Geological characterisation of the Bamboesberg Member within the Molteno-Indwe coalfield, Eastern Cape Province, South Africa

Abstract

The Bamboesberg Member forms the basal member of the Late Triassic Molteno Formation. It hosts the two economically significant coal seams in the Molteno-Indwe coalfield, namely the Guba and Indwe seams. The Molteno-Indwe coalfield is located mainly in the northern region of the Eastern Cape Province in South Africa. The Council for Geoscience (CGS) drilled thirteen vertical boreholes within the Molteno-Indwe coalfield between 1984 and 1986. These cores were retrieved from the National Borehole Core Depository in order to characterise the sandstones of the Bamboesberg Member. To achieve this objective, the cores were logged in detail and scanned using a hyperspectral imaging scanner. Additionally, sandstone samples were collected for physio-chemical analysis. The Bamboesberg Member consists of fining-upwards cycles of sandstones, interbedded with argillaceous units and, in certain areas, coal. The coal seams typically occur on top of the argillaceous successions. These sandstones are classified as sub-litharenite and are sourced from the recycled orogenic provenance. Through SEM-EDX analysis it was observed that the sandstones contain large euhedral detrital garnets and sub-rounded zircons. The coal samples from the Bamboesberg Member were assessed for their rare earth element potential. Most samples displayed a rare earth element concentration ranging from 100 to 200 ppm, while five samples exhibited a higher concentration of over 350 ppm. The minerals hosting the rare earth element were observed to be monazite, parisite, and xenotime. Three-dimensional (3D) modelling revealed that the Bamboesberg Member has a concave shape at its centre and the maximum thickness observed is 120 meters. It tapers off to the north, pinching out to 20 m north of Aliwal North. The Bamboesberg Member is more exposed in the western and central parts of the Molteno-Indwe coalfield. Consequently, the western and central portions of the coalfield have potential for further coal prospecting within the Molteno-Indwe coalfield.