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

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 being 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), resulting from direct Cu-Mo precipitation triggered by cooling of single-phase fluids. Stage II mineralization occurred under hydrostatic pressure (∼300 bar), 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^98/95Mo 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^98/95Mo) provide a novel vectoring tool to locate mineralization centers.