OVERVIEW OF THE ELK CREEK CARBONATITE, SOUTHEASTERN NEBRASKA, USA

The Elk Creek carbonatite, southeastern Nebraska, not only displays rare earth element (REE) enrichment, but also is reported to be the largest Nb resource in the United States. In 1970, a geophysical anomaly associated with the carbonatite was identified, and the following year the carbonatite was discovered when drilled by the state of Nebraska. Between 1973 and 1986, Molycorp undertook an extensive exploration program at the site. In 2010, Quantum Rare Earth Developments Corp acquired exploration rights to the property through April 2015. Our study utilizes drill core from Molycorp’s exploration program that is now housed at the University of Nebraska. The carbonatite intruded Precambrian granite, granite gneisses, and amphibolite biotite gneisses. In addition, a syenitic unit occurs with the carbonatite, but the association is unclear. Molycorp geologists divided the carbonatite into lithologies based on mineralogy, textural characteristics, and crosscutting relationships. The primary rock type in the Molycorp classification was beforsite: a dolomitic carbonatite. The lithologies described in core included apatite beforsites, barite beforsites, magnetite beforsite, breccias, and mafic units. REE-bearing mineral phases include bastnäsite, parisite, synchysite, and monazite, whereas pyrochlore is the primary Nb mineral phase.

In our study, the chemical composition of 14 samples of the syenite and carbonatite were determined to evaluate general features of the carbonatite. The carbonatite samples lie in the magnesio- and ferrocarbonatite fields on a carbonatite ternary plot. Elemental variations include: Nb content ranging from 15 to 12,400 ppm, La from 160 to 18,700 ppm, Ba from 500 to 76,200 ppm, Sr from 1330 to 4400 ppm, F from 1250 to 39,100 ppm, and P₂O₅ from 0.54 to 3.65%. REE patterns display light REE enrichments (La_cn/Yb_cn= 33 to 1370) and no Eu negative anomalies, a pattern similar to other carbonatites. Samples with high Nb contents tend to have relatively low LREEs and Mn. Similar to many carbonatites, petrographic examination reveals a complex history including relic primary textures, brecciation, recrystallization, and varying redox conditions (reducing to oxidizing). Studies are underway to constrain the petrogenetic evolution.