From Melt to Fluid: Tracking <scp>REE</scp> Enrichment in the Huangshui'an Carbonatite‐Hosted Deposit via Calcite Trace Element and Sr Isotope

ABSTRACT The Huangshui'an deposit in the Qinling orogenic belt represents one of the most distinctive carbonatite‐hosted Mo‐REE deposits globally. The specific role of magmatic–hydrothermal evolution in the Huangshui'an deposit for the enrichment of rare earth elements (REEs) remains uncertain. This study focuses on the REE enrichment mechanism during the carbonatite magma and fluid evolution processes through detailed field investigation, petrography, in situ trace element geochemistry and Sr isotopic analysis of calcite. Based on textural and paragenetic relationships, calcites are categorised into four distinct types: Type I (Cal‐I) occurs in the brecciated carbonatite, type II (Cal‐II) is related to the coarse‐grained pure carbonatite, type III (Cal‐III) is intergrown with fluorite and molybdenite and type IV (Cal‐IV) is characterised by an assemblage of calcite, fluorite and bastnäsite. During the magmatic stage (Cal‐I to Cal‐III), REE concentrations progressively increase in the residual melt accompanying fractional crystallisation. The marked LREE depletion in Cal‐III relative to Cal‐II is attributed to the enhanced fluorine activity, which preferentially retains LREE as stable fluoride complexes in the residual melt, thereby inhibiting their incorporation into the calcite lattice. The 87 Sr/ 86 Sr ratios gradually increase from 0.7056 to 0.7061, recording progressive melt– and wall–rock interaction during carbonatite magma evolution. In the hydrothermal stage, Cal‐IV is characterised by remarkably high Y/Ho ratios (37.1–42.7) that lie beyond the charge and radius control (CHARAC) range, indicating hydrothermal origin. The markedly low ΣREE contents in Cal‐IV are attributed to precipitation of bastnäsite at the hydrothermal stage. The distinctly elevated 87 Sr/ 86 Sr ratios of Cal‐IV (0.7062–0.7065), together with lower and more variable Sr concentrations, reflect increased signature from the crustal wall rock. Fluid–rock interaction is probably the main mechanism to introduce crustal component into the carbonatite system, triggering the precipitation of bastnäsite, calcite and fluorite at the hydrothermal stage.