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 Illinois-Kentucky 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 mu m in size and associated with an unidentified aluminum-strontium (Al-Sr) phosphate. The fluorocarbonate was identified as synchysite [Ca(Ce,La,Nd,Y)(CO (sub 3) ) (sub 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 (sub 3) ) (sub 2) F], bastnasite [(Ce,La,Y)CO (sub 3) F], and rontgenite [Ca(Ce,La)(CO (sub 3) ) (sub 2) F]. These fluorocarbon-ate minerals can occur together as inter-growths 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 concentrations 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.