The geochemistry of the sedimentary rocks of the Pretoria Group, Transvaal Sequence

Abstract

A total of 1203 samples from all formations of the Pretoria Group, including mainly sedimentary rock samples, some volcanic rocks from the Hekpoort Formation and Machadodorp Volcanic Member as well as diabase sills, were taken from surface outcrops and drillholes in the main Transvaal basin. All samples were analysed for major and trace elements with XRF, 275 selected samples were analysed for boron with ICP-AAS, and 50 selected shale samples were analysed for REE, organic carbon and CO2 using liquid chromatography and LECO. The mineralogy of selected shale samples was established with XRD. The sedimentary and volcanic sample populations were treated statistically after differentiation in stratigraphic and regional sub-groups, and average element concentration estimates of the sub-groups were established. Generally, two distinct groups of shales can be differentiated geochemically and mineralogically, i.e. high-Al shales with a predominant kaolinite-mica clay mineral assemblage, and high-Mg-Ca-Na shales with a micaplagioclase mineral assemblage. The high-Al shales dominate the succession from the base of the Pretoria Group up to the Daspoort Formation, the high-Mg-Ca-Na shales occur mainly in the Silverton and post-Magaliesberg Formations. The shale samples from the Botswana sampling area are somewhat different from this stratigraphic trend, as they contain significant amounts of chlorite, which is only an accessory mineral in other parts of the basin. The geochemistry of the shales generally points to some regional differences, independent of the stratigraphic signature. The Pretoria Group shale estimate (640 samples) is enriched in Al, Th, Cr and Sc, and depleted in Mn, Na, Ca, Sr and some base-metals compared to average shale estimates. The Pretoria Group sandstone estimate (335 samples) is enriched in Cr, Ni and Fe, and depleted in Ca, Na, Ti and K compared to average sandstone estimates. The geochemistry of the sedimentary rocks of the Pretoria Group shows evidence for complex source terrains, with predominantly granitic and sedimentary sources, as well as, possibly, ultramafic sources. Transport fractionation seems to have influenced the evolving pattern to a certain degree. The syndepositional palaeoclimate is thought to have been humid-hot, at least for the middle part of the Pretoria Group (Hekpoort to Daspoort Formations). The basal part of the Pretoria Group has a similar geochemical pattern (i.e. points to a similar climate), but the possible introduction of reworked and redeposited weathering profiles, related to the depositional hiatus between the Chuniespoort and Pretoria Groups must be considered in any interpretation. Penecontemporaneous volcanic activity and proposed tectonic activity obviates an evaluation of syndepositional climate for the Post-Daspoort formations. The geochemical signature of Pretoria Group sedimentary rocks is indicative of a divergent margin tectonic setting (intracratonic sag, failed rift/aulacogen or passive margin). Asthenospheric upwelling related to a rift tectonic setting could provide a simple explanation for the composition of Pretoria Group volcanic rocks. The geochemistry of boron and REE point to a strong influence of postdepositional processes, namely diagenesis and/or hydrothermal alteration. Mineral assemblages and trace element patterns provide some evidence for local source areas, for at least the lower part of the Pretoria Group. The described geochemical pattern cannot solve the marine/lacustrine nature of the basin.