REE ENRICHMENT IN THE LOWER WEATHERING PROFILE OF THE BULL HILL CARBONATITE AT BEAR LODGE, WYOMING

Rare earth element (REE)-bearing carbonate, fluorocarbonate, phosphate, and oxide minerals occur within near vertical carbonatite dikes on the western margin of the Paleogene Bull Hill diatreme within the Bear Lodge, WY alkaline complex. The Σ REE concentrations are generally higher in the weathered portion of the dikes compared to the unweathered dikes at depth. The weathering profile displays a mineralogically zoned array of REE-bearing phases, correlating with decreased supergene alteration with depth. We present mineralogical, geochemical, and textural evidence of small-scale REE enrichment within approximately 400 meters of drill core from the lower portion of the weathering profile. The carbonatite contains magmatic burbankite, which is present as inclusions within manganoan calcite. The major REE-bearing minerals include ancylite, bastnäsite with synchysite/parisite, and an unidentified Sr-Ca-REE-phosphate, which pseudomorhpically replace unidentified hexagonal phenocrysts. The replacive REE-minerals are largely stable in the lower portion of the weathering profile, where additional colloform Sr-Ca-REE-phosphate and cerianite occur as supergene phases. Weathering of the carbonatite dikes caused oxidation of pyrite to Fe-oxides and Fe-hydroxides, dissolution of calcite and strontianite, and replacement of Mn-calcite with Mn-oxides. These mineralogical changes resulted in an increased porosity. Scanning electron microscope-based automated phase analyses on selected samples show a 40% porosity gain with weathering. The volumetric concentration of resistant minerals from the removal of gangue carbonate from the carbonatite resulted in relative REE enrichment, with concentrations ranging from an average of 5.4 wt. % Σ REE in the least weathered carbonatite to an average of 12.6 wt. % Σ REE in the moderately weathered carbonatite. Chondrite-normalized REE patterns demonstrate overall REE enrichment in the moderately weathered carbonatite with no major differentiation or fractionation compared to the least weathered carbonatite. Isocon plots confirm the increased concentration of REE in the weathered carbonatite and demonstrate that REE, along with the oxides and elements (TiO₂, Ta, Nb, Zr, and Hf) of resistant minerals, are conserved in the lower weathering zone.