Distribution of yttrium and rare-earth elements in the Penge and Kuruman iron-formations, Transvaal Supergroup, South Africa

Abstract

Shale-normalized rare-earths and yttrium (REY SN; Y inserted between Dy and Ho) patterns for detritus-free samples from both the Kuruman and Penge Iron-Formations (IFs) in the Late-Archaean to Early-Palaeoproterozoic Transvaal Supergroup display pronounced heavy rare-earth element (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. Excepting Ce SN and Eu SN anomalies, the Transvaal IFs yield all the features that are typical of the REY distribution in Modern 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 distinctly higher ratios. Within a sequence of eleven adjacent samples (each comprising less than ten microbands) from the Kuruman IF, ( Eu Eu∗ ) SN ratios were found to vary significantly. Positive Eu SN anomalies reveal the presence of a high-temperature hydrothermal component in Transvaal seawater. The absence of positive Ce SN anomalies rules out the existence of an alkaline ‘soda-ocean’ with pH considerably above the Recent 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 layers cannot be due to post-depositional modification of initially homogeneous 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 indicate short-term variability of ( Eu Eu∗ ) SN ratios of Transvaal seawater, probably resulting from temporal variation of the activity of high-temperature 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 shelf environment occurred very rapidly. Hence, REY adsorbed on the surface of iron-oxyhydroxide particles that eventually became Fe-rich IF microbands, were not in exchange equilibrium with REY dissolved in ambient seawater. Higher ( Eu Eu∗ ) SN ratios in the Penge IF compared to the Kuruman IF suggest significantly more important REY input from high-temperature solutions to the REY budget of bottom waters in the Eastern Transvaal than in the Griqualand West sub-basin. The REY distribution in Penge and Kuruman IFs is compatible with a palaeogeographic setting which invokes the existence of a rather small basin in the northeast (the Eastern Transvaal sub-basin) in which spreading-related high-temperature fluid-rock interaction occurred. The basin widened towards the southwest (the Griqualand West sub-basin) where it was connected to the open ocean.