Abstract Organism-environment feedbacks in Precambrian platformal carbonates and reefs were strongly influenced by the activities of diverse microbial ecosystems. Microfacies studies of representative platformal microbial carbonates, comprising cyanobacterial mat, stromatolites and giant ooids, from the Late Archaean Ghaap Group of South Africa have provided compelling evidence for an intimate relationship between taphonomic evolution, fabric development and mineralogy in rocks of the Gamohaan and Boomplaas formations. Cements, both in fold hinges and between the limbs of slump-folded and contorted, partially-degraded, pyritiferous stromatolitic laminae, were precipitated after deformation of organic fabrics, but before or during their compaction, indicating that cementation took place at the same time as anoxic organic degradation involving bacterial sulphate reduction. Bundles and strands of the organic remains of filamentous cyanobacteria, in varying states of degradation in both stromatolites and ooids, have been preserved by mineralization. Structural detail is usually best preserved in calcite, where cyanobacterial sheaths, 10 μm to 25 μm in diameter and hundreds of micrometres in length, can be clearly seen. Petrographic analysis of the microfabrics using cathodoluminescence reveals dolomicrite nucleated along the outer margin of some sheaths. Dolomicrospar and dolospar fabrics developed progressively in association with increasing sheath degradation, as evidenced by the sequential loss of structural detail, culminating in a xenotopic fabric comprising brown, inclusion-rich, anhedral crystals with irregular boundaries. Biogeochemical modelling supports a genetic link between bacterial sulphate reduction and (1) calcite precipitation in the contorted laminae, and (2) replacive dolomitization of the calcific matrix in the stromatolites and ooids. The evidence indicates that anoxic organic diagenesis was an essential and major process in controlling carbonate precipitation and mineralogy in widespread microbialitic sediments of the Ghaap Group, a depositional environment analogous to many other Archaean, Proterozoic and, during periods of biotic stress, some Phanerozoic carbonate platforms.

Abstract This paper provides a critical review of advances made in understanding of sedimentary environments, geochemical processes, and biological systems that contributed to the deposition and diagenetic evolution of the exceptionally well-preserved and large iron formations of the late Neoarchean to very early Paleoproterozoic Ghaap-Chuniespoort Group of the Transvaal Supergroup on the Kaapvaal craton (South Africa) and the time equivalent Hamersley Group on the Pilbara craton (Western Australia). These iron formations are commonly assumed to have formed coevally but in separate basins, and they are often used as proxies for global ocean chemistry and paleoenvironmental conditions at ~2.5 Ga. However, lithostratigraphic and paleogeographic reconstructions show that the iron formations formed in a single large partly enclosed oceanic basin along the margins of the ancient continent of Vaalbara. Furthermore, although large relative to other preserved iron formations, the combined Transvaal-Hamersley basin is miniscule compared to marginal basins of the modern ocean system so that the succession probably documents secular changes in depositional environments of that basin rather than of the global ocean at the time. The iron formations comprise a large variety of textural and mineralogical rock types that display complex lateral and vertical facies variations on basinal scale. Based on detailed analyses of these variations it is concluded that the iron formations were deposited in environments that ranged from very deep-water basinal settings far below storm-wave base and the photic zone to very shallow-platform settings above normal wave base. Precipitation of both iron and silica took place from hydrothermal plumes in a dynamically circulating ocean system that was not permanently stratified. Ferric oxyhydroxide was the primary iron precipitate in virtually all of the iron formation facies. This primary precipitate is now represented by early diagenetic hematite in some of the iron formations. However, in both deep- and shallow-water iron formations most of the original ferric oxyhydroxides have been transformed by dissimilatory iron reduction to early diagenetic siderite and/or magnetite in the presence of organic carbon. Precipitation of ferric oxyhydroxides in very deep water below the photic zone required a downward flux of photosynthetically-derived free oxygen from the shallow photic zone. In these deep-water environments, under microaerobic conditions, chemolithoautotrophic iron-oxidizing bacteria may have played an important role in precipitation of ferric oxyhydroxides and acted as a source of primary organic matter. With basin fill even shallow-shelf embayments were invaded by circulating hydrothermal plume water from which ferric oxyhydroxides could be precipitated in oxygenated environments with high primary organic carbon productivity and thus iron reduction to form hematite-poor siderite- and magnetite-rich clastic-textured iron formations. Depositional models derived from the study of the iron formations along the Neoarchean-Proterozoic boundary can be applied to iron formations of all ages in both the Archean and later Paleoproterozoic. The facies architecture of the iron formations determines to a large degree the textural attributes, composition, and stratigraphic setting of high-grade iron ores hosted by them. Detailed facies information thus would assist in improving genetic models for high-grade iron ore deposits. Future research should be guided in this direction, especially in some of the very large iron ore districts of Brazil and India where very little is known about the composition and facies variations of the primary iron formation hosts and possible controls on localization of high-grade ores.