Tracing missing copper: ultrapotassic rocks record isotopic evidence of crustal Cu depletion

Post-subduction reactivation of metasomatized lithospheric mantle generates high-K shoshonitic magmas in the continental crust and has been suggested to be potentially associated with porphyry Cu mineralization, yet its role in the continental crustal Cu budget remains ambiguous. Here, we use Cu isotope analyses and geochemical modeling of Cenozoic ultrapotassic rocks from southern Tibet to show that variations in Th/La record a transition from mantle partial melting to magmatic differentiation, during which Cu behavior was controlled by sulfide retention in the source and fluid activity, respectively. During partial lithospheric mantle melting, residual solid sulfide phases retain Cu in the mantle source, generating Cu-depleted melts with slightly heavier δ⁶⁵Cu and potentially contributing to the Cu depletion of continental crust. Subsequently, exsolved fluids preferentially scavenge isotopically heavy Cu, causing both Cu contents and δ⁶⁵Cu values to decrease in the residual magmas. The Cu isotopic evolution of these ultrapotassic magmas differs from that of ore-bearing porphyries in the belt, potentially reflecting differences in magma source or evolutionary history. This study shows that residual sulfides in the lithospheric mantle can retain copper during ultrapotassic magmatism, limiting its transfer to the crust, while fluid activity controls Cu redistribution in evolved magmas.