normalized REE plots from fluorite associated with carbonatite complexes in other regions of the world. Previous isotopic studies, homogenization temperatures, and salinities of fluid inclusions in fluorite strongly suggest that the ore within the IKFD mixed with brine fluids prior to deposition. While the ore deposits of the IKFD are similar to other Mississippi Valley-Type (MV-T) ore deposits in that primary fluid inclusions have similar homogenization temperatures and salinities, the anomalous concentration of fluorite in the IKFD ore is difficult to explain solely to be related to MV-T basinal brine migration. Therefore, it is likely that the development of a carbonatite beneath the Tolu Arch is the source of fluorine for the IKFD. We suggest that late stages of carbonatite activity provided the fluorine, which then mixed with regional brines to form the ore deposits of the IKFD. A deep seated carbonatite body in this mineralized region may have potential for future mineral exploration in the IKFD.