The geochemistry of sedimentary rocks from the Fig Tree Group, Barberton greenstone belt: Implications for tectonic, hydrothermal and surface processes during mid-Archaean times

Abstract

The ca. 3.25Ga old Fig Tree Group in the southern part of the Barberton greenstone belt consists of deep- to shallow-water shale, greywacke, jaspilitic banded iron formation (BIF), and carbonaceous chert. The sequence is more than 1200m thick and crops out as tectonically duplicated, but stratigraphically distinct tectono-stratigraphic units. Chemical weathering of the source terrain of Fig Tree strata was minor. Instead, hydrothermal-metasomatic events affected the sedimentary rocks in the study area, resulting in the depletion of alkaline earth elements and K metasomatism. Provenance modelling using REEs and trace element ratios indicate varying contributions from ultramafic to mafic greenstones, TTGs and HREE-undepleted granites. A clear stratigraphic control on the composition, in the form of increasing trace metal contents and decreasing LaN/YbN ratios, reflects progressive increase of ultramafic and mafic volcanic rocks in the source. Banded iron formation shows REE-Y patterns consistent with its precipitation in a marine environment from Eu-enriched seawater. Carbonaceous cherts at the base of the Fig Tree Group do not represent chemical precipitates out of a hot Archaean ocean, but represent carbonaceous sediments that were silicified during low-temperature hydrothermal fluid emanations on the seafloor.