The carbonatite dike swarm and vein stockwork at the center of the Paleogene Bear Lodge alkaline complex (BLAC), Wyoming, USA, is host to diverse REE mineral assemblages that are largely a result of subsolidus modification and REE redistribution. Pseudomorphic replacement of primary burbankite by an assemblage of ancylite, strontianite, and barite is the result of interaction with late-stage hydrothermal fluids that added Sr, Ba, S, F, and REE, analogous to the replacement processes described for some carbonatite complexes of Russia's Kola Peninsula. Carbon and oxygen stable isotope ratios indicate that the primary carbonatite mineralogy experienced degassing/pneumatolysis and alteration by fluids of variable temperature, CO₂/H₂O ratios, and/or meteoric water content. Isotopic differences of matrix calcite between Group 1 carbonatites (avg. δ¹³C = −7.3‰; δ¹⁸O = 9.1‰) and Group 2 carbonatites (avg. δ¹³C = −9.9‰; δ¹⁸O = 10.2‰) are consistent with loss of CO₂ during degassing. The open-system alteration of burbankite caused a pronounced positive δ¹⁸O-shift in bulk ancylite pseudomorphs (δ¹⁸O: 14.3–25.7‰) relative to matrix calcite (δ¹⁸O: 8.7–11.2‰). Oxygen isotope compositions of biotite (δ¹⁸O: 4.5–5.9‰) and K-feldspar (δ¹⁸O: 7.3–7.9‰) in unoxidized carbonatite are typical of primary magmatic silicates and suggest that fluids responsible for the burbankite-to-ancylite conversion remained predominantly magmatic (carbohydrothermal). Concomitant increases toward the surface in ¹³C and ¹⁸O, oxidation, matrix carbonate dissolution, and the replacement of REE carbonates (ancylite, carbocernaite, and burbankite) by Ca-REE fluorocarbonates (bastnäsite, parisite, synchysite) suggest interaction with late-stage, low temperature (<250 °C) fluids characterized by lower CO₂/H₂O ratios, and an increasing meteoric water component. The first ⁴⁰Ar/³⁹Ar ages from carbonatite-hosted biotite and K-feldspar at the BLAC are between 51.45 ± 0.08 and 51.89 ± 0.14 Ma. Although carbonatite is commonly observed as the final intrusive phase in alkaline igneous complexes, relative-age relationships and previously published geochronology for Bear Lodge rocks indicate that alkaline silicate magmatism both preceded and followed carbonatite emplacement.