RARE EARTH ELEMENT VARIATIONS IN THE ELK CREEK CARBONATITE, S.E. NEBRASKA

Carbonatites are the world’s primary source of light rare earth elements (LREEs), but of the over 500 known carbonatite occurrences, only 5 are currently in production. Chondrite-normalized REE patterns for most carbonatites display LREE enrichment (La~800 ppm), no negative Eu anomaly, and a steep negative slope. Understanding processes that control ore-grade REE enrichment in carbonatites is a key to identifying viable exploration targets. This study focuses on the buried Elk Creek carbonatite in southeastern Nebraska, a multi-lithologic carbonatite highly enriched in Nb. Although most of the Elk Creek carbonatite does not contain ore-grade concentrations of REEs, zones of REE enrichment have been identified. Our study utilizes drill core from Molycorp’s 1970s and 1980s exploration program; this core is now housed at the University of Nebraska. Molycorp geologists divided the carbonatite into lithologies based on mineralogy, textural characteristics, and crosscutting relationships. The carbonatite lithologies described in the core included apatite beforsites, barite beforsites, magnetite beforsite, and breccias.

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.