Extensive oxidative weathering in the aftermath of a late Neoproterozoic glaciation – Evidence from trace element and chromium isotope records in the Urucum district (Jacadigo Group) and Puga iron formations (Mato Grosso do Sul, Brazil)

摘要

Abstract The massive Fe and Mn deposits of the Urucum district (Banda Alta Formation) and the iron formations from Fazenda S?o Manoel (Puga Formation) in Mato Grosso do Sul, Brazil, are associated with glacigenic deposits and represent the youngest and largest sedimentary Fe and associated Mn formation (IF;MnF) deposits of Cryogenian age in the world. The Urucum district IFs studied are predominantly composed of pure classical plane-parallel and stratified hematite–chert–iron - and intercalated manganese (cryptomelane) micro- and mesobands, whereas the IFs at Fazenda S?o Manoel are closely associated with diamictites and shales. Although the precise depositional age is unknown, maximum age constraints for the Puga IFs are defined by the youngest detrital zircon with an U-Pb age of 706±9Ma (), a result which we here confirm by a U-Pb age of the youngest zircon of 695±17Ma from within shaly beds in the Urucum district IF succession. Redox-sensitive trace element signatures and tendency to hump-shaped Rare Earth Element+Yttrium (REY) patterns with negative Ce- but without Eu anomalies support the presence of an oxic surface water layer, fertilized by low temperature hydrothermal fluids injected from submarine thermal springs and/or, alternatively, by significant fresh water input directly derived from glacial meltdown. Strongly positively fractionated, authigenic chromium isotope signatures (average δ53Cr=1.10±