The Marathon copper and platinum-group element (PGE) deposit is hosted by the Two Duck Lake intrusion, located at the eastern margin of the Port Coldwell alkalic complex, northwestern Ontario. The Two Duck Lake intrusion is subdivided into an upper layered zone and a lower unlayered zone that consists of olivine gabbro and pegmatite with trace amounts of granophyre. The sulfides are hosted in the lower zone which is referred to here as Two Duck Lake gabbro. Petrography, mineral-chemistry, and geochemical data imply that Two Duck Lake gabbro crystallized in situ from a plagioclase crystal mush. There is no geochemical evidence for metasomatic alteration of the gabbro. The sulfides are disseminated in gabbro and pegmatite and consist of approximately equal proportions of chalcopyrite and pyrrhotite and minor amounts of pentlandite and pyrite. Samples of Two Duck Lake gabbro exhibit a significant correlation between sulfur and Ni, Ir, Rh, Pt, Pd, Au, and Cu. Further, the Pd/Ir and Pd/Ni ratios increase with increasing concentrations of Rh, Pt, Pd, Au, and Cu. Consequently, bulk sulfide compositions are best explained by the separation of a sulfide liquid from a gabbroic magma. This origin is contrary to that proposed by previous workers. They presented detailed petrographic and mineral-chemical data that imply inter-action of Cl-bearing hydrous fluids and sulfides at low temperatures. In the light of our study, we interpret these observations to indicate that elements migrated over very short distances and that bulk sulfides were not enriched with Cu and PGE, as previously proposed. A model for the genesis of the Marathon deposit is proposed.Many characteristics of the Marathon deposit bear a striking resemblance to observations reported in disseminated sulfide deposits at the base of the Duluth Complex, Minnesota. The features include mineral textures, abundances, and compositions; crystallization paths for the host gabbroic bodies; silicate-sulfide associations; trace element trends; and chalcophile element fractionation trends. The similarities imply that the Marathon deposit and deposits in the Duluth Complex formed by analogous processes.