Provenance and Geochemical Variations in Shales of the Mesoarchean Witwatersrand Supergroup

Abstract

Major, trace, and rare earth element data from shales of the Witwatersrand Supergroup in South Africa were analysed to evaluate their provenance, weathering, and tectonic setting. The studied rocks were affected by minimal hydrothermal and biogenic modification. Geochemical analyses indicate that the Witwatersrand shales are compositionally highly variable, both laterally and within individual stratigraphic shale units, and are characterised by high Chemical Index of Alteration (CIA) values. High CIA, Chemical Index of Weathering (CIW) and Index of Compositional Variability (ICV) values suggest that the source area was subjected to prolonged intense chemical weathering associated with fluctuating palaeo-climate. The Zr/Sc and Th/Sc ratios revealed that Witwatersrand shales were derived from a mixed, predominantly mafic source area. Noteworthy enrichment in incompatible elements in some Witwatersrand formations indicates the presence of granitic sources. Tectonic discrimination diagrams based on immobile trace elements (Th-La-Sc, Sc-Th-Zr/10) and diagnostic trace element ratios (Th/Sc, Zr/Hf, La/Th, La/Sc) indicate that the West Rand Group source area was a passive margin, whereas the Central Rand Group shales were derived from an active continental margin, thus supporting a foreland basin setting. Geochemical signatures illustrated by Hf and Zr content show various cycles of crustal recycling that influenced source area sediment composition, diagenetic and provenance history.

Aquatic geochemistry of a major freshwater lake in the Kashmir Himalaya: solute acquisition and denudation process in the lacustrine system

“…6a). As discriminant function is based on most mobile major elements, the sedimentary and post-deposition phenomena including weathering, diagenesis, and metamorphism govern variability in the mobile elemental concentration, and hence, does not support to discriminant between samples from the key tectonic provenances (Garzanti et al, 2010;Hou et al, 2018;Nwaila et al, 2017). The Al 2 O 3 /TiO 2 ratios of the clastic rocks are principally used to deduce the source rock compositions (Rashid et al, 2015;Taylor & McLennan, 1985).…”

Recycling of Paleoplacer Gold through Mechanical and Postdepositional Mobilization in the Neoarchean Black Reef Formation, South Africa

“…All of the studied samples have very low CaO (!0.35 wt%) and Na 2 O (!0.6 wt%) concentrations, probably indicating the breakdown of plagioclase in the source region. The BRF shale is distinguished from the underlying Witwatersrand shales (Nwaila et al 2017) particularly by its high K/Na due to the possible addition of K.…”

“…All major-and trace-element data were normalized against the post-Archean Australian shale (PAAS) composite values of Taylor and McLennan (1985) because of the absence of reliable Archean reference data. Most of the recent literature also utilizes the PAAS reference for comparative reasons, rather than an Archean shale composite, because of the consistency of the former standard (Smith et al 2013;Nwaila et al 2017).…”

Data-Driven Predictive Modeling of Lithofacies and Fe In-Situ Grade in the Assen Fe Ore Deposit of the Transvaal Supergroup (South Africa) and Implications on the Genesis of Banded Iron Formations

“…The samples were analyzed for the following major elements and oxides: SiO 2 , Al 2 O 3 , FeO (total), TiO 2 , CaO, MgO, K 2 O, MnO and P 2 O 5 , all expressed in weight percent (wt%). An ARL Advant XP Sequential XRF (Thermo Fisher) instrument was used to analyze the bulk chemical composition of the samples (Nwaila et al, 2017). The analytical error for most elements is less than 0.10%, with the exception of MgO (0.50%).…”