In-situ trace element and sulfur isotope analyses of sulfides as indicators of ore-forming fluid evolution in the Lakang’e porphyry Mo-Cu deposit, Tibet, China

The Lakang’e Mo-Cu deposit is a Mo-dominated porphyry deposit located in the Gangdese metallogenic belt of southern Tibet, which is known for Cu-dominated porphyry deposits such as Jiama and Qulong. The magmatic-hydrothermal evolution responsible for the development of ore-forming fluids in this deposit has not been systematically examined, thus limiting our understanding of the genetic link between Cu and Mo mineralization throughout the metallogenic belt. This study addresses this problem through in-situ analysis of S isotopes and trace elements of pyrite, chalcopyrite and molybdenite from hydrothermal veins of different stages (A, B1, B2, D), which reflect the evolution of the hydrothermal system in terms of S source and temperature, pH and redox condition. The overall range of δ³⁴S_CDT values (–7.68 ‰ to +0.75 ‰) of the sulfides and the lack of systematic variation from one stage to another are consistent with a common magmatic source for the sulfur. The decrease of Co/Ni ratios in pyrite from A and B1 to B2 and D veins indicates an overall cooling trend. The elevated As concentrations in sulfides and the lack of calcite in A and B1 veins versus the relatively low As in sulfides and presence of calcite in the D and B2 veins suggest that the fluids became less acidic from early to late stages. The relatively elevated Te concentration in pyrite in B1 and B2 veins compared to those in A and D veins suggests that B1 and B2 veins formed under relatively reducing conditions, whereas A and D veins formed under relatively oxidizing conditions. The consumption of Fe³⁺ due to precipitation of large amounts of magnetite in A vein may be responsible for the decrease in fO₂, which induced significant Mo mineralization in B1 veins. As the hydrothermal system continued to evolve with decreasing temperature and increasing influx of meteoric water, the fO₂ increased again, which promoted the precipitation of pyrite-chalcopyrite instead of molybdenite in the D veins. At last, another phase of distinct Mo mineralization may have been triggered by the release of residual metalliferous fluids from a dormant magma chamber due to tectonic reactivation, subsequently migrating upward and forming the B2 veins. Our study highlights the dynamic evolution of magmatic-hydrothermal fluids during the formation of the Lakang’e Mo–Cu deposit and demonstrates that fluid temperature, oxygen fugacity (fO₂), and the involvement of meteoric water were the primary controls on mineralization. Molybdenite precipitation in the B1 and B2 veins was driven by decreasing temperature and fO₂, whereas chalcopyrite precipitation in the D vein was triggered by fluid cooling due to mixing with meteoric water. Such dynamic magmatic-hydrothermal processes may have also operated in other Cu-Mo deposits within the Gangdese metallogenic belt, implying that separate episodes of Mo-dominated mineralization may have been potentially developed in the Cu-dominated deposits as well.