Abstract

Carbonate samples from exposures of the Pongola Supergroup in the Buffalo River gorge, South Africa, preserve high Sr contents and relatively low Mn concentrations. Some of the Buffalo River gorge samples have substantially higher Sr, lower Mn, lower 87Sr/86Sr and similar δ13C and δ18O compositions relative to previously reported Pongola carbonates from the White Mfolozi inlier.

δ13C values for the Buffalo River gorge samples do not display any variation whereas δ18O values vary by up to 5 per mil. These variations fall within the trends previously reported for carbonate lithologies from the Pongola Supergroup. The variations noted can not be explained by metamorphic decarbonation reactions but are consistent with fluid/rock exchange systematics involving a fluid such as meteoric water. Water-rock exchange modelling demonstrates that the Sr isotope composition of high-Sr carbonate samples is substantially less prone to change than those of carbon and, particularly, oxygen.

The Sr isotope system in some of the samples probably preserves original sea water signatures. Initial 87Sr/86Sr values at 3 Ga of ~0.70305 indicate formation from Archaean sea water with a large mantle component, consistent with formation of these carbonates in an open-sea environment.

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