RARE EARTH ELEMENT VARIATIONS IN THE ELK CREEK CARBONATITE, S.E. NEBRASKA
In our study, the chemical compositions of 25 samples of various carbonatite lithologies were determined to characterize general features of the carbonatite. Much of the carbonatite has REE patterns similar to other carbonatites (moderate LREE enrichment with a negative slope); however, zones of high LREE enrichment (La to 53,100 ppm) and zones of enrichment of the more valued middle and heavy REEs (Eu, Gd, Tb, and Dy) have been identified. The zones of relatively high LREE enrichment generally occur within narrow sections of the barite beforsite and the apatite beforsite. Petrographic and microprobe analysis of samples with the greatest LREE concentrations have identified late-stage, fine-grained, LREE-rich phosphates (monazite) and LREE-rich fluocarbonates (parasite and synchysite). The zones containing middle and heavy REE enrichments (and a relative depletion in LREEs) occur within the magnetite beforsite, which is the Nb-rich lithology of Elk Creek. Mineralogical studies are underway to characterize the REE-hosts of this recently identified, anomalous middle and heavy REE enrichment.