Reactions between carbonatitic melt and syenitic wall rocks drive REE mineralization in the Maoniuping deposit, southwest China

Abstract Carbonatite-related rare earth element (REE) deposits commonly form by complex magmatic−hydrothermal processes. In this study, REE-mineralized metasomatic alteration selvages surrounding carbonatite dikelets in the Oligocene Maoniuping REE deposit, southwest China, were investigated to determine the role of metasomatic interactions between evolved carbonatitic magmatic–hydrothermal fluids and silicate wall rocks in controlling REE mineralization. The alteration selvages contain primary assemblages of clinopyroxene, amphibole, phlogopite, feldspars, calcite, and bastnäsite, along with accessory apatite, titanite, thorite, and zircon. Electron probe microanalysis, laser ablation–inductively coupled plasma–mass spectrometry, whole-rock geochemical, and sensitive high-resolution ion microprobe oxygen isotopic methods were used to investigate the origins of the carbonatite dikelets by examining the mineral assemblages in the metasomatic selvages peripheral to the dikelets. The widespread occurrence of alkali silicate minerals in both the metasomatic selvages and carbonatites in the Dagudao section in the Maoniuping deposit indicates that silica enrichment in the evolved carbonatitic melts resulted from interactions with syenite. Calcite in the selvages has near-chondritic Y/Ho ratios (25.8–31.3), consistent with limited fluid–rock interactions and the preservation of magmatic compositions. Titanite associated with clinopyroxene, has high Fe/(Fe + Al) ratios (0.65–0.84) and elevated REE contents (1.95–3.59 wt.%), This result, and the textural association and distinctive negative Eu anomalies (Eu/Eu* = 0.58–0.98) of titanite are indicative of crystallization from an Fe- and REE-rich carbonatitic melt that had interacted with silicate wall rocks. Zircon within the selvages has textures and compositions indicative of hydrothermal overprinting, including low Th/U ratios (< 0.4), variable Ce anomalies (Ce/Ce* =0.94–9.8) and (Sm/La)N values (0.24–126), and low δ18OV-SMOW values (2.63‰–4.41‰). The composition is similar to that of zircon from the altered syenitic wall rocks and reflects interactions with late-stage, low-δ¹⁸OV-SMOW fluids. Petrographic and geochemical evidence indicate two stages of mineralization. The initial stage involved metasomatic interactions between evolved carbonatitic melt and syenite, leading to Na–K depletion and SiO₂ enrichment in the residual melt, and the precipitation of alkali silicate minerals, titanite, apatite, and bastnäsite. The subsequent hydrothermal stage is evident from the dissolution of apatite and bastnäsite overgrowths, indicative of localized REE remobilization rather than the large-scale introduction of REE. The results show that early magmatic metasomatism (i.e., antiskarn development) between carbonatitic melt and wall rocks is key to establishing the primary REE endowment, whereas later hydrothermal activity mainly causes local redistribution of the REE.