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Genetic model for a residual brick clay deposit, western Cape Province, South Africa

Department of Geology, University of Pretoria, Pretoria 0002, South Africa. e-mail: htheart{at}postino.up.ac.za

The Joostenberg kaolin deposit is a residual clay overlying Malmesbury Group strata and is located on the farm Joostenberg Vlakte 724, some 12 km northeast of Bellville. By means of field observations and inference derived from the chemical analyses of clay and its precursor rock types, it is suggested that this deposit, which is suitable for brick making, formed as a result of two independent geological processes. The intrusion of the approximately 550 to 510 Ma syn- to post tectonic granitoids of the Cape Granite Suite into micaceous, feldspathic shale of the Malmesbury Group created a hydrothermal vein structure. These veins developed along pre-existing zones of weakness such as fault or shear zones. Hydrothermal fluids migrating along these fractures were initially dominated by magmatic water and later by meteoric or seawater. The hydrothermal fluids were also responsible for extensive alteration of the surrounding shale but the original rock fabric survived. The second geological process comprised a phase of in-situ weathering between the Cretaceous and Neogene Periods, resulting in the development of a deep soil profile and the formation of an iron-rich layer (ferricrete) at surface.