A genetic link between magmatic evolution and indium mineralization in the Dulong Sn-polymetallic deposit, Southwest China: Constraints from the compositions of mica and apatite

Abstract

The Dulong deposit is a world-class Sn-polymetallic deposit with indium mineralization, which is associated with highly evolved Late Cretaceous S-type granites, indicating the magmatism is crucial for the indium enrichment. Through integrated in-situ geochemical analyses of mica and apatite across variably fractionated granites and country rocks, this study reveals a systematic evolution of redox conditions, halogen chemistry, and metal enrichment during magmatic differentiation. As magmatic evolution progresses, indium and tin contentratons in mica increase progressively, reaching peak vales in the late stage of evolution at Dulong deposit. Notably, indium concentration in muscovite is greater than that in biotite, when muscovite converts into phengite. Apatite and mica compositions indicate low oxygen fugacity (fO₂) magmatism. Fe-rich biotite in Sn-In-bearing granites contrasts with Mg-biotite in Sn-poor systems, with Fe³⁺/(Fe²⁺+Fe³⁺) ratios < 0.2 confirming reduced fO₂ conditions conducive to indium melt incorporation. Apatite geochemistry reinforces this: low SO₃ (0.01–0.08 wt%), As depletion (14.54–42.10 ppm), and marked negative Eu anomalies (Eu/Eu* <0.4). Multi-proxy evidence collectively confirms a reduced magmatic system. Integrated proxies confirm a reduced magmatic system where progressive redox reduction during differentiation fosters indium enrichment, enhancing metallogenic potential. Concurrently, apatite halogen signatures track magmatic evolution: sustained F enrichment with late Cl spikes aligns with mantle-derived inputs evidenced by mafic enclaves. Although Cl concentrations remain low in the early magmatic stages, their subsequent dramatic increase correlates temporally and genetically with marked enrichment of In and Sn during later evolutionary phases. The mixing of mantle-derived magmas may bring in Cl-rich fluids/melts and related ore-forming materials, which promotes indium mineralization. The concurrent enrichment of halogen elements, F and Cl, plays a crucial role in the extraordinary accumulation of indium.