The nickel deposits of Mount Prospect are small concentrations of pyrite, pyr-rhotite, pentlandite, and chalcopyrite. The ores for the most part are sulphide-rich facies of olivine norite, quartz norite, and hypersthene pyroxenite intruded in the order named, but at certain deposits wall rocks and xenoliths also contain sulphides. Field and laboratory studies indicate that the sulphides formed by fracture filling and metasomatic replacement of silicates after intrusion of a series of dikes which cut the ore-bearing rocks. The deposits are therefore regarded as hydrothermal, rather than magmatic or late magmatic as previously classified. Hornblende and biotite, developed in the host rocks prior to sulphide deposition, probably are due in considerable part to alteration during an initial stage of mineralization.