Indium mineralization of the Chaganbulagen Ag-Pb-Zn deposit in Inner Mongolia, Northeast China: Evidence from geochemistry of sphalerite and chalcopyrite

The Chaganbulagen large AgPbZn deposit in the southwestern segment of the Derbugan metallogenic belt contains significant indium (In) and cadmium (Cd), though their enrichment mechanisms remain unclear. Field geological investigations and petrographic observations revealed well-developed mineralization zoning. Furthermore, four mineralization stages were identified, comprising four generations of sphalerite (SpI-SpIV) and chalcopyrite (CcpI-CcpIV). LA-ICP-MS analysis shows In and Cd mainly occur in sphalerite via isomorphism, followed by chalcopyrite. Cadmium exhibits consistent enrichment across all mineralization stages, whereas indium displays a marked decline from early-stage (SpI: averaging 656 ppm; CcpI: averaging 270 ppm) to late-stage sulfides (SpIV: averaging 1.15 ppm; CcpIV: averaging 0.36 ppm). This depletion trend correlates with decreasing temperatures (336-208 °C) and diminishing CuFe concentrations in the ore-forming fluids. Spatially, shows two trends in indium distribution: (1) vertical increasing with depth in No. X orebody, and (2) lateral zoning with decreasing In content away from the porphyry Mo (Cu) mineralization, highest in proximal CuZn stage sphalerite (up to 1112 ppm at 1.5–3 km), moderate in intermediate CuPbZn and PbZnAg stages orebodies (up to 76.8 ppm at 3–5.5 km), and negligible in distal Ag stage orebodies (< 5 ppm at 5.5–8 km). Indium in chalcopyrite follows a similar enrichment pattern as in sphalerite. Based on this, the Jia-Cha ore field, centered on the porphyry body within 3 km to the northwest and southeast, and the deeper parts of the field are favorable areas for future indium ore exploration.