Paleoenvironmental reconstruction of the Griquatown Granular Iron Formation, South Africa, and its implications for the Great Oxidation Event

ABSTRACT The Paleoproterozoic Griquatown Iron Formation of South Africa is one of the few examples of a granular facies iron formation (GIF) from the sedimentary rock record preceding the Great Oxidation Event (GOE). The authors' previous research focused on the geochemical attributes of the Griquatown GIF, with an emphasis on the origin of manganese carbonates in its uppermost stratigraphic portion and their possible paleoenvironmental implications. In this study, the same rocks are revisited by closely examining the petrography, mineralogy, mineral chemistry and distribution of various types of granules that occur within the Griquatown strata. In comparison to sedimentologically similar facies from the modern and ancient sedimentary record, the granules in the Griquatown GIF are classified as ooids, peloids, microaggregates/microglobular clusters, intraclasts and stilpnomelane‐rich granules. The first three types are dominantly composed of Fe carbonates that occasionally preserve traces of a high‐Mg calcite precursor. They are interpreted to have formed under the influence of microbial activity in a low‐energy to high‐energy, carbonate‐supersaturated shallow‐marine environment. Intraclasts are composed of a variety of carbonates, Fe silicates, Fe oxide (mainly magnetite) and chert, and represent the reworking of other granules and rip‐up clasts during high‐energy events. Stilpnomelane‐rich granules point to a mixed chemical and volcanogenic deposition. The microglobular clusters resemble calcified coccoidal colonies of possible cyanobacterial origin, with alternating rings of high‐ and low‐Mn Mg‐calcite. These textural and chemical signatures are interpreted as possible evidence for the presence of the water‐oxidizing complex, a critical component in the evolution of oxygenic photosynthesis. It is hypothesized that a set of favourable environmental conditions—along with elevated concentrations of aqueous Mn 2+ and Ca 2+ in ambient waters during the late stages of Griquatown GIF deposition—may have provided an ideal chemical setting for biological innovation, which would have led to the evolution of the Photosystem II (PSll) metabolic pathway at the latest by the dawn of the Great Oxidation Event at ca 2.4 Ga.