Initial crystallization of monosulfide solid solution (MSS) from magmatic sulfide liquid occurs at about 1150˚C. For typical Ni-rich sulfides as found in komatiites, pentlandite ((Fe, Ni)9S8) first starts to exsolve from MSS at around 600˚C, and continues to exsolve down to below 250˚C, changing in morphology from intergranular blebs to fine lamellae with falling temperature Sulfide assemblage in komatiite-hosted deposits consist of intergrowths of pyrrhotite (Fe1-xS), pentlandite, pyrite (FeS2) and minor chalcopyrite (FeCuS). To understand the relationship between these phases we used optical microscopy, electron backscatter diffraction (EBSD) and microprobe analyses on samples from high and low grade metamorphic terrenes in Western Australia. The sulfide assemblages at low metamorphic grade preserve pockets of annealed “foam” textures inherited from original post-magmatic cooling, whereas sulfides that have been exposed to higher metamorphic grade and high strain retain high-temperature deformation textures. Pyrrhotite from both types shows deformation features (kink bands, subgrains and new grains) closely related to the presence of pyrite and pentlandite.
EBSD is used as a tool to understand crystalo-plastic deformation in both isotropic (pyrite and pentlandite) and anisotropic minerals (pyrrhotite). We succeed in producing for the first time EBSD maps on pentlandite grains. EBSD maps showed that the majority of the deformation is accommodated by pyrrhotite, followed by pentlandite and pyrite respectively. EBSD maps of pentlandite grain show the presence of internal subgrains and enable determination of active slip systems in all sulfide phases. Combined EBSD analysis and microprobe elemental mapping shows that some of the pentlandite exsolutions from pyrrhotite are closely related to low angle subgrain boundaries in pyrrhotite. Ongoing research is addressing the question of whether exsolution is governed by the deformation microstructures and if so, at what stage in the cooling and deformation history does exsolution takes place. Results are significant for the interpretation of tenor variations within nickel sulfide ores.