The use of X-ray tomography in defining the spatial distribution of barite in the fluvially derived palaeosols of Vaalputs, Northern Cape Province, South Africa

Abstract

The distribution pattern of barite in large (15×10×5cm) undisturbed aggregates of calcite/clay laminae from a Petric Durisol was investigated using micro-computerised tomography (micro-CT). The petroduric horizons are part of truncated palaeosols which have formed in the Vaalputs sediments located in the Northern Cape Province of South Africa. In these soils clay and calcite laminae encase large polygonal petroduric units. The distribution pattern of barite in relation to the calcite/clay laminae can be used as an indicator to determine the sequence of barite and calcite formation. Samples from two typical profiles were collected for micro-CT analysis. The 2D and 3D visualisation of undisturbed aggregates show an intricate vein network of barite, which runs in a predominantly vertical direction. The barite is intimately associated with the calcite/clay laminae and largely follows the topography of the wavy laminae. In certain instances, along the interface between the petroduric matrix and the laminae, the barite accumulations appear to have wedged the laminae apart. This, together with the unlikelihood that the intricate vein networks of barite would be able to remain intact during the crystallisation of calcite, suggests that barite accumulation in the laminae occurred at the same time or after the calcite laminae were formed.