Summary
This study presents the first documented granular iron formation (GIF) of Archaean age and provides textural and geochemical evidence that it formed through microbial iron oxidation in shallow marine environments. The GIF exhibits distinctive oncoidal structures with laminated domal magnetite rims around chert cores, with positive δ⁵⁶Fe values consistent with incomplete oxidation of hydrothermal Fe²⁺ by iron-oxidizing bacteria. The distinct Fe isotope signatures between the shallow oncoidal GIF and deeper-water interbedded iron formations indicate different Fe²⁺ sources and biogeochemical pathways, providing insight into early Earth ocean chemistry and microbial metabolic activity.
UK applicability
This research is palaeontological and geochemical in nature, addressing early Earth geobiology rather than contemporary farming systems or soil health. It has no direct applicability to UK agricultural practice or policy.
Key measures
Iron isotope compositions (δ⁵⁶Fe values), mineralogical texture analysis, trace element contents (Al, silica, Fe), petrographic and geochemical characterisation of oncoid structures
Outcomes reported
The study documented textural and geochemical evidence of microbial iron oxidation in Archaean-age granular iron formations from the Pongola Supergroup. Iron isotope compositions and domal laminated textures in oncoidal magnetite rims suggest incomplete oxidation of hydrothermal Fe²⁺ by iron-oxidizing bacteria in shallow, wave-base environments.
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