GEOCHEMISTRY OF PRETORIA GROUP (2.1-2.35 MA) SHALES, SOUTH AFRICA

The Pretoria Group, exposed in Southern Africa, is between ca. 2.1-2.35 billion years old. The deposition of its lower units overlapped a major Paleoproterozoic glaciation followed by the Great Oxidation Event. The slightly metamorphosed 7000m thick stratigraphic section is dominated by shale, sandstones, and diamictites and occurs within an ovoid intracratonic basin extending over 100,000 km2. Depositional models encompass two major epeiric seas and fluvial braid plains punctuated by volcanism. The geochemistry (in ascending stratigraphic order) of the Lower Timeball Hill (n = 120), Upper Timeball Hill (n = 124), Strubenkop (n = 55), and Silverton (n = 203) shale Formations were investigated. Compared to Upper Continental Crust (UCC), the Lower and Upper Timeball Hill shales are enriched slightly in Al2O3 and Fe2O3 while depleted in MnO, MgO, CaO, and NaO (likely due to a highly weathered source) whereas the Strubenkop shales are enriched in TiO2, Al2O3, and Fe2O3, and depleted in SiO2, MgO, CaO, and NaO. The Silverton shales closely follow the trends of the UCC, perhaps due to less source rock weathering. However, all units have relatively high Chemical Index of Alteration (CIA) values with distinctive trace element signatures greater than UCC averages. The Strubenkop shales are enriched in Cu, Ni, Mo, Cr, and V, possibly due to the weathering of sulfides associated with the underlying Hekpoort basaltic andesites and/or other mafic/ultramafics source(s). Organic carbon and sulfur concentrations are low and redox indicators (U/Th, V/Sc, Ni/Co, V/Cr) are dominantly oxic suggesting deposition in shallow, well-circulated water. Only the Silverton Formation shales have appreciable organic carbon (2.21%) and sulfur (1470 ppm). REE plots generally follow UCC trends but have strong to pronounced negative Ce anomalies with the Silverton shales' being the largest. The Lower Timeball Hill is most enriched in REE while the Upper Time ball Hill is most depleted. REE patterns are flat or have a slight slope (enrichment in LREEs, except Ce) when normalized to UCC. Various elemental and ratio plots suggest a dominantly evolved, felsic cratonic source with trace element concentrations indicating the admixing of mafic detritus for at least deposition of the Strubenkop Formation shales.