A chemical, mineralogical and petrographic study of the Koppies bentonite deposits within the Volksrust Formation, South Africa

Abstract

The Koppies bentonite deposits are hosted within the Permian Volksrust Formation, Ecca Group, Karoo Supergroup, deposited in a deep to shallow marine environment, while the locally associated Greenlands Formation represents a deformed remnant of the Vredefort impact (ca. 2.02 Ga). Fieldwork suggests that the Greenlands Formation, comprising amphibolites, komatiites, actinolite-chlorite schists, dykes and sills, did not weather in-situ (as suggested by some authors) to form bentonite due to the lack of allophane material. It rather served as a palaeo-high, assisting in structural control of the deposits. Low energy mechanisms transported pyroclastic ash into palaeo-low embayments within the Greenlands Formation. The source and alteration process responsible for the formation of the Koppies bentonite deposits was uncertain, primarily due to ambiguity in analytical results obtained in previous studies. This study aims to elucidate how these deposits were formed and incorporate this understanding into a basic exploration model. Samples obtained from the Blaauwboschpoort pit, Oceaan pit and overlying shales of the Koppies bentonite deposits underwent mineralogical, geochemical and optical investigation. Mineralogical results suggest that smectite clay accounts for up to 99% of the samples with minor proportions of quartz, mica, chlorite/kaolinite, plagioclase, calcite and K-feldspar. Geochemical analyses suggest a felsic, trachyandesitic ash, derived from an intra-plate granitic/anorogenic setting. Small concentrations of zircons and apatite were recovered with the majority of zircon grains occurring as euhedral, small, oscillatory zoned and colourless grains, possibly illustrating a singular source. Zircons with minor rounded edges indicated some transport, while a few large, zoned, rounded grains likely reflect external inherited detritus. A comparison between the geochemical signature of the Koppies bentonites and other known Gondwanan lithologies showed similarities to the Choiyoi Group volcanics located in South America, which is believed to have supplied the ash that lead to the neoformation of the Koppies bentonite. Strong comparative evidence is seen in age (265 – 251 Ma), mineralogy, chemistry and location. The lack of opal-CT within the bentonite also points to a lower thermal gradient between the water and tuffaceous ash, which occurred due to aeolian transport from a distant volcanic event. Aeolian processes involved both high-altitude drift and dominant prevailing tropospheric winds, which kept large quantities of ash particles in suspension over a long distance. The low energy marine environment of the Main Karoo Basin would have provided favourable conditions for ash to settle out of suspension and deposit on the floor of the laterally extensive basin controlled by palaeo-lows and embayments. The interaction of initial seawater and subsequent meteoric water within the ash resulted in leaching of Si, K and Na, and enrichment of Ca and Mg. Favourable factors for secondary supergene alteration and diagenesis of the felsic ash is evidenced by a strong weathering signature within the chemical index of alteration graph. A stratigraphic interval within the Volksrust Formation units needs to be defined. The geographic position along current drainage lines, with shallow water tables to promote chemical alteration, is a crucial factor in localised bentonite formation. Such parameters can be considered as integral in further exploration. Older lithologies, which created palaeo-highs with accompanying palaeo-lows acted as a suitable embayment for ash accumulation, alteration and preservation and in-situ formation of bentonite.