Copper-nickel sulfide blebs in disseminated sulfide ore in quartz diorite from the Frood mine, Sudbury, Ontario, Canada, show very marked variations in chemical composition. For blebs within one hand specimen the calculated proportions of pentlandite and chalcopyrite vary from 3 to 23 and 2 to 45 weight percent, respectively. Such hetero-geneity is inconsistent with direct formation from immiscible sulfide droplets in quartz diorite magma. Moreover, inclusions of quartz diorite minerals within the sulfide blebs strongly suggest bleb formation by a replacement process. The order of increasing resistance of quartz diorite minerals to replacement appears to be plagioclase, quartz, biotite, amphibole, chlorite, magnetite, and chromite. The physicochemical conditions of sulfide emplacement are defined as equivalent to middle greenschist facies by inclusions of metamorphic chlorite (ripidolite) within the sulfide and rims of chrysotile (filling contraction voids) around the bleb margins.