Rare Earth Mineral Concentrations in Ultramafic Alkaline Rocks and Fluorite Within the Illinois-Kentucky Fluorite District: Hicks Dome Cryptoexplosive Complex, Southeast Illinois and Northwest Kentucky (USA)

The rare earth elements (REE) are composed of the Lanthanide Series of 15 elements with atomic numbers (57) through (71) and yttrium (39). Because of their geochemical behavior, REE rarely form their own minerals and more commonly are found incorporated within the structure of other minerals. There is growing concern over the economical availability of REE because of their diverse and expanding array of technological applications and limited supply. This paper presents the results of a mineralogical and geochemical study of fluorite ore, ultramafic rock, and igneous breccia from the IllinoisKentucky Fluorite District (United States). Igneous rocks and fluorite ore were analyzed through whole-rock trace element geochemistry and electron microscopy to determine the relative abundance of REE. Geochemical analysis (inductively coupled plasma atomic emission spectroscopy and inductively coupled plasma mass spectrometry) from outcrop samples at the Sparks Hill Diatreme (Hardin County, Illinois) detected elevated concentrations of cerium group or light REE: lanthanum (La, 293 ppm), cerium (Ce, 467 ppm), praseodymium (Pr, 45.5 ppm), and neodymium (Nd, 143 ppm). Results of electron microprobe analyses indicate that the source of REE anomalies in the Sparks Hill Diatreme is an REE fluorocarbonate mineral. Electron microscopy detected several tabular fluorocarbonate grains that are less than 20 μm in size and associated with an unidentified aluminum-strontium (Al-Sr) phosphate. The fluorocarbonate was identified as synchysite [Ca(Ce,La,Nd,Y)(CO 3 ) 2 F] and was found in both the well-rounded clasts and the matrix within the diatreme/breccia. Synchysite is similar to other rare earth fluorocarbonates, such as parisite [Ca(Ce,La,Nd)(CO 3 ) 2 F], bastnasite [(Ce,La,Y)CO 3 F], and rontgenite [Ca(Ce,La)(CO 3 ) 2 F]. These fluorocarbonate minerals can occur together as intergrowths and thus are difficult to distinguish. The identification of a rare earth fluorocarbonate mineral within the igneous diatreme/breccia at Sparks Hill can have significant economic implications relating to the concentration of REE within the Illinois-Kentucky Fluorite District. Future studies will focus on the fluorite, igneous breccia, and rare earth fluorocarbonate through additional geochemical and electron microprobe analyses. RELEVANCE OF THE STUDY In 2010, as part of an Illinois statewide geologic mapping program (STATEMAP), a geochemical analysis of an igneous breccia detected 293 ppm of lanthanum and 467 ppm of cerium (Denny et al. 2010). The rare earth element (REE) values within this sample were elevated, and an effort was undertaken to identify the source of the REE. The igneous breccia “Sparks Hill Diatreme” is located within the Illinois-Kentucky Fluorite District (IKFD) approximately 5 mi (8 km) northwest of Hicks Dome. Although the IKFD has been examined and studied for many years, few geologic investigations have been focused on determining the concentrations of REE within the ultramafic igneous rocks, igneous breccia, and fluorite bodies of this district. This paper presents geochemical and electron microprobe results of REE and other trace elements from fluorite ore and igneous rocks associated with Hicks Dome in southeastern Illinois. These data are preliminary, but because of the apparent elevated REE values and the present interest in these elements, the preliminary results are presented to focus future research and mineral exploration efforts. The authors are continuing to investigate these rocks and ideally will provide additional quantitative analyses. This article was previously presented at the 47th Forum on the Geology of Industrial Minerals, held May 15–17, 2011, in Champaign, Illinois. Page numbers shown are not final. *Corresponding author: [email protected].