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

Abstract

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