Mode of mineralization and relationship with igneous intrusions of rare earth elements at Hicks Dome, Illinois

Rare earth elements (REEs) are an increasingly important group of metals due to their role in the development of modern technologies. Despite being abundant within the Earth's crust recoverable ores are uncommon and their mineralization isn't as well understood as other abundant ore types. In this work, the mineralization of REEs at Hicks Dome, southeastern Illinois, was studied to determine the mode of mineralization and the origin of the REEs. Hicks Dome is a Permian intrusive center of alkaline ultramafic dikes and breccias with F-Zn-Pb-Ba-REE mineralization, located within the northern section of the Illinois-Kentucky-Fluorspar District (IKFD). 29 samples of intrusive rocks, and fractured sedimentary host rock from Hicks Dome were analyzed using X-ray diffraction, optical petrography, and scanning electron microscopy. The textures observed through optical petrography were largely brecciation. A majority of the samples had angular to sub-angular clasts (1-10 mm) comprised of country rock, with various amounts of secondary quartz or calcite infilling. Multiple stages of cross cutting was observed, indicating at least two phases of mineralization. Most of the samples had been silicified or were highly siliceous. The mineral xenotime-(Y) was identified in two samples from this locality. The element thorium was found to be evenly dispersed throughout clasts and groundmass. Samples had radioactivity as high as 4.6 mu Sv/hr. Based on the multiple stages of cross cutting, there were at least two stages of mineralization that occurred during or after brecciation of the country rocks, and REEs were likely to have been mobilized during this event and therefore likely crystallized after the original intrusive event.