Origin of coexisting carbonates and iron oxides in the 2.42 Ga Gandarela Formation, Minas Supergroup, Gerais, Brazil

A detailed mineralogical, petrological and geochemical study of quartz and carbonate itabirites and massive carbonates from the lower Gandarela Formation has been performed to understand the evolution of the depositional environment that resulted in the facies changes seen today. Nano-hematite droplets, positive Eu and Y anomalies in both the quartz and carbonate itabirites and a negative Ce anomaly in the carbonate itabirite reveal precipitation from an oxygenated mixture of marine and hydrothermal fluids. In both itabirites, iron oxides aggregated into bands. Quartz itabirite underwent reductive diagenesis forming magnetite and promoting dissolution of iron oxides releasing Ce, thus losing its negative Ce anomaly with resulting diagenetic fluids precipitating ankerite and sideroplesite. Micritic-size dolomite (<10 mu m) and a lack of recrystalisation textures in the carbonate itabirite are interpreted as a primary precipitation resulting from higher ocean temperatures and an increased seawater Mg/Ca ratio interpreted due to diagenetic sideroplesite in the quartz itabirites. A cessation of hydrothermal fluids to the local area ended iron oxide precipitation but not dolomite.