Possible origin of authigenic to early diagenetic magnetite through ‘Dissimilatory Iron Reduction’ (DIR) within Late Archean Banded Iron Formation from Chitradurga Schist Belt (CSB), Western Dharwar Craton (WDC), India

Late Archean Banded Iron Formations (BIFs) serve as exceptional reservoirs of primordial aquatic precipitates, offering a valuable window into the ancient ocean water chemistry and biogeochemical cycles that operated prior to the Great Oxygenation Event (GOE) around 2.4 Ga. It is generally believed that the primordial mineralogy of these BIFs was dramatically modified to Fe-oxides (magnetite and/or hematite) during subsequent hydrothermal and metamorphic episodes. While the earlier consensus does not clearly support an authigenic to early diagenetic origin for magnetite, some experimental studies suggest its stability within microbially influenced primary authigenic to early diagenetic environments. Despite being affected by several post-diagenetic alteration events, the central part of the Chitradurga Schist Belt (CSB) in the Western Dharwar Craton (WDC), particularly around the Chitradurga district, adequately preserves a wide array of primary mineral assemblages, with locally developed dispersed magnetite grains. Detailed petrographic observations supported by SEM-EDS analysis of the cherty Banded Iron Formation (BIF), stratigraphically positioned atop the shallow-water unstable shelf association of the Vanivilas Formation within the Chitradurga Group (3.0–2.6 Ga), offer a valuable opportunity to investigate the origin of these magnetite grains, their association with primary mineral assemblages, and their diagenetic modifications.

The primary mineral assemblages are present as submicron-scale lump-like structures (10–50 µm) embedded within silica (SiO₂) matrix, intervened by a network of silica-filled shrinkage cracks. Based on mineralogy and texture, three microfacies have been identified: a) silicate-carbonate-phosphate-bearing green lumpy microfacies (greenalite + siderite + apatite ± magnetite), b) silicate-oxide-bearing red lumpy microfacies (greenalite + hematite ± siderite), and c) silicate-sulphide-bearing black lumpy microfacies (greenalite + pyrite). Magnetite occasionally appears as a primary lump-forming mineral in the first microfacies, whereas in the second variety, it develops along the periphery of associated Fe³⁺-bearing mineral phases. The coexistence of euhedral-shaped, submicron-sized magnetite (1–5 µm) within these primary lumps, along with greenalite, suggests their origin through the reduction of a primary Fe³⁺-bearing oxy-hydroxide phase, formed in near-surface `oases' of O₂-rich seawater through cyanobacterial oxidation of hydrothermally sourced Fe²⁺. The reduction of this Fe³⁺-bearing oxy-hydroxide phase to form a metastable Fe²⁺-bearing hydrous green clay (greenalite) and more stable magnetite can occur either during settling through the water column or during authigenic to early diagenetic stages via dissimilatory iron reduction (DIR) at the sediment-water interface.

The possibility of DIR is further supported by textural evidence within silicate-oxide-bearing microfacies, where subhedral to anhedral magnetite is present along the periphery of these Fe³⁺-bearing lumps. The presence of Fe³⁺-bearing phases in the core reflects the signature of an incomplete reaction involving Fe³⁺ oxy-hydroxides and organic matter to form magnetite. Our findings reevaluate the debate over the origin of magnetite in Late Archean BIFs, suggesting that magnetite can form within biologically influenced microenvironments, even during authigenesis and/or early diagenetic stages.