Mineralization and hydrothermal remobilization of heavy rare earth elements in carbonatites at Huayangchuan, Qinling orogenic belt, Central China

• Identification of primary heavy rare earth element mineralization in carbonatites. • A general model for heavy rare earth element behavior in carbonatite systems. • Geochemistry of sulfate minerals serves as petrogenetic indicators. Carbonatites are primary sources of light rare earth elements (LREEs), yet their potential for hosting heavy rare earth elements (HREEs) enrichment has garnered growing attention; the geochemical behavior of HREEs in carbonatite systems, however, remains poorly understood. This study investigates magmatic mineralization and hydrothermal remobilization of HREEs in the Huayangchuan carbonatites in the Qinling orogenic belt, Central China. In this deposit, HREEs are hosted by oxide minerals (samarskite, euxenite) that are closely associated with rock-forming minerals, indicating their early precipitation from carbonatitic magma. These primary ore minerals have undergone alteration along their rims and fractures. Baryte-celestine is widespread in this deposit and is classified into three types based on its textural characteristics. Early strontium-rich baryte (Brt-1) occurs in carbonatite rocks and crosscuts primary minerals, and hosts high total rare earth element (ΣREE) contents (1069–1809 ppm) and primary Sr isotopic compositions ( 87 Sr/ 86 Sr = 0.70497–0.70510). Celestine from late zeolite-bearing veins (Brt-2) exhibits elevated radiogenic Sr isotopic compositions (0.70587–0.70608) and significantly lower ΣREE (5.56–17.5 ppm), reflecting modification by mixing with noncarbonatic fluids. Brt-3 is poorly crystallized, with high and variable 87 Sr/ 86 Sr ratios (0.70862–0.71077), representing postmineralization metasomatism that degraded primary HREE mineralization. Sulfur isotope equilibrium temperatures of baryte–galena pairs are 437–603°C for Brt-1 and 316–366°C for Brt-2. The bulk sulfur isotopic compositions of early primary stage are close to the mantle value (∼ 0 ‰), while the distinctly negative sulfur isotope values in later hydrothermal veins (∼ −10 ‰) indicate intensive SO 2 degassing. This study identifies three key factors controlling carbonatite-related HREE mineralization: (1) initial HREE enrichment in carbonatite magma is an essential prerequisite; (2) low fluorine availability inhibits HREE mobility, facilitating their early crystallization within carbonatite dikes; (3) postmineralization metasomatism disrupts primary HREE mineralization. These findings enhance our understanding of HREE behavior in carbonatite systems and provide critical insights for exploring HREE resources associated with carbonatites.