A geochemical and micro‑textural comparison of basalt‑hosted chalcedony from the Jurassic Drakensberg and Neoarchean Ventersdorp Supergroup (Vaal River alluvial gravels), South Africa

摘要

South African basalt-hosted chalcedonies provided an ideal case study, to (a) achieve information about the origin of mineral-forming fluids, the process of chalcedony formation and weathering alteration and (b) to verify if the geochemical and micro-textural characterization of chalcedony allowed multiple provenances to be distinguished. For the first time, Drakensberg chalcedonies from the Karoo Jurassic basalts and Windsorton chalcedonies from the alluvial environment of the Vaal River (traversing the Allanridge basaltic-andesitic lavas) were investigated by optical microscopy, laser ablation inductively coupled plasma mass spectroscopy and isotope ratio mass spectrometry (O-16, O-18). The results showed that the compositional differences observed in Drakensberg and Windsorton chalcedonies could not be explained by changes in host rocks composition. The tholeiitic basaltic-andesitic lavas of the Allanridge Formation (Pniel Group, comparison term for Windsorton samples) and the tholeiitic basaltic-andesitic lavas of the Golden Gate Unit (Drakensberg Group, comparison term for Drakensberg specimens) proved to be very similar, also in terms of alteration degree. Conversely, both geochemical and oxygen isotope compositions clearly supported a relatively low temperature, hydrothermal origin for all the investigated chalcedonies, further indicating that differences mostly occurred during their formation (fluid circulation). Moreover, several characteristics suggested deposition in non-equilibrium conditions, although oxidizing conditions were most likely. Weathering processes were effective in the alluvial environment only, leading to a wider compositional heterogeneity of Windsorton chalcedonies with respect to Drakensberg samples. Lastly, a correlation between color and chromophores (Fe and Mn) amounts was lacking while a correlation between the geochemical composition and the texture was clearly observed in several specimens.