Magmatic Ni–Cu–Fe sulphide mineralization from the Halaguru area, Eastern Dharwar Craton, southern India

Here we report sulphide mineralization in a spinel-bear­ing orthopyroxene hornblendite from the Halaguru area, Eastern Dharwar Craton (EDC), southern India. The hornblendite contains approximately 3% olivine, 38% orthopyroxene, 56% amphibole and 2% oxide/sulphide phases. Petrographic and mineral chemistry studies show its formation as a result of the reaction between early formed olivine and hydrous silicate melt without any metamorphic imprint, despite its proximity to amphibolites–granulite isograd in the EDC. The sulphides comprise the disseminated form of pyrrhotite–pentlandite–chalcopyrite assemblage, which is well crystallized and the primary magmatic features are preserved as contact and granular textures. Absence of crustal xenoliths and lower SiO2 (48 wt%), Sr (35 ppm), Pb (<5 ppm), U (<0.1 ppm) and Th (0.10 ppm) content coupled with higher Mg# (~82), Ni (700 ppm), Cr (2500 ppm) concentration in the bulk sample rules out the possibility of sulphide saturation by the addition of crustal-derived sulphur. Based on the combined textural and compositional criteria, the pyrrhotite–pentlandite–chalcopyrite assemblage is a product of the cooling and crystallization of immiscible sulphide globules. The initial high monosulphide solution, upon cooling, exsolves to pyrrhotite and pentlandite and later, at lower temperature, intermediate sulphide solid solution gives rise to chalcopyrite which is dominantly present around the edges of the pyrrhotite–pentlan­dite assemblage.