The occurrence and enrichment mechanism of indium in sphalerite from the Dachang Sn polymetallic ore field, South China: Part I. The Tongkeng deposit

Indium, a dispersed and strategically important element, typically exists as an associated element within various polymetallic deposits. The Dachang ore field, renowned for its substantial Sn reserves, also hosts significant high-grade In resources. However, the occurrences and enrichment mechanisms of In in this field remain unclear. The Tongkeng Sn polymetallic deposit of the Dachang ore field in Guangxi was chosen as an example to decipher the above issues. Three generations of sphalerite (Sp1-Sp3) were identified based on different mineral assemblages. Sp1 is mainly associated with chalcopyrite and galena, whereas Sp2 is always associated with cassiterite and stannite. Sp3 mainly coexists with cassiterite and pyrite. The average In contents of Sp1, Sp2, and Sp3 are about 99, 954 and 689 ppm, respectively. Sp1 is enriched in Fe, Cd, Pb, and Bi, while Sp2 and Sp3 are predominantly enriched in In, Cu, Ga, and Sn. Indium may be present in sphalerite via the substitution of In³⁺ + Cu⁺ ↔ 2Zn²⁺ and (In³⁺, Sn³⁺) + (Cu⁺, Ag⁺) ↔ 2Zn²⁺. Copper exhibits a consistent positive correlation with In enrichment across all sphalerite generations, while the effects of Ag and Sn show generation-specific variations. A significant increase in In content of sphalerite when Cd content ranges from 0.35 wt% to 0.45 wt%, consistent with the “Indium Window Effect”. Using the GGIMFis geothermometer, the formation temperatures for Sp1, Sp2, and Sp3 were estimated to be 329–392 °C, 321–354 °C, and 325–357 °C, respectively. The relationship between In content and temperature indicates that the effect of temperature on In enrichment is content-dependent and should be evaluated within specific compositional ranges.