Vanished evaporites and carbonate formation in the Neoarchaean Kogelbeen and Gamohaan formations of the Campbellrand Subgroup, South Africa

Field, petrographic and stable isotopic evidence indicate the former presence of widespread evaporites in the Neoarchaean Campbellrand Subgroup of South Africa. Calcitization of the vanished but once laterally-extensive evaporites was apparently driven by bacterial sulphate reduction of solid sulphate in association with organic diagenesis and pyrite precipitation within platform-wide microbialites and sapropels. This counters current interpretations that much of the calcite was precipitated directly on the seafloor or in primary voids in open marine conditions controlled by regional seawater chemistry. Rather, large-scale microbial mediation of ambient waters across a shallow to emergent platform raised carbonate alkalinity and removed kinetic inhibitors to carbonate formation. The low preservation potential of Precambrian solid sulphate is related in part to bacterial sulphate reduction within the microbially-dominated ecosystems of which cyanobacteria were a major component. Evidence for the former presence of solid sulphate in shallow Neoarchaean seas includes pseudomorphs after selenite, also recorded from the contemporaneous Carawine Dolomite of Australia, together with rock fabrics and textures typical of evaporite dissolution. Importantly, sulphur isotopes of pyrite samples from the Cambellrand carbonates show a wide range of values indicating biogenic fractionation of sulphate, a signature also seen in the Neoarchaean Belingwe Greenstone Belt of Zimbabwe, and the Mt McRae and Jeerinah shales of Western Australia. Mass microbial colonization across extensive Neoarchaean epeiric seas witnessed the microbiogeochemical transformation of the Earth's hydrosphere, atmosphere and biosphere. The consequences for a reducing ocean would be the progressive oxidation of the major dissolved species in surface seawater, most notably of reduced sulphur and iron. Cyanobacterial photosynthetic oxidation of surface seawater drove formation of aqueous sulphate and permitted the precipitation of extensive evaporites in restricted basins, perhaps beginning the process of ridding the oceans of reduced sulphur. The first dramatic explosion of carbonate precipitation can be related to intense bacterial sulphate reduction in association with anoxic organic diagenesis and pyrite formation within the decaying interiors of microbialites and in sapropels. (c) 2005 Elsevier Ltd. All rights reserved.