The Lac des Iles Complex is a 2.74-Ga-old composite intrusion emplaced into a granite and/or granite-gneiss terrane of the Wabigoon subprovince, northwestern Ontario. The complex consists of a gabbroic and ultramafic part; the latter has been subdivided into northern and southern intrusive phases. Both the gabbroic and ultramafic parts contain platinum-group element-bearing Ni-Cu sulfide mineralization, although only the Roby zone in the gabbro is of economic importance.The southern ultramafic complex consists of a wehrlite to olivine-clinopyroxenite core, which is surrounded by websterite and gabbronorite cumulates. Petrographic and geochemical studies suggest that the northern ultramafic complex consists of at least eight cyclic units, which ideally are composed of dunite and wehrlite to olivine-clinopyroxenite cumulates at the base, followed by clinopyroxenite and websterite, locally grading into gabbronorite. This indicates similar crystallization sequences in both ultramafic centers: olivine, olivine + clinopyroxene, clinopyroxene, clinopyroxene + orthopyroxene + or - plagioclase.In the ultramafic complex, platinum-group element-bearing sulfide mineralization occurs at the top of cyclic units in websterite and orthopyroxenite cumulates. Sulfide-bearing samples (up to 1 wt % sulfide) are enriched in Au, Pd, Pt, and Cu relative to Ni, Ir, Os, and Ru, with Pd/Ir ratios on the order of 250. Sulfide immiscibility probably was caused by mixing of fractionated residual melt with more primitive liquid, which periodically intruded the magma chamber. Evidence of mixing is seen in gradational chemical variations across the interface of cyclic units and in the occurrence of inclusions of olivine crystals and crystal aggregates in websterite and gabbronorite cumulates. These inclusions are not in equilibrium with their present host rock and presumably formed before mixing in the more primitive magma. A mineralized orthopyroxenite cumulate at the top of cyclic unit 2 could not have been produced by the normal crystallization sequence. Consideration of the phase relationships in the ternary system olivine-clinopyroxene-plagioclase-quartz suggests that these cumulates are the crystallization product of a hybrid magma formed during magma mixing at the interface of cyclic units.The gabbroic complex consists mainly of largely altered gabbro and gabbronorite cumulates and is intruded by mafic-ultramafic dikes. The center of the gabbroic complex contains the economically important Roby zone, with its platinum-group element-bearing Cu-Ni sulfide mineralization. This is a lithologically and texturally very complex zone in which high-grade platinum-group element mineralization is associated with pegmatite and mafic-ultramaflc dikes. The Roby zone is characterized by strong enrichments of Au, Pd, and Pt relative to Ir and Os, and by variable but extremely high Pd/Ir ratios of greater than 10,000. High Au/Pt and Pd/Pt ratios indicate a fractionation of Pt from Au and Pd.It is suggested that the high volatile content of the residual magma of the gabbroic complex triggered partial remelting of surrounding gabbro cumulates, a process called "constitutional zone refining" by McBirney (1987). During the partial melting event, compatible elements such as Ir and Os are buffered by the residual silicate and oxide mineralogy, whereas incompatible elements such as Au, Pd, and Pt, as well as volatile components such as H 2 O and H 2 S, become highly concentrated in the partial melt phase. Eventually this newly formed magma may become sulfide-saturated, either by cooling and fractional crystallization or due to the continuous dissolution of sulfides when the zone of melting proceeds through the gabbro cumulates. Ratios such as Pd/Ir and Pd/Pt would strongly depend on the degree of remelting and fractional crystallization until the time of sulfide saturation. The matrix of the varitextured zone is interpreted as the residuum of this melting process, whereas pegmatoids and pegmatites, segregationlike gabbro phases, and quartz-bearing plagioclase- and incompatible element-rich dikes are regarded as the final crystallization products of the partial melting.