DISTRIBUTION OF YTTRIUM AND RARE-EARTH ELEMENTS IN THE PENGE AND KURUMAN IRON-FORMATIONS, TRANSVAAL SUPERGROUP, SOUTH-AFRICA

Shale-normalized rare-earths and yttrium (REY(SN); Y inserted between Dy and Ho) patterns for detritus-free samples from both the Kuruman an d Penge Iron-Formations (IFs) in the Late-Archaean to Early-Palaeoprot erozoic Transvaal Supergroup display pronounced heavy rare-earth eleme nt (REE) enrichment, and positive anomalies of La-SN, Eu-SN, Gd-SN, Y- SN, and Er-SN, but neither positive nor negative Ce-SN anomalies. Exce pting Ce-SN and Eu-SN anomalies, the Transvaal IFs yield all the featu res that are typical of the REY distribution in Modem seawater. (Eu/Eu )(SN) ratios in the Kuruman IF correspond to ratios observed in other IFs of similar age, whereas the Penge IF is characterized by distinct ly higher ratios. Within a sequence of eleven adjacent samples (each c omprising less than ten microbands) from the Kuruman IF, (Eu/Eu)(SN) ratios were found to vary significantly. Positive Eu-SN anomalies reve al the presence of a high-temperature hydrothermal component in Transv aal seawater. The absence of positive Ce-SN anomalies rules out the ex istence of an alkaline 'soda-ocean' with pH considerably above the Rec ent value of 8.2. Small-scale variation of (Eu/Eu)(SN) ratios within the Kuruman IF as well as alternation of iron- and silica-dominated la yers cannot be due to post-depositional modification of initially homo geneous material showing homogeneous REY distribution, because neither diagenetic nor metamorphic conditions were suitable for decoupling of Eu from the other REY. The observed small-scale variation may indicat e short-term variability of (Eu/Eu)(SN) ratios of Transvaal seawater, probably resulting from temporal variation of the activity of high-te mperature venting at the seafloor. Preservation of this feature in IF microbands and the presence of positive Y-SN anomalies suggest that IF precipitation from upwelling marine bottom waters in an oxygenated sh elf environment occurred very rapidly. Hence, REY adsorbed on the surf ace of iron-oxyhydroxide particles that eventually became Fe-rich IF m icrobands, were not in exchange equilibrium with REY dissolved in ambi ent seawater. Higher (Eu/Eu)(SN) ratios in the Penge IF compared to t he Kuruman IF suggest significantly more important REY input from high -temperature solutions to the REY budget of bottom waters in the Easte rn Transvaal than in the Griqualand West sub-basin. The REY distributi on in Penge and Kuruman IFs is compatible with a palaeogeographic sett ing which invokes the existence of a rather small basin in the northea st (the Eastern Transvaal sub-basin) in which spreading-related high-t emperature fluid-rock interaction occurred. The basin widened towards the southwest (the Griqualand West sub-basin) where it was connected t o the open ocean.