Agradecimentos: Thanks to the mining companies Vale and Rio Tinto for access to critical sample material. We would like to thank Toby Leeper for always maintaining the mass spectrometers in perfect running conditions and thank Toni Larsen for lab-assistance. We thank Dirk Frei (Geological Survey of...

Agradecimentos: Thanks to the mining companies Vale and Rio Tinto for access to critical sample material. We would like to thank Toby Leeper for always maintaining the mass spectrometers in perfect running conditions and thank Toni Larsen for lab-assistance. We thank Dirk Frei (Geological Survey of Denmark and Greenland, GEUS, at the time of the project period) for LA-ICP-MS analyses of zircons and with final U-Pb isotope data reporting. Financial support through the Danish Agency for Science, Technology and Innovation grant number 11-103378 to RF and through the Danish National Research Foundation's center of excellence NordCEE (DNRF grant number DNRF53) is highly appreciated. We also are indebted to CNPq (a research supporting foundation of Brazil), project Proc 312293/2013-5 and to FAPESP through which we received funding under project Proc 2014/01233-0. We are thankful for the thorough, helpful and insightful review by D. Papineau which substantially improved our manuscript

Abstract: The massive Fe and Mn deposits of the Urucum district (Banda Alta Formation) and the iron formations from Fazenda São 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...

Abstract: The massive Fe and Mn deposits of the Urucum district (Banda Alta Formation) and the iron formations from Fazenda São 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 São 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 ± 9 Ma (Babinski et al., 2013), a result which we here confirm by a U-Pb age of the youngest zircon of 695 ± 17 Ma 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 d53Cr = 1.10 ± 0.4‰; n = 16; 2s) prevailing throughout the entire stratigraphic section indicate that riverine supply of continentally-derived Cr, remained more or less constant throughout the glacier meltdown and IF depositional period. Cycling across a redoxcline and predominant deposition of the IF in anoxic deeper seawater of the Jacadigo basin is supported by the peculiar presence of subchondritic Y/Ho ratios, by decreased negative Ce anomalies and by shifts of LREE patterns towards higher values in the Urucum district IFs that are independent of detrital contamination. The strongly positively fractionated Cr isotope signatures measured in these iron formations are compatible with those from other iron formations and black shales deposited during the late Neoproterozoic and Precambrian-Cambrian transition worldwide and are in support of prevailing high atmospheric O2 levels following the late Neoproterozoic glaciations that accompanied the evolution of macroscopic multicellular organisms

FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP

2014/01233-0

CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ

312293/2013-5

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