Evidence for Hydrothermal Superimposed Mineralization from Ore-controlled Structure, Geochronology, and Isotope Geochemistry in the Precambrian Dahongshan Iron Oxide-Copper-Gold Deposit, Yunnan, China

The Precambrian Dahongshan Iron Oxide-Copper-Gold (IOCG) deposit, which represents the largest Cu-Fe polymetallic deposit in the Kangdian region, exhibits pronounced contrasts between hydrothermal vein-type Cu–Mo orebodies formed through superimposed hydrothermal mineralization and stratiform. Re–Os isotopic analyses of molybdenite samples from the deposit in the hydrothermal superimposition period, conducted using N-TIMS (Negative-Thermal Ionization Mass Spectrometry), yielded an isochron age of 795 ± 34 Ma (MSWD = 0.24, σ=2) and a weighted mean age of 777 ± 20 Ma (MSWD = 0.53). These age results closely correspond to the quartz Ar–Ar isochron age of 770 ± 5.4 Ma reported for the Taoyuan copper deposit in the adjacent Dongchuan area, where copper mineralization is quartz-hosted, indicating that hydrothermal mineralization at Dahongshan took place during the Chengjiang period. The δD values of hydrothermally overprinted orebodies range from –81.7‰ to –57.7‰, whereas the calculated δ18OH₂O values span 0.87‰ to 12.99‰, collectively indicating a dominantly magmatic-hydrothermal origin for the ore-forming fluids. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of S and Pb isotopic compositions in representative sulfide minerals reveal that the measured δ34S values (molybdenite: –0.34‰; pyrite: 5.18‰; chalcopyrite: 5.03‰) are consistent with a predominantly magmatic sulfur source. The progressive increase in δ34S values during the late mineralization stage is attributed to two principal processes: sulfide precipitation under relatively elevated oxygen fugacity conditions and thermochemical sulfate reduction (TSR) involving seawater-derived sulfate (bornite: 9.68‰; chalcocite: 10.74‰). The Pb isotopic compositions of hydrothermal vein-type ores indicate that ore lead plots between mantle and upper-crustal Pb isotopic fields, implying a mixed crust–mantle source that is clearly different from the isotopic signatures of stratiform orebodies and surrounding country rocks. An average Re concentration of 3.035% further supports a predominantly mantle source for rhenium in the Dahongshan deposit. Overall, the results suggest that the final breakup of the Rodinia supercontinent triggered the Chengjiang tectono-magmatic event in the Kangdian region, ultimately driving hydrothermal overprinting and the formation of vein-type orebodies in the Dahongshan deposit. These findings provide important insights into the genesis of IOCG deposits and offer a valuable geological framework for guiding future mineral exploration strategies.