PETROLOGICAL AND GEOCHEMICAL ANALYSIS OF THE GRANT’S INTRUSION FEATURE 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 explosive igneous intrusions, causing uplift and clear structural damage to the area. These intrusive episodes failed to breach the surface during emplacement; remaining dormant but mineralizing economic quantities of precious ore metals. The petrology of these intrusions are unique, containing amalgamations of alkaline, carbonatitic, and ultramafic compositions. Due to the highly unusual geochemistry of the magma, the deposit contains rare earth elements, as well as high concentrations of Ba and Th.

We collected samples of igneous material to gain a better understanding of the Grant’s Intrusion feature (a prominent lamprophyre dike) and its role in economic mineralization. Samples were analyzed by XRD and SEM to analyze potential ore minerals for elements of interest. We also constructed a 3D model of the intrusion using Vulcan Software with previous geomagnetic data collected from the region by the United States Geological Survey. By mapping igneous bodies within the subsurface, a better timeline of the overall dome formation and subsurface ore zone sites have been inferred. Results show extensive emplacement of ultramafic material throughout the Hicks Dome region, and strong potential for economic REE ores in breccias proximal to the intrusions.