Formation of Core-Rim Magnetite with a Carbonaceous Core in Mid-Archean Banded Iron Formation from the Barberton Greenstone Belt, South Africa

Through the analysis of core-rim magnetite, we demonstrate that the core contains carbonaceous materials (CMs) derived from a 3.2-billion-year-old banded iron formation within the Barberton Greenstone Belt in South Africa. Using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy, we establish a direct association between these CMs and the magnetite. Although the possibility that CMs formed from the hydrothermal decomposition of siderite cannot be ruled out, several lines of evidence indicate a likely microbial origin for the CMs. Firstly, Raman spectroscopy reveals that the CMs exhibit characteristics of low-maturity biogenic organic matter (OM) featuring aliphatic carbon chains, which supports the notion that organic carbon compounds mature during burial metamorphism at temperatures below approximately 200 °C. Secondly, phosphorus and sulfur detected in the CMs suggest a microbial origin. Lastly, the formation of the unique texture of core-rim magnetite can be conceptualized as follows: Fe2+ is oxidized through anoxygenic photosynthesis, leading to the precipitation of ferrihydrite. This ferrihydrite is then transformed into magnetite by iron-reducing microorganisms. Subsequently, the magnetite grows larger through oriented attachment, which also confines OM. Ultimately, smooth magnetite rims may have preserved the OM for up to 3.2 billion years.