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 petrograph ical and chemical data was available, we selected samples which best r epresent the pure chemical sediment and still carry the primary rare-e arth 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 epsilon(Nd) (2.5 Ga) values more radiogenic than published values for epiclastic s ediments from the slightly older Black Reef Formation (-5.1 to -3.2). This suggests that REE and iron in the Transvaal Ifs were predominantl y, though not exclusively, derived from hydrothermal alteration of oce anic crust. The epsilon(Nd) (2.5 Ga) value of the Kuruman IF is simila r to that proposed for the Hamersley IF in western Australia, suggesti ng that epsilon(Nd)(t) values of +1 +/- 1 were typical of IF-depositin g open ocean waters at the Archean/Proterozoic boundary.