Early fluid exsolution suppressed sulfide saturation in arc volcanic rocks from the Gangdese belt: Implications for Cu depletion in arc magmas and mineralization potential

Cu abundance in arc magmas is an important factor controlling the formation of porphyry Cu deposits. Globally, primitive arc basaltic magmas contain elevated Cu concentrations, which gradually decrease with magma differentiation. It is widely believed that this variation is due to sulfide saturation in magmas. The sulfides extracted the Cu and removed from the magma. However, our study found that the same gradual decrease in Cu content with magma differentiation occurs in sulfide-unsaturated arc magmas. The initial arc magmatism generated by the Neo-Tethyan subduction is characterized by water-rich and oxidized features and early aqueous volatile exsolution during magmatic differentiation. These exsolved fluids dissolve the S in the magma, resulting in a reduction in S content and inhibiting sulfide saturation. As elements such as Cu could also move into the fluids, the Cu content gradually decreases in the magma. These exsolved fluids, which contain elevated metal, also become a potential source for the formation of porphyry Cu-Au deposits.