Native iron in the Palaeoproterozoic Chaibasa Formation: primary or secondary?

Abstract

AbstractNative Fe in the Chaibasa Formation may be the oldest native Fe ever found. The Chaibasa Formation contains pre-1.6 Ga offshore shelf sediments from the Singhbhum Craton in India. The sediments are light green in colour, consistent with a low content of organic matter and a metamorphic grade of greenschist–lower amphibolite facies. The native Fe grains are enclosed in fibrous mica. The native Fe occurs with magnetite crystals, evidence of relatively oxidizing conditions in the Chaibasa Formation during diagenesis. (Mn,Fe)S, a compound characteristic of reducing conditions, is found within the native Fe. The native Fe contains small amounts of native Si. Most grains of native Fe have rough edges. One bulbous grain has smooth edges and an internal wavy structure that is suggestive of a drop of melted iron. The melting temperature of pure Fe is 1535 °C. We also found native Fe with small amounts of carbon in it that appears to have a plate-like, pearlitic structure. Pearlite typically forms at temperatures of 723–500 °C. All of these temperatures are too high to be achieved during greenschist-facies metamorphism. As a result, we infer that the native Fe is primary rather than secondary. Our work and that of others suggests a primary, impact origin for the native Fe.