Date of Award
8-1-2026
Degree Name
Master of Science
Department
Geology
First Advisor
Hummer, Daniel
Abstract
The Hicks Dome structure of Hardin County, Illinois, located within the Illinois-Kentucky Fluorspar District (IKFD), is one of the most perplexing geological features in the mid-continental U.S. Composed of uplifted Devonian strata intruded by a Ca-rich magma and ultramafic dikes, the dome is recognized as part of the Wauboukigou Igneous Province (WIP) and has been shown to host an abundance of rare earth element (REE) mineralization, particularly Dy, Y, and Sc. Despite extensive prior studies on REE concentrations and the link between fluorite mineralization in the IKFD and igneous activity at Hicks Dome, the absolute timing and relationship among REE emplacement, fluid evolution, and mineralization remain poorly understood. This study aims to directly date REE-bearing mineralization at Hicks Dome using U-Pb isotopic analysis of xenotime. The U-Pb chronometer enables the determination of crystallization and alteration ages in REE-enriched systems, given the preferential partitioning of uranium into xenotime.Methods of study include powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, petrological analysis, electron microprobe analysis (EMPA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to characterize mineral phases and isotopic compositions in the samples obtained. By determining the age of REE-bearing mineralization, this research will yield isotopic data suitable for U-Pb geochronology and clarify whether enrichment is linked to Early Permian, Jurassic, or Cretaceous events while establishing any potential genetic relationships between mantle-derived carbonatite melts and Mississippi Valley Type (MVT) hydrothermal fluids. Results from this study will improve understanding of REE ore genesis and further advance domestic critical mineral exploration and resource development. The results suggest that REE mineralization at Hicks Dome is mainly hosted within phosphate-bearing minerals, particularly xenotime-(Y), and was likely associated with evolved alkaline/carbonatitic fluids that interacted with the brecciated host rocks. The MREE- and HREE-enriched patterns, REE anomalies, association with phosphorus, and Y/Ho ratios observed in REE-bearing vein material all support the interpretation that REEs were transported by hydrothermal fluids and later concentrated then remobilized within the breccia and quartz vein system. Although U-Pb dating of xenotime-(Y) did not provide a definitive age for the REE mineralization, the presence of U, Th, and Pb shows that xenotime-(Y) may still be useful for future geochronological work. Overall, the results indicate that REE mineralization at Hicks Dome was likely influenced by multiple stages of brecciation, alkaline/carbonatitic fluid activity, hydrothermal alteration, and REE remobilization associated with carbonatitic magmatism. however, additional dating is needed to determine which fluid event was responsible for the main stage of REE mineralization.
Access
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