The origin of stilpnomelane lutite layers in Banded Iron Formations from the West Transvaal Basin, South Africa

Abstract

Banded iron formations (BIFs) provide a record of chemical changes in the oceans during the first accumulation of atmospheric oxygen during the Paleoproterozoic great oxygenation event (GOE). They frequently contain cm- to m-scale shaly intervals called stilpnomelane lutites that are of poorly constrained origin. Various hypotheses exist to explain their genesis, including: volcanism, sea-level variations and biological processes. In this study, I investigated the origin of lutites from the ~2.43 Ga Griquatown BIF in the Western Transvaal Basin, South Africa using samples obtained from drill cores. I conducted chemical analysis of bulk rock samples involving major- and trace element XRF and ICPMS measurements, mineralogical XRD measurements, (in)organic carbon and quadruple sulphur isotope analysis. The main mineralogy of stilpnomelane lutite consists of stilpnomelane, biotite, magnetite, quartz, siderite, ankerite and pyrite as well as various minor mineral phases. The samples are composed mainly of SiO2 and Fe2O3 (60-90 wt%) with lesser amounts of Al2O3, MgO, CaO and K2O. Shale normalized rare earth element and yttrium (REY) plots lack strong La and Ce anomalies while Eu anomalies vary. Large variability was also observed for the degree of LREE enrichment. The fluctuation in REY depends strongly on the mixing of the marine component with a felsic component. Trace element variability indicated that this component originates from distal felsic volcanism and that it represents ash fall following episodic eruptions. Quadruple sulphur isotope analysis, together with total organic carbon (TOC) contents, showed that microbial sulphate reduction was also influencing the deposition of stilpnomelane lutites. An increase of δ34S with decreasing age indicates that deposition occurred in a closed basin with Rayleigh fractionation governing the isotopic composition of sulphur. Mass independent fractionation (MIF) of sulphur isotopes indicate that the entire Griquatown BIF was deposited during the onset of the GOE when sulphur MIF continued to be produced or recycled in near-surface environments. Furthermore, variations in sulphur MIF slopes reveal temporal changes in Archean atmospheric chemistry.