Hicks Dome drill cores: mineralogical and geochemical observations

Hicks Dome, a feature created by the release of volatiles from calc-alkaline magmas, is located within the Illinois-Kentucky Fluorspar District, a fluoritic Mississippi-Valley Type deposit (MVT). Surrounding the dome are numerous ultramafic igneous dikes that have gone through varying degrees of brecciation and hydrothermal mineralization. The intrusions host a variety of critical minerals, as they are enriched in rare earth elements, beryllium, and niobium, which has prompted numerous coring projects throughout the past six decades. Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), cathodoluminescence microscopy, and optical petrography, along with pre-existing geochemical analysis, are being used to characterize the mineral phases and textures of cores taken from both hydrothermally altered breccias and igneous material from a variety of depths and locations. These results are based on samples from cores drilled by Ozark Mahoning Co., Hicks Dome Corp., and Asarco Inc.: FSB-7, FSB-21, FSB-20, FSB-12W, FSB-27, and FSB-10. These methods have confirmed that the breccia material is largely made up of sandstone, limestone, and shale fragments, partially to fully replaced by carbonates and fluorite. Major phases are fluorite, calcite, dolomite, and quartz. Minor phases are pyrite, galena, barite, fluorapatite, bertrandite, brockite, monazite, xenotime, and ilmenorutile. The first three are common MVT gangue minerals. The REEs are believed to be held dominantly in the phosphates. Although the REE concentrations of fluorite are much lower, the abundance of fluorite in the breccias may represent a significant portion of the REE budget. However, portions of cores with the highest REE concentration show lower fluorite abundance and higher phosphate mineral abundance. Though apatite is the most ubiquitous of the phosphate minerals, the REE-phosphate to apatite ratio varies within the cores, sometimes exceeding one.