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.