Peter B. Larson Michael A. Cosca Allen K. Andersen 2019 <p><span>The&nbsp;carbonatite&nbsp;dike swarm&nbsp;and vein stockwork at the center of the&nbsp;Paleogene&nbsp;Bear Lodge alkaline complex (BLAC), Wyoming, USA, is host to diverse&nbsp;REE&nbsp;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&nbsp;barite&nbsp;is the result of interaction with late-stage&nbsp;hydrothermal fluids&nbsp;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&nbsp;stable isotope&nbsp;ratios indicate that the primary carbonatite&nbsp;mineralogy&nbsp;experienced degassing/pneumatolysis and alteration by fluids of variable temperature, CO</span>₂<span>/H</span>₂<span>O ratios, and/or&nbsp;meteoric water&nbsp;content. Isotopic differences of matrix&nbsp;calcite&nbsp;between Group 1 carbonatites (avg. δ</span>¹³<span>C = −7.3‰; δ</span>¹⁸<span>O = 9.1‰) and Group 2 carbonatites (avg. δ</span>¹³<span>C = −9.9‰; δ</span>¹⁸<span>O = 10.2‰) are consistent with loss of CO</span>₂<span>&nbsp;during&nbsp;degassing. The open-system alteration of burbankite caused a pronounced positive δ</span>¹⁸<span>O-shift in bulk ancylite&nbsp;pseudomorphs&nbsp;(δ</span>¹⁸<span>O: 14.3–25.7‰) relative to matrix calcite (δ</span>¹⁸<span>O: 8.7–11.2‰).&nbsp;Oxygen isotope&nbsp;compositions of&nbsp;biotite&nbsp;(δ</span>¹⁸<span>O: 4.5–5.9‰) and K-feldspar (δ</span>¹⁸<span>O: 7.3–7.9‰) in unoxidized carbonatite are typical of primary magmatic&nbsp;silicates&nbsp;and suggest that fluids responsible for the burbankite-to-ancylite conversion remained predominantly magmatic (carbohydrothermal). Concomitant increases toward the surface in&nbsp;</span>¹³<span>C and&nbsp;</span>¹⁸<span>O,&nbsp;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 (&lt;250 °C) fluids characterized by lower CO</span>₂<span>/H</span>₂<span>O ratios, and an increasing meteoric water component. The first&nbsp;</span>⁴⁰<span>Ar/</span>³⁹<span>Ar 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&nbsp;geochronology&nbsp;for Bear Lodge rocks indicate that alkaline silicate&nbsp;magmatism&nbsp;both preceded and followed carbonatite&nbsp;emplacement.</span></p> application/pdf 10.1016/j.lithos.2018.11.030 en Elsevier C–O stable isotope geochemistry and 40Ar/39Ar geochronology of the Bear Lodge carbonatite stockwork, Wyoming, USA article