Biogenesis of the neoproterozoic kremydilite manganese ores from urucum (brazil) – a new manganese ore type

Abstract 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, Significance statement The Neoproterozoic Urucum manganese deposit (Brazil) is a ∼600 Mt microbially-mediated sedimentary Mn ore. Proto-ore formation via sedimentation and diagenesis occurred under suboxic-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.