Unusual speleothem formation in the Thabazimbi mine cave, Limpopo Province, South Africa, and its chronology

An unusual cave is located in the hills southeast of the town of Thabazimbi in the Limpopo Province. This cave, a V-shaped shallow chamber measuring 110 × 40 m, was discovered serendipitously when an adit was driven into the hill in connection with iron ore mining. It has no natural entry point and no externally derived sediment is found in it. It is situated within the upper part of the Malmani Subgroup and although it was formed as a solution cavity, requiring a phreatic environment, dripwater flow must subsequently have been low: no large speleothems (stalactites, stalagmites or flowstones) were ever formed in the cave. Instead, evaporation produced crystallictitic frostwork bushes consisting of aragonite (metastably grown due to enhanced Mg-content) which were in part redissolved followed by the secondary formation of calcite corals and encrustations. This sequence of events can be understood in the context of an enclosed cave where the CO2 content of the air is high. U/Th disequilibrium dating of two aragonite frostwork samples from the cave yielded two very different ages (155.1 ± 7.4 and 67.2 ± 3.4 ka), thus showing that the formation of this type of frostwork occurred (probably episodically) over a long period of time. The two dates broadly coincide with the waning stages of periods of low total inorganic carbon (TIC) as well as high clastic input in the Tshwaing crater sediment record, both indicating enhanced rainfall at that locality. The younger age coincides with a brief negative excursion in chlorine content and a positive potassium anomaly at Tshwaing, both also indicating high rainfall. However, both ages are also coeval with glacial maxima, thus underlining earlier observations from the Tshwaing record that in southern Africa, wet periods are not necessarily coeval with northern hemisphere interglacials.