The temporal and spatial evolution of ore-forming fluids during the formation of the Dulong deposit, Southwest China: Linkage with in mineralization

The Dulong deposit is a major cassiterite-sulfide skarn deposit in Southwest China, hosting significant In resources (7,000t). Although the occurrence of In has been studied in detail, its spatiotemporal distribution and controlling factors remain poorly understood. This study therefore systematically analyzes fluid inclusions, chlorite geochemistry, and combined with published In contents from sphalerite to comprehensively examine ore-forming fluid evolution and its impact on In mineralization. The fluid inclusions data show that S-type, L-type, and V-type can be recognized at Dulong. The S-type inclusions predominantly occur in pyroxene from prograde stage and coexist with L-type inclusions, suggesting that fluid boiling occurred. The homogenization temperature and salinity of fluids show a significant decrease from prograde stage (468–560 °C, 44.7–49.6 % NaCl_eqv) to retrograde stage (365–375 °C, 3.1–3.7 % NaCl_eqv), indicating potential mixing with meteoric waters. From retrograde to sulfide stage, both temperature and salinity decrease slightly (from 365–375 °C to 210–365 °C, and 3.1 %–3.7 % NaCl_eqv to 0.3 %–2.6 % NaCl_eqv, respectively), inferring that fluid cooling probably was the key factor for sulfide precipitation and associated In enrichment. The chlorite results show that, from bottom to top of the open pit, this variation in trace element concentrations is dominantly related to the decrease in temperature that occurs during the migration of hydrothermal fluid. The fluid temperatures correlate with increased In concentrations in sphalerite, suggesting temperature controls In enrichment. Additionally, the Ti/Sr ratio in chlorite indicate the possible hydrothermal center at the bottom of the Manjiazhai ore block (approximately 1152 m-depth), implying that the Dulong deposit is probably a distal skarn deposit.