REE mineralization in the Saima peralkaline complex (China): Constraints from magmatic-hydrothermal processes recorded in apatite Ca-Sr solid solutions

Rare earth elements (REE) are strategic resources critical for advanced technologies. Although their enrichment in peralkaline systems is widely attributed to magmatic-hydrothermal processes, the key mechanisms controlling REE enrichment and LREE-HREE fractionation remain poorly constrained. This study investigates a near-continuous Ca-Sr solid solution series in apatite from the Saima peralkaline complex to track REE behavior from magmatic through hydrothermal stages. All analyzed apatites exhibit homogeneous initial ⁸⁷Sr/⁸⁶Sr (0.7083–0.7089) and εNd(t) (-10.5 to -14.4) values, indicating a stable Sr-Nd isotopic system during both crystallization and late alteration. Throughout the early magmatic sequence from primitive lamprophyre to syenite and nepheline syenite, the REE budget and LREE-HREE fractionation were dominantly controlled by apatite crystallization. This is evidenced by positive correlations between whole-rock P₂O₅ and total REE contents, together with whole‑rock REE patterns that resemble those of magmatic apatite. Concurrent decreases in apatite SO₃ contents and volatile ratios (XCl/XOH, XCl/XF) record a decline in melt oxygen fugacity and an increase in H₂O saturation, which efficiently promoted REE and volatile enrichment in the residual melt. These processes culminated in the most evolved lujavrite, where intense REE mineralization produced relatively HREE-enriched silicates such as eudialyte. Subsequently, an autometasomatic fluid enriched in Na⁺, Sr2⁺, LREE3⁺, CO₃2⁻, PO₄3⁻, and F⁻ exsolved from the lujavrite system, decomposing earlier REE-bearing phases and precipitating hydrothermal LREE-phosphates and carbonates at 220.0 ± 2.4 Ma. Hydrothermal apatite from this stage exhibits coupled Sr and REE enrichment, accomplished through fluid-mediated substitutions Ca2⁺ ↔ Sr2⁺, 2Ca2⁺ ↔ REE3⁺ + Na⁺, and Ca2⁺ + P⁵⁺ ↔ REE3⁺ + Si⁴⁺. The pronounced LREE-HREE fractionation observed in hydrothermal apatite is ascribed to the prior magmatic removal of HREE into early silicates and the preferential compatibility and mobility of LREE in alkaline, carbonate-rich fluids. This study highlights the utility of apatite as a petrogenetic indicator for deciphering REE mineralization processes in alkaline igneous systems.