The Hidaka Metamorphic Belt, in southeastern Hokkaido, Japan, provides insights into how magmatic sulfide deposits may form through magma mixing deep within arcs. Here, a near-complete cross-section of arc crust is exposed, with large mafic igneous complexes preserved at deeper levels. Magmatic sulfide mineralization occurs within the Opirarukaomappu Gabbroic Complex (OGC), which preserves a record of crustal contamination of mafic magmas via assimilation and magma mixing involving introduction of crust-derived tonalite. Assimilation–fractional crystallization modelling suggests that the gabbro evolved through a combination of approximately 10 wt % mixing and 14 wt % fractional crystallization. Magmatic sulfides and associated gabbros, diorites and tonalites at this locality contain graphite, with carbon isotope signatures consistent with derivation from the surrounding partially melted carbonaceous shales. This indicates that crust-derived carbon was added to the mafic magma through the magma mixing and assimilation process. Sulfur isotope data suggest that sulfur was also added from crustal sources during assimilation and magma mixing. The relationships observed in the OGC suggest that intrusion of basalt into a segment of deep arc crust drove partial melting of carbonaceous metamorphic rocks, producing graphite-bearing felsic magmas with high reducing potential. Redox budget modelling shows that mixing of only small proportions of these magmas is sufficient to lower the oxidation state of oxidized basaltic magmas enough to induce sulfide saturation and consequent exsolution of immiscible sulfide melt. Magmatic sulfide deposits are likely to form by this reduction-induced sulfide saturation mechanism deep within other arcs where magma mixing is thought to be common.