PETROLOGICAL AND GEOCHEMICAL ANALYSIS OF THE GRANT’S INTRUSIVE BRECCIA WITHIN HICK’S DOME, HARDIN COUNTY, ILLINOIS

Hick’s Dome is a cryptovolcanic feature found in the southeastern corner of Hardin County, Illinois. The dome formed around 270 Ma (late Permian) through a series of explosive igneous intrusions that caused uplift and clear structural damage to the area. These intrusive episodes failed to breach the surface during emplacement; remaining dormant but mineralizing valuable quantities of precious ore metals. Grant’s Intrusive is a predominant lamprophyre dike that contains amalgamations of alkaline, carbonatitic, and ultramafic gabbros, due to the highly unusual geochemistry of the magma(s). The peculiar petrology of this intrusion contains viable economic concentrations of barium (Ba), titanium (Ti), and thorium (Th), as well as Heavy Rare Earth Elements (HREE) such as scandium (Sc), and yttrium (Y). The ore minerals hosting these critical elements include apatite, barite, ilmenite, and xenotime-(Y). (Badbury and Baxter, 1997).

Samples were collected and provided by the Illinois State Geological Survey (ISGS). X-Ray Diffraction (XRD) and Scanning Electron Microscope - Electron Dispersive Spectroscopy (SEM-EDS) were utilized to analyze potential ore minerals for elements of interest; X-Ray Fluorescence (XRF) was used to determine bulk geochemistry and overall ore grade. I also constructed a 3D model of the intrusion using Vulcan Software, in conjunction with geomagnetic data collected from the region. By mapping igneous bodies within the subsurface, ore zone localities have been 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’s Intrusion.