The Petrological and Geochemical Analysis of the Grant Intrusive Breccia and Vaughn Intrusive Within Hicks Dome, Hardin County, Illinois

Hicks Dome is a crypto-volcanic feature found in the southeastern corner of Hardin County, Illinois. The dome formed around 270 Ma (late Permian), as part of the Permian Wauboukigou Igneous Province (PWIP), through a series of explosive igneous intrusions that caused uplift and clear structural deformation to the area. There was not enough magma feeding the intrusions to breach the surface, but due to the unique geochemistries of the magma(s), these intrusions formed amalgamations of alkaline, carbonatitic, and ultramafic gabbros. The overall geochemistry of these intrusions have high amounts of iron (Fe) and calcium (Ca), with relatively low amounts of silicon (Si). A previous characterization of the Grant Intrusive revealed the potential for economic concentrations of barium (Ba), titanium (Ti), and thorium (Th), as well has Heavy Rare Earth Elements(HREEs) including scandium (Sc) and yttrium (Y). Not all of the intrusions found at Hicks Dome have been studied, and new intrusions are still being discovered as the topsoil erodes away revealing the crystallized dikes that have been preserved for the past 270 million years. Of these intrusions the Grant Intrusive has been cited in several studies Bradbury and Baxter, (1992); Reynolds et. al. (1997); and Trela et. al. (2024). However the Vaughn Intrusive, has not been previously studied or cited in any publications or papers until now. The composition and mineralogy of the Vaughn Intrusive has been unknown, but it still leaves the question of whether or not this intrusion is possibly connected to the Grant Intrusive, due to its close vicinity, or were these intrusions formed from different magmatic episodes? Initial samples for the Grant Intrusive were provided by the Illinois State Geological Survey (ISGS), and additional samples for the Grant Intrusive and initial samples for the Vaughn Intrusive were collected during a field expedition on October 19th 2024. X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM-EDS), and an optical petrographic analysis were all utilized to analyze potential ore and gangue minerals. X-Ray Fluorescence (XRF) was used to determine bulk geochemistry and make inferences on element partitioning and economic viability, Electron Dispersive Spectroscopy (EDS) was used in conjunction to confirm the presence of critical elements such as titanium and niobium. A magnetic anomaly model of the intrusion was also constructed using a dual sensor cesium vapor magnetometer, and Surfer Pro Modeling Software. By mapping magnetic anomaly localities, igneous bodies within the subsurface have been located and inferred. Results show extensive emplacement of ultramafic material throughout the Hicks Dome region, and strong potential for economic HREE ores in breccias proximal to Grant Intrusive. Geochemical and mineralogical data also show many differences between the Grant Intrusive and Vaughn Intrusive magmas, indicating that multiple magma sources, or episodes of magmatic differentiation have taken place.