Origin of Fe–Ni–Cu (Co) sulfide and Fe–Ti oxide minerals in the c. 1.77 Ga dolerite dyke, Singhbhum Craton (eastern India)

Abstract

Dolerite dyke swarms are widespread within the Singhbhum Craton (eastern India) that emplaced from the Neoarchean to Paleoproterozoic era just after the stabilization of crust before c. 3 Ga. These dyke swarms are oriented in NE–SW to NNE–SSW, NW–SE to WNW–ESE, east–west and north–south directions. The WNW–ESE-trending c. 1.77 Ga Pipilia dyke swarm is sampled from the Satkosia area of Orissa state. The dyke shows a noticeable disparity in terms of the modal proportion and grain size of pyroxenes, plagioclase, Fe–Ti oxide minerals and texture across the trend. In places the primary silicates are altered to secondary hydrated mineral assemblages of amphibole, chlorite and sericite. The primary silicates are clinopyroxene (augite – Mg# = 65.7–82.6; En_37–48Fs_11–17Wo_36–41), orthopyroxene (clinoenstatite – Mg# = 68.5–78; En_63–70Fs_20–29Wo_4–5) and plagioclase (An_11–39Ab_44–82Or_1–7) and the Fe–Ti oxides are titanomagnetite (FeO, 34.38–39.50 wt%; Fe₂O₃, 48.26–56.21 wt%; TiO₂, 5.05–9.60 wt%) and ilmenite (FeO, 40.75–43.79 wt%; Fe₂O₃, 3.54–10.03 wt%; TiO₂, 47.82–50.87 wt%). Application of two-pyroxene thermometry yields an equilibration temperature range of 1065–978°C, and coexisting titanomagnetite–ilmenite pairs reveal 731.39–573.37°C under the oxygen fugacity (fO₂) condition NNO + 0.3 to FMQ − 1.03. The dyke contains disseminated sulfides at the interstices of Fe–Ti oxides and silicates. Major sulfide minerals are pyrite, chalcopyrite and vaesite; pyrite–vaesite assemblages occur in association with secondary silicate minerals. Pyrite grains contain variable concentrations of Co (0.01–5.70 wt%) and Ni (0.02–1.95 wt%). Coexisting vaesite contains Co (2.42–10.44 wt%), Ni (26.40–47.88 wt%) and Fe (7.32–26.55 wt%). Texture, sulfide–silicate assemblage and the presence of low metal/S sulfides such as the pyrite–vaesite assemblage indicate that primary Fe–Ni sulfides (pyrrhotite–pentlandite) that segregated from immiscible sulfide liquid at high temperature are modified by late magmatic/hydrothermal fluid activities. Numerous sulfide-bearing deposits hosted in ultramafic–mafic intrusions of Paleoproterozoic age have been recorded globally and the occurrence of Fe–Ni sulfides in the c. 1.77 Ga Pipilia dyke swarm in the Singhbhum Craton enhances the exploration potential of this craton in eastern India.