RARE EARTH ELEMENT ZONATION AND FRACTIONATION AT THE BEAR LODGE REE DEPOSIT, WYOMING

The Bear Lodge Mountains of Wyoming host one of North America's largest REE deposits. Bear Lodge carbonatites are generally LREE-enriched. However, recent exploration reveals zones of HREE- and HFSE-enrichment, much of it hosted by hydrothermal veins peripheral to the carbonatites. A district-wide zonation from LREE to HREE is reflected by mineralogy. Central carbonatites host REEs in the fluorocarbonate minerals bastnäsite, synchysite, and parisite, and the carbonate mineral ancylite. Peripheral veins host the REE+Th-phosphate minerals, monazite, xenotime, and brockite. Bear Lodge carbonatites have low concentrations of HFSE (Ti and Nb), which precipitated in peripheral veins as ilmenorutile and Nb-anatase. REE fractionation is the result of fluid evolution, which likely occurred during and shortly after the emplacement of carbonatites, resulting in the district-wide outward increase in HREE/LREE ratios.

Carbonate and fluoride complexes may have been important in fluids precipitating LREE-dominant minerals in the central carbonatites, while phosphate and fluoride complexes may have been important ligands as fluids circulated peripherally. HREEF²⁺ complexes are less strongly associated than LREEF²⁺ complexes at hydrothermal temperatures (>150°C), suggesting the fluids cooled below 150°C or that a different ligand, such as phosphate or chloride was involved. Phosphate enrichment in late-stage fluids from carbonatites may be due to dissolution of apatite in carbonatite. An external Paleozoic sedimentary source of phosphate from detrital apatite is also possible.

Complex textural relationships suggest multiple stages of REE precipitation. Parts of the central carbonatites are overprinted by HREE mineralization, often seen as yttrium replacement in parisite. LREE/HREE zonation has recently been discovered between adjacent dikes within individual targets. Electron microprobe data show a similar REE zonation to that observed district-wide (LREE to HREE). Unusual phosphate minerals are zoned from monazite to xenotime compositions.

Stable carbon and oxygen isotope data show a complex history of carbonate and fluorocarbonate crystallization in Bear Lodge carbonatites. Data suggest degassing and low-T alteration were important during carbonatite crystallization and REE-enrichment.