A high-resolution geochemical analysis of the Kuruman Banded Iron Formation: The identification of significant meter-scale variability in REE+Y

Abstract

A geochemical analysis was performed on the Kuruman banded iron formation (BIF) from the Griqualand West Basin in South Africa, in order to study the depositional environment and controls on BIF deposition. This study was supplemented with a preliminary spectral analysis to identify a possible Milankovitch cyclicity in the Kuruman formation. The Kuruman BIF (~2.46 Ga) was deposited pre-GOE (Great Oxygenation Event) and provides a window into Pre-Cambrian conditions on Earth. High-resolution sampling (1 meter-scale) was performed on the lower half of the UU BH-1 core, which comprises the lower half of the Kuruman formation. An optimised carbonate extraction scheme was applied on the samples in order to more closely study this fraction, as it has been found to most closely resemble a primary seawater signal. The Kuruman formation is characterised by an absence of Ce-anomalies, positive Eu-anomalies and Y/Ho values, and low abundances of continentally derived elements (i.e. Al, Zr and Ti). These observations indicate that the Kuruman formation was deposited in a highly reducing environment, with a strong hydrothermal component and limited input of continentally derived material. A gradual shift from an oxide dominated lower half of the core to a carbonate dominated upper half reflects the shallowing of the depositional setting of the Kuruman formation. Rare-earth elements + Yttrium (REE+Y) and Al concentrations were highly variable at the meter-scale and were found to show a periodicities of 2.8 and/or 3.7-3.9 meters, along with periods of 4-6 and ~9 meters. The periodicities observed in the elemental data, along with the dependency of BIF deposition on climatologically controlled factors such as nutrient availability and organic productivity, leads me to conclude that climate exerts a primary control on BIF deposition in the form of Milankovitch cycles.