MAGMATIC RARE EARTH ELEMENT OCCURRENCES IN THE ILLINOIS-KENTUCKY FLUORSPAR DISTRICT

The Illinois-Kentucky Fluorspar District is a unique location in the U.S. with proven potential for rare earth elements (REEs) and other critical mineral resources, including fluorine (fluorspar) and barium (barite). Current research funded by the Earth Mapping Resources Initiative (Earth MRI) program of the U.S. Geological Survey focuses on providing a better understanding of the occurrence and distribution of REE-bearing minerals in the fluorspar district in western Kentucky, especially regarding the REE content of igneous dikes. The igneous rocks in the district show varying degrees of alteration of primary olivine + pyroxene, including extensive carbonation that produced calcite and magnesite pseudomorphs and late cross-cutting veins, resulting in visually distinct rock types. Whole-rock geochemical analysis on dike samples collected in the district indicate that the total REE content (including scandium and yttrium) ranges from 37 ppm up to 605 ppm, with an average of 337 ppm (n=34). All of the analyzed dark gray dikes contain higher total REE (343–605 ppm, avg.=426, n=12 ), whereas the other color variants associated with varying degrees of alteration have somewhat lower total REE content, with wide and overlapping ranges: light greenish-gray dikes: 149–575 ppm (avg.=341 ppm, n=6), light gray dikes: 183–488 ppm (avg.=293 ppm, n=12), and brown dikes: 37–389 ppm (avg.=293 ppm, n=5). Preliminary interpretation of the geochemical data did not reveal obvious spatial trends in the distribution of the REEs with regard to distance from assumed igneous centers, such as Hicks Dome in southern Illinois and the Coefield Anomaly in western Kentucky. One of four analyzed brecciated calcite veins in the vicinity of the igneous dikes yielded 280 ppm total REE whereas the other three samples’ total REE ranged from 11–54 ppm. Cross-cutting relationships revealed by high-resolution aeromagnetic mapping indicate that the pervasive NE–SW-trending faults in western Kentucky postdate the emplacement of NW–SE-striking igneous dikes. The occurrence of the region’s fluorspar deposits along these faults implies the interaction of later hydrothermal (basin brine) mineralizing fluids with the REE-bearing dikes, possibly remobilizing REEs and allowing further migration of REE-bearing fluids between fluorspar deposits.