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Chlorite Thermometry in the Witwatersrand Basin: Constraints on the Paleoproterozoic Geotherm in the Kaapvaal Craton, South Africa

A series of post‐depositional alteration events can be distinguished in the late Archean Witwatersrand Basin. They range from thrust‐controlled metamorphism during pre‐Transvaal crustal thickening, burial metamorphism and hydrothermal infiltration during crustal thinning and associated deposition of the 2.6‐2.1 Ga Transvaal Supergroup sediments, thermal metamorphism related to the 2.06 Ga Bushveld Igneous Complex, and hydrothermal alteration as a consequence of the 2.023 Ga Vredefort impact event. The relative importance of these events varies locally, with different events responsible for the maximum temperatures achieved in different parts of the basin: the pre‐Transvaal metamorphism affected the northern margin of the basin, whereas the syn‐Bushveld thermal metamorphism is particularly pronounced in the lower parts of the Witwatersrand Supergroup. Chlorite geothermometry, in conjunction with fluid inclusion microthermometry and the thermodynamic modeling of equilibrium mineral assemblages, was applied to siliciclastic rocks to set constraints on the geothermal gradients. The best estimate for the steady‐state Paleoproterozoic geotherm in the Witwatersrand Basin is 27°C km−1. An increase of the geotherm to ∼40°C km−1 is indicated for the time of the emplacement of the Bushveld Igneous Complex and is explained by mafic underplating. Less than 30 myr later the geotherm had relaxed to the steady‐state value. Study of chlorites from a number of localities and stratigraphic horizons revealed that sudoite is a common and petrologically significant constituent of Witwatersrand siliciclastic rocks. The application of existing chlorite geothermometers to sudoite compositions yields erroneous results, emphasizing the fact that these calibrations may only be used for tri, trioctahedral chlorites.