Zinc isotope constraints on magma source and fluid evolution of post-collisional Qulong porphyry Cu-Mo deposit, southern Tibet

• Zn isotopes are applied to post-collisional porphyry deposits. • Zn isotopes indicate carbonate input into magma sources of the Qulong intrusions. • Exsolved magmatic fluids preferentially enrich light Zn isotopes. • Zn isotopes effectively trace magma source and fluid evolution. Porphyry-type deposits are widely recognized as major contributors to global Cu resources and are traditionally linked to subduction-related magmatic arcs. Recent exploration has revealed that substantial porphyry mineralization can also occur in post-collisional tectonic environments, but the ore enrichment mechanism in such settings remain insufficiently understood. Deciphering the roles of magma source, fluid evolution, and metal transport remains critical for understanding their ore fertility. Here we present whole-rock Zn isotope data and Cl concentrations for barren and mineralized rocks from the post-collisional Qulong porphyry Cu deposit in the Gangdese belt, southern Tibet. The barren host rocks and associated mafic microgranular enclaves (MMEs) display systematically elevated δ 66 Zn values relative to mantle-derived magmas, which cannot be explained by magmatic differentiation or crustal assimilation. Instead, these heavy Zn isotopic signatures are best interpreted to reflect incorporation of recycled carbonate-rich sediments into their magma sources, resulting in magma oxidation and volatile enrichment to enhance ore fertility in post-collisional settings. In contrast, mineralized porphyries exhibit significantly lower δ 66 Zn values and variational Zn concentrations, coupled with elevated K 2 O, Rb/Ba, and Pb/Ce ratios, indicating extensive interaction with exsolved magmatic fluids. We propose that exsolved fluids preferentially extracted isotopically light Zn from the melts, generating metal-rich hydrothermal fluids with high mineralization potential. Subsequent fluid-rock interaction and sulfide precipitation induced kinetic Zn isotope fractionation, progressively removing light Zn into sulfides, driving remaining fluids toward heavier Zn isotopic compositions, and rapidly diminishing Cu-carrying capacity. Our results demonstrate that Zn isotopes sensitively record both magma source composition and fluid evolution, providing novel insights into metal transport mechanisms in post-collisional porphyry systems.