MINERALOGICAL CHARACTERIZATION OF HICKS DOME INTRUSIVE CORE SAMPLES AND RELATED FLUORITE MINERALIZATION, SOUTHERN ILLINOIS

The Illinois-Kentucky Fluorspar District (IKFD) represents a fluoritic subclass of Mississippi Valley-Type deposits (MVT), which are low-temperature lead-zinc ore deposits formed by hydrothermal fluids. Typically MVTs lack contribution from and do not occur near igneous activity, as they are epigenetic in sedimentary basins. However, the northwestern portion of the IKFD hosts Hicks Dome, a cryptoexplosive feature created by the release of volatiles dissolved in the carbonatite intrusion that it is situated over. Surrounding the dome are numerous mafic and ultramafic igneous intrusions, most of which have undergone at least one episode of brecciation—the breccias are classified on the basis of geometry and mineralogy by Bradbury and Baxter in to three types: shatter, vent, and carbonatitic. The area is set apart from the larger IKFD due to its radioactivity, which prompted searches for rare earth minerals and eventually the coring of several intrusions. Thin sections from cores of the breccias have been obtained and studied by powder X-ray diffraction (PXRD), cathodoluminescence (CL) microscopy, optical petrography, and semi-quantitative Rietveld analysis. These techniques have shown there to be a variety of different fragments and unique mineralization in the cement of the breccias, with quartz, calcite, fluorite, and dolomite making up the dominant mineral phases. Fluorapatite, barite, bertrandite, brockite, xenotime, pyrochlore, and ilmenorutile have also been identified as accessory phases.

The major phases of these drill cores are representative of the greater IKFD, which genetically links the breccias to this MVT deposit. This also calls into question the origin of the fluorine and REEs that are present in the cement, as it is unlikely that the depositing hydrothermal fluids would have leached them from the crust. Instead, it is possible that they are of igneous origin, with the carbonatite intrusion beneath the dome supplying the fluorine, REEs, and a source of heat for the hydrothermal fluids that ultimately mineralized the brecciated intrusions. A mineralogical characterization of these breccias focusing on the fluorite, REE phosphates, and fluorcarbonates will ultimately provide the setting for a larger investigation on asthenospheric contributions of fluorine in MVTs.