The Dalucao deposit, located in western Sichuan Province, southwestern China, in the western part of the Yangtze Craton, is one of the largest and most extensive rare earth element (REE) deposits in the Himalayan MianningâDechang REE belt. Moreover, the Dalucao deposit is the only deposit identified in the southern part of the belt. The Dalucao deposit contains the No. 1, 2, and 3 orebodies; the No. 1 and 3 orebodies are both hosted in two breccia pipes, located in syeniteâcarbonatite host rocks. Both pipes have elliptical cross-sections at the surface, with long-axis diameters of 200â400m and short-axis diameters of 180â200m; the pipes extend downwards for >450m. No. 1 and No. 3 have total thickness varying between 55 and 175m and 14 to 58m respectively. The REE mineralization is associated with four brecciation events, which are recorded in each of the pipes. The ore grades in the No. 1 and 3 orebodies are similar, and consist of 1.0%â4.5% rare earth oxides (REOs). The No. 1 orebody is characterized by a Type I mineral assemblage (fluorite+barite+celestite+bastnĂ€site), whereas the No. 3 orebody is characterized by a Type II assemblage (fluorite+celestite+pyrite+muscovite+bastnĂ€site+strontianite). Argon (40Ar/39Ar) dating of hydrothermal muscovite intergrown with REE minerals in typical ores from the No. 1 and 3 orebodies yielded similar ages of 12.69±0.13 and 12.23±0.21Ma, respectively, which suggest that both mineral assemblages formed coevally, rather than in paragenetic stages. Both ages are also similar to the timing of intrusion of the syeniteâcarbonatite complex (12.13±0.19Ma). The ore-mineral assemblages occur in breccias, veinlets, and in narrow veins. The ore veinlets, which usually show a transition to mineralized breccia or brecciated ores, are commonly enveloped by narrow veins and stringer zones with comparable mineral assemblages. The brecciated ores form 95% of the volume of the deposit, whereas brecciated ores are only a minor constituent of other deposits in the MianningâDechang REE belt. The carbonatite in the syeniteâcarbonatite complexes contains high concentrations of S (0.07â2.32wt.%), Sr (16,500â20,700ppm), Ba (3600â8400ppm), and light REEs (LREE) (2848â10,768ppm), but is depleted in high-field-strength elements (HFSE) (Nb, Ta, P, Zr, Hf, and Ti). The syenite is moderately enriched in large-ion lithophile elements (LILE), Sr (155â277ppm), and Ba (440â755ppm). The mineralized, altered, and fresh syenites and carbonatites exhibit similar trace element compositions and REE patterns. Brecciation events, and the Dalucao Fault and its secondary faults around the deposit, contributed to the REE mineralization by facilitating the circulation of ore-forming fluids and providing space for REE precipitation. Some hydrothermal veins composed of coarse-grained fluorite and quartz are distributed in the syeniteâcarbonatite complex. The oxygen isotope compositions of ore-forming fluids in equilibrium with quartz at 215°C are â4.95â° to â7.45â°, and the hydrogen isotope compositions of fluid inclusions in coarse-grained quartz are â88.4â° to â105.1â°. The syeniteâcarbonatite complex and carbonatite are main contributors to the mineralization in the geological occurrence. Thus, the main components of the ore-forming fluids were magmatic water, meteoric water, and CO2 derived from the decarbonation of carbonatite. According to the petrographic studies, bastnĂ€site mineralization developed during later stages of hydrothermal evolution and overprinted the formation of the brecciated fluoriteâquartz hydrothermal veins. As low-temperature isotope exchange between carbonates of the carbonatite and water-rich magmatic fluids will lead to positive shifts in ÎŽ18O values of the carbonates, CâO isotopic compositions from the bulk primary carbonatite to hydrothermal calcite and bastnĂ€site changed (ÎŽ18OV-SMOW from 8.0â° to 11.6â°, and ÎŽ13C V-PDB from â6.1 to â8.7â°). According to the chemical composition of syenite and carbonatite, REE chloride species are the primary complexes for the transport of the REEs in the hydrothermal fluids, and the presence of bastnĂ€site and parisite means the REE were precipitated as fluorocarbonates. High contents of Sr, Ba and S in the syeniteâcarbonatite complex led to the deposition of large amount of barite and celestite.