Fluid evolution and metal precipitation mechanisms in the Yulong porphyry Cu-Mo deposit, Eastern Tibet: insights from the two-stage mineralization process

• The Yulong deposit records two distinct mineralization stages linked to evolving magmatic-hydrothermal fluids • Significant Mo isotope fractionation implies multiple episodes of intensive fluid boiling. • Superimposed sericitic alteration and heavy δ 98/95 Mo isotopes in halos could serve as exploration indicators. The Yulong porphyry Cu-Mo deposit, the largest deposit in the Eocene Yulong porphyry copper belt of eastern Tibet, China, preserves three mineralization events triggered by multiple magmatic intrusions. Despite most Cu-Mo sulfides are concentrated in the first two mineralization stages, however, the pressure–temperature conditions and metal precipitation mechanisms of those remain poorly constrained. This study integrates field observations, ore microscopy, cathodoluminescence (CL) imaging, fluid inclusion microthermometry, and coupled sulfide sulfur isotope/molybdenite Mo isotope analyses to constrain ore-forming fluids evolution during two mineralization stages. Stage I Cu-Mo mineralization was associated with high-temperature (∼400–450 °C) K-silicate alteration under lithostatic to hydrostatic transitional conditions (∼450 bar, paleodepth 2–4 km), resulting from direct Cu-Mo precipitation triggered by cooling of single-phase fluids. Stage II mineralization occurred under hydrostatic pressure (∼300 bar, paleodepth ∼ 3 km), where fluid phase separation into high-salinity brine and low-salinity vapor ultimately facilitated extensive Cu-Mo precipitation accompanied by moderate-temperature sericitic alteration (∼350–400 °C). Mo isotopic compositions of molybdenite reveal pronounced δ 98/95 Mo fractionation (0.05 to 0.48 ‰) attributable to multistage fluid boiling and Rayleigh fractionation during Stage II mineralization. These findings establish that the overprinting of sericitic alteration on K-silicate alteration could serve as an indicator for high-grade mineralization zone. Systematic Mo isotopic variations in alteration halos (enriched in heavy δ 98/95 Mo) provide a novel vectoring tool to locate mineralization centers.