Abstract The formation of magmatic Cu-Ni-Co-platinum-group element (PGE) sulfide deposits is dependent on mantlederived silicate magmas (komatiites and basalts) attaining sulfide saturation. Because the sulfur content of the upper mantle is considered to be low (250 ppm; McDonough and Sun, 1995), it is likely that, for moderate to high degrees of partial melting (≥25%), much of earth’s mantle-derived magmatism is sulfide undersaturated at the time of separation from the mantle residue (Morgan and Baedecker, 1983; Keays, 1995; Lesher and Stone, 1996). Thus, many models for the petrogenesis of giant magmatic sulfide deposits associated with mafic-ultramafic rocks propose that sulfide saturation and immiscible sulfide ore formation were a consequence of assimilation of crust or crustally derived sulfur into sulfide-undersaturated mafic-ultramafic magmas which transported base and precious metals from the mantle to the upper crust (see Naldrett, 1989, and Barnes et al., 1997a, for summaries). Constraining the sources of sulfur and metals in magmatic ore deposits is important for understanding the dynamic and potentially open-system behavior of their parental magmatic systems. This information can lead to improved or enhanced exploration strategies in prospective new terranes based on the recognition of key geodynamic processes of ore formation (Duke, 1990; Barnes et al., 1997a; Lambert et al., 1998a). Important tools that can be used in this regard are the stable and radiogenic isotope systems. The study of the sulfur isotope composition of magmatic sulfides is the best method of directly tracing the source(s) of sulfur in ore deposits. In many cases, S isotope data demonstrate that assimilation of crustally derived sulfur into sulfide-undersaturated mantle-derived magmas is an important step in the genesis of magmatic sulfide deposits