MULTI-GENERATIONAL NIOBIUM MINERALIZATION AT THE ELK CREEK CARBONATITE, ELK CREEK, NE

The Elk Creek Carbonatite Complex (ECCC) is a compositionally-diverse dolomitic carbonatite (beforsite) and alkali igneous complex located in Southeast Nebraska. Major sub-units include beforsite dominated by apatite, barite, or magnetite, as well as mafic dikes and syenites. Original exploration work by Molycorp showed that the ECCC contains significant niobium mineralization, typically occurring as pyrochlore, and that there were multiple generations of pyrochlore. Petrographic and electron microprobe analysis has allowed us to characterize several distinct suites (generations) of pyrochlore mineralization. The pyrochlore grains in each suite have distinctive sizes, morphologies, and compositions.

The first generation consists of large (2mm-0.1mm diameter), euhedral grains that display complex concentric zoning controlled by substitution of tantalum for niobium. Some concentric zones display skeletal texture. This generation occurs principally in apatite-dominated beforsite and appears as 3-10m thick intercepts of high-grade niobium mineralization in core separated by tens-hundreds of meters of unmineralized carbonatite rock. The second generation is more diverse in its setting and occurs as accessory minerals in most carbonate rocks in the ECCC. These grains are unzoned, and occur typically as inclusions in dolomite or along dolomite grain boundaries. These pyrochlore grains are euhedral to subhedral and fine grained, ranging from 50-10 μm in longest dimension. The chemistry of these pyrochlore grains is similar to the stoichiometric pyrochlore formula with very minor Ti and Sr. The third generation of pyrochlore is associated entirely with a magnetite/ilmenite-dominated beforsite rock and appears as a common accessory mineral in nearly all intercepts of this rock. Based on Molycorp’s resource estimates, these pyrochlore grains comprise the bulk of the niobium budget at Elk Creek. Individual pyrochlore grains within the magnetite beforesite are typically less than 10μm in longest dimension, occuring as anhedral inclusions and aggregates in ilmenite, rutile, and magnetite or as subhedral-to-anhedral disseminations in massive rock. Electron microprobe analysis indicates that these pyrochlores are Ca-rich rather than Na-rich and contain minor amounts of Fe, Sr, Ti, Th, and U.