DEEP CRUSTAL BARRIERS TO THE FLUX OF COPPER DURING ARC MAGMATISM

Understanding the mechanisms governing the flux of copper (Cu) and sulfur (S) during the genesis and evolution of arc magmas is critical to the refinement of models for the formation and distribution of porphyry Cu deposits (PCD’s). Although oxidized and water-rich arc magmas in subduction zones are critical drivers to the genesis of PCD’s, an increasing body of evidence indicates that they typically evolve towards low Cu contents during differentiation. The exact processes causing Cu depletion and the fate of the missing Cu during arc magmatism remain an open question. Here, we tackle this issue by studying deep-seated (~1.1 GPa) sub-arc cumulate rocks from the Acadian orogen (~410 Ma). These rocks represent the residues left behind during the differentiation of Acadian magmas, and thus provide a direct window into the early processes mediating the behavior of Cu and S during arc magmatism in the deep crust. We show that the Acadian cumulates contain abundant sulfides and are enriched in Cu (up to ~730 ppm), providing direct evidence for the formation of sub-arc Cu-rich cumulate reservoirs. Furthermore, their S and Hf isotope systematics reveal that the evolving magmas parental to the Acadian cumulates assimilated surrounding sulfidic and graphite-bearing metasedimentary crustal rocks in the deep crust. We demonstrate that crustal assimilation caused a strong decrease in the oxygen fugacity of the evolving magma, leading to sulfide saturation and the formation of Cu-rich cumulates. Our findings indicate that crustal assimilation processes in the deep crust can significantly limit the flux of Cu and S in evolving arc magmas. However, we hypothesize that Cu-rich cumulate reservoirs produced after assimilation-driven sulfide saturation may serve as key sources for the production of unusually Cu-rich magmas during subsequent tectono-magmatic events, thus underpinning the formation and distribution of giant PCD’s in arcs.