Indium distribution and substitution mechanisms in the skarn mineralization process of world-class Dulong Sn–Zn–In deposit, Yunnan Province (Southwest China)

• Indium enrichment in Dulong skarn linked to selective substitution. • Sphalerite, cassiterite, chalcopyrite host most indium via isomorphism. • Tetrahedral sites and high-T conditions favor indium incorporation. • Sulfur-rich fluids enhance indium enrichment during mineralization. • Substitution mechanisms explain scarcity of discrete indium minerals. As a representative deposit of indium (indium)-rich skarn mineralization systems, the Dulong world-class Sn-Zn-In deposit offers an ideal natural laboratory for investigating the occurrence patterns, spatial distribution, and substitution mechanisms of indium through systematic in-situ analysis using EPMA and LA-ICP-MS. Our analytical results reveal significant and selective enrichment of indium in minerals, governed by complex substitution mechanisms. Indium preferentially concentrates in sphalerite, cassiterite, and chalcopyrite, while other minerals (e.g., magnetite, arsenopyrite, and pyrrhotite) exhibit anomalously elevated indium contents. However, the predominance of sphalerite in the deposit inhibits the formation of discrete indium mineral phases. The incorporation of indium primarily occurs through isomorphic substitution involving multiple coupled mechanisms, including: (i) (Cu, Ag) + + In 3+ ↔ 2(Zn, Fe) 2+ in sphalerite and chalcopyrite; (ii) In 3+ ↔ Fe 3+ in cassiterite and chalcopyrite; (iii) In 3+ + Sn 4+ ↔ Fe 2+ + (Nb, Ta) 5+ in cassiterite; (iv) Ag + + Sn 2+ + In 3+ ↔ 3Zn 2+ in chalcopyrite. During skarn mineralization processes, the distribution and enrichment of indium are principally controlled by four key factors: (i) crystallographic constraints of host minerals, (ii) physicochemical properties of ore-forming fluids, (iii) thermodynamic conditions, and (iv) specific substitution mechanisms. Indium preferentially partitions into minerals with tetrahedral coordination sites and those formed under high-temperature conditions. Furthermore, sulfur-rich hydrothermal systems with elevated sulfur fugacity demonstrate enhanced indium enrichment capacity. Temporally, indium enrichment exhibits distinct stage-specific characteristics, with principal concentration occurring in oxide-stage cassiterite and early sulfide-stage Fe-rich sphalerite and chalcopyrite developed under high sulfur fugacity conditions. We provide critical insights into the microscale occurrence states and atomic-scale substitution mechanisms of indium in the Dulong deposit. By systematically elucidating the geochemical behavior and enrichment processes of indium during skarn mineralization, our findings advance the understanding of critical metal enrichment mechanisms and expand the theoretical framework of skarn deposit formation.