PETROLOGIC AND GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF LAMPROPHYRES AND CARBONATITES IN THE MIDWEST PERMIAN ULTRAMAFIC DISTRICT AND THEIR RARE EARTH ELEMENT ECONOMIC POTENTIAL

The Midwest Permian Ultramafic District (MPUD) of Illinois and Kentucky hosts igneous rocks with elevated rare earth element (REE) concentrations. While these rocks show geochemical similarity to lamprophyres (alnöites) and carbonatites, their petrogenesis remains enigmatic. Here, we present new geochemical and petrologic data from drill core samples from the Hicks Dome REE prospect and the Suttner and Soward diatremes in Illinois. All MPUD samples show a strong (r²=0.7) negative linear correlation between CaO and SiO₂. However, several samples from the Soward diatreme trend towards high CaO (>30 wt.%) and low SiO₂ (<10 wt.%) similar to classic carbonatites. This correlation is also observed in multiple petrologic melting experiments of carbonated garnet-peridotite. Recent parameterization of these experimental data can help estimate primary CO₂ contents of carbonated melts. Using this approach, we estimate that MPUD melts contained up to 40 wt.% CO₂ for the most calcic Hicks Dome and Soward samples, comparable to carbonatites from Bear Lodge, WY and Songwe Hill, Malawi. Our results indicate Suttner diatreme primary melts contained approximately 15 wt.% CO₂. Petrologic modeling using major element abundances shows that Hicks Dome and Soward melts equilibrated at pressures and temperatures close to 2.5 GPa and 1150 °C, while Suttner melts equilibrated at 5 GPa and 1550 °C. Therefore, we suggest that MPUD melts derived from carbonated peridotite sources at variable depths in the upper mantle. Chondrite-normalized spider diagrams reveal that MPUD rocks show strong similarity to classic Aillik Bay (Labrador) carbonatites but are not as enriched in REE as Mountain Pass (California) and Bear Lodge carbonatites. Trace element patterns of Hicks Dome diatreme breccias show elevated heavy REE (HREE) concentrations with respect to light REE (LREE). We hypothesize that late-stage hydrothermal oxidizing fluids released HREE from early crystallizing phases. These fluids transported and reprecipitated the HREE as Ca-REE fluorocarbonates along brecciated pathways. Late-stage hydrothermal processes have been invoked to explain HREE/LREE enrichment Bear Lodge and Songwe Hill carbonatites. Although continued exploration and research is needed, MPUD related rocks show potential as global REE prospects.