The occurrence and origin of pentlandite-chalcopyrite-pyrrhotite loop textures in magmatic Ni-Cu sulphide ores

Pentlandite can sometimes form as the result of peritectic reaction between early formed monosulphide solid solution (MSS) and residual Ni-Cu–rich sulphide liquid during differentiation of the sulphide melt. This suggests that at least some loop textures may be genuinely magmatic in origin. Microbeam X-ray fluorescence mapping has been used to image pentlandite-pyrrhotite-chalcopyrite intergrowths from a range of different deposits including slowly cooled magmatic environments (Nova, Western Australia; Sudbury, Canada), globular ores from shallow-level intrusions (Norilsk, Siberia), and extrusive komatiite-hosted ores from low and high metamorphic-grade terranes. Laser ablation-inductively coupled plasma-mass spectrometry analysis of palladium in varying textural types of pentlandite was also carried out. Pentlandite forming coarse granular aggregates, together with loop-textured pentlandite where chalcopyrite also forms part of the loop framework, consistently has the highest Pd content compared with pentlandite clearly exsolved as lamellae from MSS or pyrrhotite. This is consistent with granular and loop pentlandite being formed by peritectic reaction between Pd-rich residual sulphide liquid and early crystallised MSS. The wide range of Pd contents in pentlandite reflects a continuum of processes between peritectic reaction and grain boundary exsolution. The presence of loop textures can be taken as evidence of a lack of penetrative deformation and remobilisation at submagmatic temperatures.
Pentlandite can sometimes form as the result of peritectic reaction between early formed monosulphide solid solution (MSS) and residual Ni-Cu–rich sulphide liquid during differentiation of the sulphide melt. This suggests that at least some loop textures may be genuinely magmatic in origin. Microbeam X-ray fluorescence mapping has been used to image pentlandite-pyrrhotite-chalcopyrite intergrowths from a range of different deposits including slowly cooled magmatic environments (Nova, Western Australia; Sudbury, Canada), globular ores from shallow-level intrusions (Norilsk, Siberia), and extrusive komatiite-hosted ores from low and high metamorphic-grade terranes. Laser ablation-inductively coupled plasma-mass spectrometry analysis of palladium in varying textural types of pentlandite was also carried out. Pentlandite forming coarse granular aggregates, together with loop-textured pentlandite where chalcopyrite also forms part of the loop framework, consistently has the highest Pd content compared with pentlandite clearly exsolved as lamellae from MSS or pyrrhotite. This is consistent with granular and loop pentlandite being formed by peritectic reaction between Pd-rich residual sulphide liquid and early crystallised MSS. The wide range of Pd contents in pentlandite reflects a continuum of processes between peritectic reaction and grain boundary exsolution. The presence of loop textures can be taken as evidence of a lack of penetrative deformation and remobilisation at submagmatic temperatures.