Biogenesis of the Neoproterozoic kremydilite manganese ores from Urucum (Brazil) - A new manganese ore type

The Urucum district in Mato Grosso do Sul (Brazil), hosts the youngest and largest sedimentary Mn ore of Neoproterozoic age; units Mn-1, Mn-2, and Mn-3 are found in jaspilites and ironstones, and represent approximately 600 Mt of extractable rock with 27-44% Mn and 12-30% Fe. High-resolution optical- and cathodoluminescence microscopy, as well as Raman and FTIR spectroscopy show that the lower Mn-1 is ferruginous, while the upper Mn-1 consists mainly of 30-75 vol% braunite, < 0.5% aegirine, 3-15% quartz, 5-10% feldspar, and 1-5% clay minerals, including apatite, chlorite, and organic matter. Here, we model the control of this ore mineralogy by homogeneous oxidation and microbial processes. Layers Mn-2 and Mn-3 contain kremydilite, as a characteristic ore structure, with 77-95 vol% cryptomelane, 0-23% hollandite, 9-19% braunite, 7-21% hematite, and 0-5% pores filled with clay minerals and organic matter. These are present within a micro-nodule matrix composed of cryptomelane and hematite in varying proportions. The first syngenetic products of microbial enzymatic oxidation were, on the Fe side, ferrihydrite and lepidocrocite, and on the Mn side, vernadite, todorokite, birnessite, and manganite. These formed under obligatory oxic (Mn) and suboxic (Fe) conditions and close to neutral pH. We describe the genesis of Urucum via complex diagenetic processes, which include the decomposition and mineralization of cellular- and extracellular-polymeric substances from Fe and Mn bacteria and cyanobacteria. The kremydilite forms in successive stages of oxidation of organic matter mediated by microbes, which generate pores and produce methane and CO2/H-2 bubbles. They are a unique type of diagenetic structure formed by heterotrophic cell colonies randomly activated in the microbialite milieu following burial in suboxic neutral/alkaline conditions, side-by-side with the lithification and stabilization of the mineral assemblages. Significance statement The Neoproterozoic Urucum manganese deposit (Brazil) is a similar to 600 Mt microbially-mediated sedimentary Mn ore. Proto-ore formation via sedimentation and diagenesis occurred under suboxicoxic-oxic and semi-neutral pH conditions in the Ediacaran ocean, wherein microbial Mn(II) oxidation ensued from the fine-grained accumulation of Mn oxides and organic matter. Oxic conditions that facilitated enzymatic Mn oxidation and overwhelmed microbial Fe oxidation appears as a sharp contact between manganese and iron beds. The Urucum deposit arose from a complex suite of diagenetic processes, including decomposition and mineralization of microbially-derived organic matter involving extracellular polymeric substances. Kremydilite - a new type of diagenetic concentric Mn mineral structure formed by randomly activated heterotrophic cell colonies that generated pores in the microbialite sediment after burial, coincident with lithification.