Sources of rare-earth elements and iron in paleoproterozoic iron-formations from the transvaal supergroup, South Africa: Evidence from neodymium isotopes

Neodymium isotopic data are reported for pure chemical sediments from the Siderian Kuruman and Penge Iron-Formations (IFs) in the Transvaal Supergroup, South Africa. From a large sample set for which petrographical and chemical data was available, we selected samples which best represent the pure chemical sediment and still carry the primary rare-earth element (REE) distribution for isotope analyses. Both the Kuruman IF (+1.9 and +1.1) and the Penge IF (+0.2 and -0.2) show ε_Nd (2.5 Ga) values more radiogenic than published values for epiclastic sediments from the slightly older Black Reef Formation (-5.1 to -3.2). This suggests that REE and iron in the Transvaal IFs were predominantly, though not exclusively, derived from hydrothermal alteration of oceanic crust. The ε_Nd (2.5 Ga) value of the Kuruman IF is similar to that proposed for the Hamersley IF in western Australia, suggesting that ε_Nd(t) values of +1 ± 1 were typical of IF-depositing open ocean waters at the Archean/Proterozoic boundary.