Metasomatic REE Mineralization at the Alkaline Vein System of the Maoniuping Carbonatite–Syenite Complex

Carbonatite intrusions host the majority of the world’s light rare earth element (LREE) deposits, the formation of which commonly requires fertile magma rich in REE, and subsequent magmatic–hydrothermal processes. However, the mechanisms driving carbonatite magmatic–hydrothermal evolution, particularly metasomatic interactions between carbonatitic melts, hydrothermal fluids, and silicate wall rocks, remain poorly understood. The Maoniuping world-class LREE deposit in southwestern China provides a unique opportunity to investigate these processes due to its excellent exposure of deeper stockwork systems revealed by recent exploration efforts. The Maoniuping deposit comprises a vertically zoned vein system with three units: (1) stockworks, (2) veinlets, and (3) thick veins distributed from bottom to top vertically in exposure, each with specific mineral assemblages of amphibole, pyroxene, phlogopite, K-feldspar, and others. The lower unit consists of low-grade (<0.5% bastnäsite) stockworks transitioning into thicker, highly mineralized veins (>4% bastnäsite) in the upper unit. These veins are composed of F-rich phlogopite, sodic pyroxenes (aegirine to aegirine-augite), sodic amphiboles (magnesio-arfvedsonite), alkali feldspar (K-feldspar and albite), fluorite, baryte, calcite, and bastnäsite. Multiple lines of evidence support an antiskarn model for Maoniuping. (1) The thin stockworks in the lower unit are dominated by pyroxene and K-feldspar crystallization, effectively clogging conduits and restricting melt flow. (2) The silicate minerals of generation I along vein margins exhibit high alkalinity (Na + K) and silica activity, forming sharp contacts with syenite and indicating limited wall–rock interaction. (3) In the upper unit, residual silicate of generation II and non-silicate minerals (e.g. fluorite, calcite, baryte, and bastnäsite) crystallized, marking an evolving carbonatite melt. The composition of pyroxene undergoes a transformation from aegirine-augite to aegirine-dominant as crystallization occurs, suggesting Na content in the carbonatite melt increases. The subsequent transition to progressively more Fe3+-rich amphibole marks the evolution of carbonatite system. Katophorite and richterite formed through the replacement of aegirine facilitated by decreasing temperature and reducing conditions with overprint of the antiskarn by magmatic-hydrothermal fluids indicated by previous fluid inclusion studies. To further trace the formation process of the deposit, the assemblages and chemical composition of minerals have been interpreted using T-activity and T-fO2 diagrams for the vein system, calculated using SUPCRTBL and published thermodynamic data. Relation of the observed mineral assemblage and compositions to phase relations in the Na–Fe–Si–O–H system suggest antiskarn formation at the bottom stockwork unit and along vein margins. Following the initial reaction, the mineral assemblage is consistent with decreasing T, ${\boldsymbol{a}}_{{\boldsymbol{Na}}^{+}}/{a}_{{\boldsymbol{H}}^{+}}$, and fO2 from magmatic to hydrothermal conditions. Significant REE mineral crystallization was initiated from the carbonatite melt, by sequestration of alkalis in pyroxene, K-feldspar, amphibole, and phlogopite. In conclusion, magmatic processes played a dominant role in REE enrichment and mineralization at Maoniuping, with minor contributions from hydrothermal overprinting. This study provides a framework for understanding similar processes in other carbonatite-related deposits, where antiskarn interactions may be underappreciated.