The 2.7-b.y.-old Abitibi greenstone belt is a crudely S-shaped zone of metamorphosed volcanic, subvolcanic, and sedimentary rocks and associated intrusions. Through it are scattered a dozen small nickel deposits in volcanic peridotites. Their hosts are volcanic and subvolcanic ultramafic rocks typically containing 40 to 45 percent anhydrous MgO, with spinifex tops and overlain by thinner peridotitic to pyroxenitic flows. At least one deposit (Dumont) is hosted by a dunitic intrusion; others (e.g., Dundonald, Langmuir 2) are partially hosted in sulfidic iron-formations.Mining of the Langmuir 2 deposit permitted extensive underground mapping, core logging, and sample collecting. The other deposits are known mainly from poor surface exposures and drill core.The Langmuir 1 and 2 deposits provided sufficient data to evaluate models for their origin and to compare them to nickel sulfide deposits associated with volcanic peridotites in Western Australia. A magmatic origin is supported by the localization of the main ore zone in a paleotrough, by the presence of a possible volcanic feeder beneath the ore, and by the regular sequence through the basal ultramafic flow of massive ore overlain by net-textured ore, then by barren peridotite, and, lastly, by a spinilex-textured cap to the flow. An unusual spinilex-textured and sulfide-bearing rock and skeletal chromite also support a magmatic origin, as do the base and precious metal contents of the ores.The high S/Se ratios (averge 23,000) and slightly lower sulfur isotopic ratios (average +0.5 per mil delta 34 S) compared to most magmatic nickel sulfide deposits are similar to the ratios for Windarra, Western Australia, and may be explained by assimilation of sulfide-rich cherty iron-formation.Other features incompatible with a purely maginatic origin include the presence of sheared and breccia ore, and the high sulfur content of some ores. These features result from deformation and greenschist facies metamorphism, but surviving features of an earlier magmatic history argue against the ores being formed metamorphically. Evidence of contemporaneous fumarolic activity was found at the south end of the mine, but its importance as an ore-forming process was not supported by detailed petrological and geochemical studies.Limited data from the other deposits in the Abitibi belt suggest a common origin for the Langmuir deposits with minor variations attributable to the volume of ultramafic magma emplaced, the volume of contained nickel sulfides, the presence of sulfidic iron-formations, and the severity of alteration and metamorphism. The deposits in Western Australia probably were formed by similar processes. In particular, the Langmuir deposits display strikingly similar sulfide chemistries to the Windarra deposits, which are probably dae to assimilation of sulfide-rich cherty iron-formation.