Nickel Sulfide Ores Related to Ultrabasic Intrusions in Canada

Abstract Canadian nickel sulfide ores related to ultrabasic rocks are divided into two categories based on geological environment. The Orogenic category is related to ultrabasic intrusions localized along or adjacent to major faults, whereas the Volcanic category is distributed throughout thick volcanic sequences as interflow sills and dikes. In both the orogenic and volcanic categories, nickel sulfide deposits occur as disseminations within the ultrabasic intrusion, as massive and stringer zones at the ultrabasic contact, and as massive and stringer zones within the adjacent wall rocks. The normal sulfide assemblage in most deposits is relatively simple, and comprises the primary minerals pyrrhotite, pentlandite, pyrite, chalcopyrite, and magnetite, usually in that order of abundance. Important exceptions are millerite, pentlandite, pyrite and chalcopyrite at Marbridge No. 2, Quebec and unusually high pyrite contents at Shebandowan, Ontario. Petrographic studies reveal a pattern of progressive recrystallization with serpentinization. Increased serpentine grain size with recrystallization leads to displacement of sulfide minerals and finally complete destruction of primary silicate-sulfide textures. In the Orogenic category, ultrabasic rocks are predominantly peridotite and pyroxenite; disseminated sulfide is the most common mode of occurrence, irregularly distributed throughout the ultrabasic intrusion; contact deposits are encountered less frequently and deposits in the wall rocks are rare. In the Volcanic category, ultrabasic rocks are predominantly peridotite with gabbro; contact deposits are most common; disseminated deposits are less common; and wall rock deposits are rare.