C, Sr and Cr isotope constraints on the depositional environment and palaeo-redox of the Neoarchean Fortescue Group, Pilbara Craton, WA

Abstract

The Tumbiana and Hardey Formations of the Fortescue group in the northern Pilbara Craton are Neoarchean sedimentary rocks dated at 2.7 Ga, with questionable palaeo-depositional environments ranging from possibly marine to continental lacustrine settings. In this study, we applied a set of traditional, novel isotopic (C, Sr and Cr) and trace element (REE, Zn/Fe) tracers which were analysed in Neoarchean carbonates from recently recovered drill core 18ABAD01 intersecting the Tumbiana and Hardey Formations. This study generated high-resolution δ13C and 87Sr/86Sr records, which provide convincing evidence for the non-marine nature of these sediments, suggesting that these deposits were formed in a Neoarchean lacustrine system. Furthermore, this study applied palaeo-redox proxies such as δ53Cr and Zn/Fe ratios to infer past redox conditions during deposition of stromatolite-bearing carbonates from the Tumbiana Formation. Overall, results show these carbonates formed in a lacustrine settings and in the presence of shallow oxic waters, the latter likely produced due to local O2 input from stromatolitic/ photosynthetic communities. Finally, the results of this study have implications for the origin of local sediment-hosted stratiform copper SSC deposits. Importantly, the palaeo-redox data from the Tumbiana Formation supports the existence of local pool oxic waters that could thus aid the formation of SSC deposits.