Understanding the behavior of chalcophile elements during the evolution of arc magmas is critical to refining models for the formation and distribution of porphyry copper deposits used in mineral exploration. Because magmas in continental arcs undergo copper depletion during their early differentiation, a widely held hypothesis posits that the removed copper is locked at the base of the crust in copper-rich cumulates that form due to early sulfide saturation. Testing this hypothesis requires direct evidence for such copper-rich reservoirs and a comprehensive understanding of the mechanisms driving sulfide saturation. Interaction between oxidized magmas and reducing crustal material in island arcs has been shown to be an efficient process causing sulfide saturation. However, the extent to which crustal assimilation impacts the flux of chalcophile elements during magmatism in thick continental arcs remains to be established. Here, we provide a deep perspective into these problems by studying a suite of subarc cumulate rocks from the Acadian orogen, New England (USA). These cumulates record the imprint of subduction zone magmatism and represent the residues left behind during the genesis of intermediate to evolved Acadian magmas (ca. 410 Ma). We find that the most primitive Acadian cumulates are enriched in copper (up to ~730 µg g–1) hosted by sulfide phases, providing direct evidence for the formation of lower crustal copper-rich reservoirs. The Acadian cumulates reveal a wide range of δ34S values, from –4.9‰ in the ultramafic rocks to 8‰ in the most evolved mafic rocks. The negative δ34S values observed in the most primitive and copper-rich cumulates (avg –3‰) reflect the assimilation of isotopically light sulfur from surrounding sulfidic and graphite-bearing metasedimentary rocks (δ34S of –19 to –12‰), whereas the more evolved cumulates with positive δ34S signatures may have formed from different magma batches that experienced less sediment assimilation. The assimilation of these reducing metasedimentary rocks caused a critical drop in oxygen fugacity (~DFMQ –2.5 to –1.9; FMQ = fayalite-quartz-magnetite buffer) in the evolving magmas, ultimately leading to extensive sulfide saturation and the consequent formation of copper-rich subarc cumulates. Assimilation-driven sulfide saturation may be a common process at the root of thickened arc crusts that triggers the formation of lower crustal copper-rich reservoirs, which play a pivotal role in the fate of copper during arc magmatism. Thus, deeply buried reducing metasedimentary crustal material at the base of continental arcs can act as a barrier to the magmatic flux of chalcophile elements and may play a crucial role in the genesis and distribution of porphyry copper deposits.