Origin of the Urucum iron formations (Neoproterozoic, Brazil): Textural and mineralogical evidence (Mato Grosso do Sul – Brazil)

Neoproterozoic banded iron formations (BIF) and ironstones occur in the Urucum mining district (Pantanal, Brazil). Microbially mediated formation as microbialites has been proposed for these deposits. This study aims to provide a high resolution characterization of micro-textural features, mineral assemblage distribution gather evidence two microbial ore forming systems: Fe- Mn-oxidizing metabolic processes cyanobacterial activity Optical- cathodoluminescence rock microscopy, Fourier Transform Infrared (FTIR), Raman spectroscopy allowed identification syngenetic diagenetic minerals. Evidence from BIF nodule samples points low temperature, suboxic, neutral/slightly alkaline conditions, but rarely acidic anoxic local conditions occurred. Direct action microorganisms is: (a) microtextural microbially beds, which microbialites; (b) types origin (ferrihydrite, todorokite, manganite) with various organic matter incorporated, (c) light isotope signal (cited negative hematite δ57Fe) and; (d) mineralized sedimentation cycles Fe, Mn, Ca Si. Two systems are dual systems, characterized by main intense activity. More ordered minerals, such hematite, goethite anatase, formed during diagenesis Fe-rich biomat. Calcite-type (calcite, Mn-calcite, rhodochrosite) dolomite-type carbonate structures (kutnohorite, ankerite, dolomite) Fine-grained is common constituent, some idiomorphic occurrences dissemination laminae nodules. The composition ankerite other carbonates highly variable. BIF’s gray consist hematite-rich part minor quartz, scarce apatite. red contain contents quartz further minerals (feldspar, apatite). Hematite occurs considerable amount, it enough only turning jasper reddish color. Nodules were segregated diapirism and/or underwent same changes microlayers they segregated. Nodule compositions fit well trends systems. Most likely differences between nodules consequences different initial fine alternation oxygen supply system.