VARIATIONS IN ECONOMIC REE DISTRIBUTION AT THE BEAR LODGE DEPOSIT, WY

Variability in the market values of individual rare earth oxides has generated interest in REE deposits that host favorable distributions of those specific rare earth elements deemed “critical” or with high market value. Many of the most valuable REEs (i.e., Eu, Tb, and Dy) are middle to high atomic number lanthanides (“HREEs”). Thus, any geological processes that may fractionate or concentrate these elements are of considerable interest to the exploration/mining industry. The distribution and concentration of individual REEs is an important economic factor in the evaluation of REE deposits. An ore zone containing 20% Ce+La may be less valuable than one with 0.5% Eu+Tb+Dy.

Carbonatite intrusions host REE mineralization at the Bear Lodge Deposit, WY, and are affected by late-stage fluids and supergene processes. Hydrothermal fluids redistribute REEs among several minerals, including fluorocarbonates, phosphates, and cerianite. Fractionation and transport by late-stage fluids results in lateral zonation with variable REE distributions. Bear Lodge carbonatites commonly contain 2-10 wt.% rare earth oxide (REO). Significant REO-enrichment occurs where leaching of matrix carbonates and oxidation leave residual Fe+Mn oxides in association with the REE minerals.

Bear Lodge carbonatites are LREE-enriched. However, significant variations in REE abundances and distribution depend on the speciation of major REE-bearing phases present. Comprehensive studies using SEM-EDS and EPMA show much of the HREE budget is sequestered in hydrated/hydroxlated REE phosphates at the Taylor and Cole areas in contrast to the LREE-rich carbonatites at Bull Hill. The Whitetail area is similarly enriched in HREEs; however, low phosphorous abundances require a different HREE host. Energy dispersive X-ray spectra indicate HREE residence in REE fluorocarbonates (bastnäsite and parisite).

In addition, the high mobility of Ce in the oxidizing supergene environment results in zones with positive and negative Ce anomalies. The distribution of Ce and other LREEs in all REE deposits is an important economic factor, as these are less valuable in today’s market. Determining the speciation of REE minerals and the distribution of REEs within those minerals is critical to identifying and processing the most valuable ore zones within a deposit.