Siliciclastic associated banded iron formation from the 3.2 Ga Moodies Group, Barberton Greenstone Belt, South Africa

Abstract

Most models proposed for banded iron formation (BIF) deposition are based on observations of well-preserved Late Archean and Paleoproterozoic BIF. Efforts to push the understanding gained from younger successions deeper in time have been hampered by the high metamorphic grades that characterize Early Archean BIF. This study focuses on a unique occurrence of well-preserved and contextualized BIF from the Early Archean (∼3.2Ga) Moodies Group, in the Barberton Greenstone Belt, South Africa. The Moodies BIF occurs thinly interbedded with fine-grained and cross-stratified sandstones, indicating deposition during times of decreased clastic sediment supply. In the Moodies BIF, chert is present as concretions, and is never observed in direct contact with the siliciclastic material but is always associated with iron minerals. This observation suggests that the processes leading to the formation of both chert and iron minerals were coupled. The dominant iron-rich minerals within unweathered Moodies BIF are hematite and magnetite, with less common occurrences of Fe–carbonate phases (mainly ankerite). Petrographic textures reveal that hematite constitutes an early mineral phase, while magnetite and ankerite display textures indicative of a late diagenetic or metamorphic origin. Carbonaceous particles are present in close association with the magnetite crystals. These C-bearing phases may be the preserved organic matter of microbes involved in the production of the ferric iron precursor phases, though it is difficult to rule out an origin from abiotic processes involving thermal decomposition of siderite to magnetite and organic carbon compounds. Nonetheless, the range of textures, mineralogies, and valence states supports the view that diagenetically-stabilized BIF mineralogies reflect the interaction of ferric iron phases with reducing fluids during diagenesis. These patterns are commonly observed in younger Archean and Paleoproterozoic iron formations, and imply a continuity of processes operating in the iron and silica cycles across both a range of paleoenvironments and long intervals of Archean time.