Iron and sulphur isotopes from the Carajás mining province (Pará, Brazil): implications for the oxidation of the ocean and the atmosphere across the Archaean-Proterozoic transition.
Résumé
Mineralogical analyses and major and trace element geochemistry combined with iron and sulfur isotopes were applied to banded iron formations (BIFs) and pyrite-bearing sandstones from the Carajás mineral province, Brazil. The BIFs belong to the 2.7 Ga old Grão Pará Group and show a rather simple mineralogy (hematite and quartz or cryptocrystalline silica). Their REE+Y patterns suggest deposition under marine suboxic conditions. Fe isotopes show highly positive values (δ57FeIRMM-14=2.06±0.34‰ in average) which can be explained by the partial oxidation of Fe2+ aq in the surface ocean, subsequent to the upwelling of deep anoxic seawater. The resulting iron oxy-hydroxides likely formed in the shallow photic zone as a seasonal process and acquired highly positive iron isotope signature values consistent with a Rayleigh distillation model. The pyrite-bearing sandstones belong to the Aguas Claras Formation of (Palaeoproterozoic) younger age and were cored at the base of the giant Azul manganese deposit. Diagenetic pyrite is characterized by positive δ57FeIRMM-14=1.02±0.2‰ and δ34SVCTD (11.97±0.12‰) average values, that are different from values of modern marine biogenic pyrite. Results likely indicate that dissolved sulfate was the limiting factor controlling biogenic pyrite formation at that time. Near-zero Δ33S values (0.013±0.003‰) indicate minimal mass-independent sulfur isotope fractionation (MIF-S), consistent with the 2.1 Ga age previously obtained from the Aguas Claras pyrites. This result suggests deposition at a time when the oxygen partial pressure in the atmosphere was already high enough (10−5 to 10−2 PAL- or Present Atmospheric Level-) to hinder MIF-S by photochemical reactions. Incidentally, this also confirms that the giant Mn deposit of Carajás is in the same age range (2.1±0.2 Ga) as other Mn ores in Gabon and South Africa, pointing to the importance of Mn precipitation in Paleoproterozoic times.