The paleoenvironmental implications of pre-Great Oxidation Event manganese deposition in the Mesoarchean Ijzermijn Iron Formation Bed, Mozaan Group, Pongola Supergroup, South Africa

The Ijzermijn Iron Formation Bed occurs in the Vlakhoek Member of the basal Sinqeni Formation of the Mesoarchean Mozaan Group of the Pongola Supergroup of southern Africa. It represents the oldest known supracratonic Superior-type iron formation that has significant manganese enrichments. Lithostratigraphic correlation between the Mozaan Group and the Witwatersrand Supergroup indicates that there was a larger Witwatersrand-Mozaan depositional basin on the Kaapvaal Craton during the Mesoarchean. Detailed stratigraphy, mineralogy, petrography, carbonate mineral chemistry, major and trace element geochemistry and carbonate oxygen and carbon isotopes are presented for two drill cores intersecting the Ijzermijn Iron Formation Bed. Drill core PMH 24, which represents a more coast-distal depositional setting, is characterized by manganese-rich siderite, higher Fe₂O₃ and lower MnO and Al₂O₃ contents, more prominent europium anomalies, lower Gd_SN/Yb_SN values and less depleted δ¹³C_PDB values. Drill core DDN 1, which represents a more coast-proximal depositional setting, is characterized by iron-rich rhodochrosite, lower Fe₂O₃ and higher MnO and Al₂O₃ contents, less prominent europium anomalies, higher Gd_SN/Yb_SN values and more depleted δ¹³C_PDB values. The presence of early diagenetic magnetite, hematite and ¹³C-depleted carbonates indicate the iron and manganese, which were sourced from hydrothermal plumes, were deposited by oxidation, with subsequent diagenetic reduction by organic carbon. The most efficient oxidative mechanism would have been biological oxidation. The increase in manganese contents towards the paleocoastline further supports manganese oxidation as it was redox buffered by Fe²⁺ in the water column and could only be deposited and preserved once Fe²⁺ had been removed through oxidation further away from the distal hydrothermal plume source. This is supported by previously published iron isotope data, where δ^56/54Fe values are more depleted closer to the paleocoastline. Deposition of iron and manganese was restricted to below the photic zone. This implies that microaerophilic chemolithoautotrophs were responsible for iron and manganese oxidation. This requirement of free oxygen in the Ijzermijn Iron Formation Bed has been supported by earlier molybdenum isotope studies. The presented work therefore further supports the idea of pre-Great Oxidation Event marine oxygen oases that did not lead to any appreciable build-up of oxygen in the atmosphere.