Making magnetite late again: Evidence for widespread magnetite growth by thermal decomposition of siderite in Hamersley banded iron formations

The relationship between magnetite and the primary precipitates, which are preserved as greenalite nanoparticles and microgranules in chert bands, suggests that much of the magnetite formed in siderite-rich, iron-silicate bands that developed by compaction of non-silicified iron-silicate muds. These bands acted as reactive pathways for fluid flow and the formation of magnetite. If most of the magnetite formed after burial then it is necessary to reassess assumptions about the chemistry of the original precipitates and the role of microbial life in the redox cycling of iron. Our results are consistent with models for the deposition of BIFs that center around a precursor sediment comprising greenalite precipitates and diagenetic siderite that was partially replaced by magnetite and hematite after burial.