Rare Earth Element Enrichment in the Weathering Profile of the Bull Hill Carbonatite at Bear Lodge, Wyoming, USA

Bull Hill is a carbonatite diatreme within the Paleogene Bear Lodge Carbonatite Complex in Wyoming, USA. Rare earth element (REE)-bearing carbonate, fluorocarbonate, phosphate, and oxide minerals occur within near-vertical carbonatite dikes on the western margin of Bull Hill. Changes in mineralogy and REE concentrations with depth are ascribed mainly to late-stage magmatic-hydrothermal and supergene alteration. Approximately 35 m of drill core from Bull Hill was analyzed and encompasses least altered, weakly weathered, and moderately weathered carbonatite. The least altered carbonatite contains magmatic burbankite, typically as inclusions within Mn-rich calcite (stage I). Secondary REE-bearing minerals, which pseudomorphically replaced unidentified hexagonal phenocrysts, include ancylite, bastnäsite with synchysite/parisite, and an unidentified Sr-Ca-REE-phosphate (stage II). These replacive minerals generated small amounts of incipient porosity (~7–8%) and are largely stable in the lower portion of the weathering profile. Progressive weathering (stages III and IV) of the carbonatite involved the oxidation of pyrite to iron oxides and iron hydroxides, dissolution of calcite and strontianite, and the replacement of Mn-rich calcite by manganese oxides. These mineralogical changes resulted in an ~40% porosity gain in the core studied here. The volumetric concentration of weathering resistant REE-bearing minerals resulted in REE enrichment from an average of 5.4 wt % in the least weathered carbonatite to an average of 12.6 wt % in moderately weathered carbonatite, and to an overall increase in REE ore tenor of two to three times compared to the least altered carbonatite. Isocon plots confirm the increased concentration of REEs in the weathered carbonatite and demonstrate that REEs, along with TiO2, Ta, Nb, Zr, and Hf, were conserved in the lower weathered zone.