REE Enrichment in Weathered Carbonatite, Bull Hill: Bear Lodge Mountains, 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 alkaline complex. The weathering profile displays a mineralogically zoned array of REE-bearing phases. Magmatic burbankite is present as inclusions within manganoan calcite. More abundant REE-bearing minerals include ancylite, bastnasite with synchysite/parisite, and an unidentified Sr-Ca-REE-phosphate pseudomorhpically replace unidentified hexagonal phenocrysts. These replacive minerals are largely stable in the weakly to moderately weathered carbonatite in the lower portion of the weathering profile. In moderately weathered carbonatite, 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 QEMSCANRTM analyses on selected samples from the lower weathering zone yielded an approximate gain of 40% porosity. The volumetric concentration of resistant minerals from the removal of gangue carbonate, mainly calcite, from the carbonatite resulted in relative REE enrichment. Rare earth element concentrations range from an average of 5.4 wt. % total REE (6.3 wt. % total rare earth oxide) in the least weathered carbonatite to an average of 12.6 wt. % total REE (14.8 wt. % total rare earth oxide) in the moderately weathered carbonatite. Chondrite-normalized REE patterns of the weathered carbonatite in the lower weathering zone show no major differentiation or fractionation compared to the REE patterns of 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, Hf) of resistant minerals, are conserved in the lower weathering zone.