RARE-EARTH ELEMENT ENRICHMENT FROM A MANTLE-DERIVED CARBONATITE AT HICKS DOME

Rare Earth Elements (REE) play a crucial role in modern technologies across industries including green energy, national defense, and electric automotive systems. Here, we show new petrological, geochemical, and P-T modeling results (collected as part Phase II of the United States Geological Survey’s Earth Mapping Resources Initiative program) from igneous samples to clarify the origin of carbonatite-lamprophyre magmatism, fluorspar, and REE enrichment in southern Illinois. Concentrations of total Light REE (LREE) for dominantly carbonate rocks range from ~10,000 up to ~80,000 ppm. Elevated REE concentrations in carbonate rich rocks further suggest they are of igneous origin and are in fact carbonatites. Major and trace elements show five distinct geochemical groups among igneous rocks in the district. P-T modeling using major element concentrations reveals that igneous rocks derive from a deep mantle carbonated source, that is consistent with trace element signatures for an F-rich transition zone origin. Major element systematics suggest that carbonatites and lamprophyres in the district are likely conjugate pairs formed by liquid-immiscibility driven by crystal fractionation and degassing during ascent through the crust. Trace element patterns of Hicks Dome breccias show relative elevated Heavy REE (HREE) concentrations (up to ~20,000 ppm) that is due to preferential partitioning and transport a by low temperature brine-melts, exsolved from a larger carbonatite body at depth. These brine-melts efficiently redistribute HREEs throughout the system through brecciated pathways and joints, where they reprecipitate as HREE-rich phosphate and fluorcarbonate minerals in the host bedrock. Shallow carbonatite occurrences at Hicks Dome provide evidence for a potentially larger LREE-body at depth. If a large carbonatite body does in fact exist at the depth, we suggest that Hicks Dome could also stand as a viable LREE prospect. Furthermore, numerous unexplored anomalies identified by the Earth-MRI program in Illinois and Kentucky highlight the potential of southern Illinois as a significant REE-bearing region and natural laboratory to study carbonated melts.