Porphyry deposits represent globally significant sources of Cu and Mo, yet the magmatic volatile controls on Cu versus Mo metallogenic divergence remain poorly quantified in collisional settings. Here, we compare porphyry Cu systems from the Gangdese belt, southern Tibet, with porphyry Mo systems from the southern margin of the North China craton—two collisional belts with contrasting magma sources and metal endowments. By integrating apatite geochemistry with thermodynamic forward modeling, we reconstructed volatile evolution during ore-forming magma differentiation. Copper-forming magmas were more hydrous (initial H₂O contents of ~5–6 wt %) than Mo-forming magmas (~3.8–4.7 wt %), whereas both were moderately oxidized. Modeled fluid-saturation thresholds are ~11 wt % H₂O for Cu systems (~0.5 GPa) and ~9.2 wt % H₂O for Mo systems (~0.4 GPa). Copper-related magmas were initially enriched in Cl (0.25–0.35 wt %) and F (0.40–0.55 wt %) but underwent significant F depletion (bulk DFc/m≈1.8？) driven by early, extensive amphibole fractionation. In contrast, Mo-related magmas had lower initial F (0.05–0.25 wt %) and comparable Cl (0.20–0.35 wt %) but became progressively enriched in both halogens DFc/m≈0.7;？DClc/m≈0.14 during prolonged crystallization with limited amphibole crystallization. This contrasting H₂O-F-Cl evolution links source-imposed volatile budgets to Cu-dominated outcomes versus Mo-dominated outcomes: Cl-enriched residual melts favor Cu-transporting fluids, whereas late-stage F enrichment likely lowers melt viscosity, prolongs differentiation, and promotes Mo enrichment in evolved residual melts.

Article link: https://doi.org/10.5382/econgeo.5257