Pb, Sr and Nd isotope systematics of uranium mineralised stromatolitic dolomites from the proterozoic Cuddapah Supergroup, south India: constraints on age and provenance

The Pb, Sr and Nd isotopic compositions were determined on uranium mineralised and barren stromatolitic dolomite samples from the Vempalle and Tadpatri Formations of the Cuddapah Supergroup in southern India. Subsamples from individual handspecimens were analyzed to minimize the potential of heterogeneous initial Pb compositions. All of the U-mineralised samples gave Pb–Pb ages close to 1800 Ma but with high MSWD values, presumably due to local redistribution of U and Pb in the samples. The best Pb–Pb age of 1756±29 Ma is interpreted as the time of U-mineralisation and as a minimum age for carbonate sedimentation and dolomitisation within the Cuddapah Supergroup. The Sr isotopic compositions of the samples vary with the mineralised samples being more radiogenic. Even the lowest 87Sr/86Sr of the barren sample exceeds the seawater 87Sr/86Sr about 1800 Ma ago. On the other hand, the Nd systematics of the samples does not appear to have been disturbed by processes subsequent to carbonate sedimentation. The calculated initial Nd isotopic compositions suggest a dominant continental crustal source that was at least 600 Ma old at the time of deposition of the carbonate sediment.