Abstract
Sedimentary black shale-hosted manganese carbonate and oxide ores were studied by high-resolution in situ detailed optical and cathodoluminescence microscopy, Raman spectroscopy, and FTIR spectroscopy to determine microbial contribution in metallogenesis. This study of the Urucum Mn deposit in Brazil is included as a case study for microbially mediated ore-forming processes. The results were compared and interpreted in a comparative way, and the data were elaborated by a complex, structural hierarchical method. The first syngenetic products of microbial enzymatic oxidation were ferrihydrite and lepidocrocite on the Fe side, and vernadite, todorokite, birnessite, and manganite on the Mn side, formed under obligatory oxic (Mn) and suboxic (Fe) conditions and close to neutral pH. Fe- and Mn-oxidizing bacteria played a basic role in metallogenesis based on microtextural features, bioindicator minerals, and embedded variable organic matter. Trace element content is determined by source of elements and microbial activity. The present Urucum (Brazil), Datangpo (China), and Úrkút (Hungary) deposits are the result of complex diagenetic processes, which include the decomposition and mineralization of cell and extracellular polymeric substance (EPS) of Fe and Mn bacteria and cyanobacteria. Heterotrophic cell colonies activated randomly in the microbialite sediment after burial in suboxic neutral/alkaline conditions, forming Mn carbonates and variable cation-bearing oxides side by side with lithification and stabilization of minerals. Deposits of variable geological ages and geographical occurrences show strong similarities and indicate two-step microbial metallogenesis: a primary chemolithoautotrophic, and a diagenetic heterotrophic microbial cycle, influenced strongly by mineralization of cells and EPSs. These processes perform a basic role in controlling major and trace element distribution in sedimentary environments on a global level and place biogeochemical constraints on the element content of natural waters, precipitation of minerals, and water contaminants.
Highlights
IntroductionIn the quest of understanding “Biogeochemical Constraints on Water Contaminants,” it is important to consider the role of microbial life (in particular, bacteria and fungi) in the geological context of mineralization and mobilization processes, because the mechanisms governing such activities often supersede purely inorganic reactions.The scavenging properties of microbially precipitated/ influenced Fe and/or Mn oxides/hydroxides and other components of (toxic) heavy metals, such as As, Cu, Co, Ni, and Ba, may highlight and influence accumulation pathways and, reduced mobility and extraction from an aquifer.Our experience in researching sedimentary Fe-Mn ores called our attention to the basic role of microbially mediated processes and the effect of cell and extracellular polymeric substance (EPS) diagenesis in determining the main and trace element composition of rocks.The Two-Step Microbial Ore Formation Model and Comparison of Case StudiesThe role of microbial processes in the formation and transformation of rocks is a topic that is gaining attention among researchers
We provide a comparison of results published in several case studies in three deposits including black shale-hosted Mn carbonate deposits of Neoproterozoic Datangpo, China (Yu et al, 2019), the Jurassic Úrkút manganese occurrences, Hungary (Polgári et al, 2012a,b, 2013, 2016a,b, 2017), and the Neoproterozoic jaspilite and ironstone-hosted basically Mn oxide deposit, Urucum, Brazil (Biondi and Lopez, 2017; Biondi et al, 2019; Figure 1)
Thin sections represent mineralized biomats based on structural observations
Summary
In the quest of understanding “Biogeochemical Constraints on Water Contaminants,” it is important to consider the role of microbial life (in particular, bacteria and fungi) in the geological context of mineralization and mobilization processes, because the mechanisms governing such activities often supersede purely inorganic reactions.The scavenging properties of microbially precipitated/ influenced Fe and/or Mn oxides/hydroxides and other components of (toxic) heavy metals, such as As, Cu, Co, Ni, and Ba, may highlight and influence accumulation pathways and, reduced mobility and extraction from an aquifer.Our experience in researching sedimentary Fe-Mn ores called our attention to the basic role of microbially mediated processes and the effect of cell and extracellular polymeric substance (EPS) diagenesis in determining the main and trace element composition of rocks.The Two-Step Microbial Ore Formation Model and Comparison of Case StudiesThe role of microbial processes in the formation and transformation of rocks is a topic that is gaining attention among researchers. Her work focuses on the principle of materialshape-process and is case study based She has created a genetic model of the Úrkút site, which cast a new light on the formation of manganese ore in a black shale environment. This two-step microbial ore formation model has a chemolithoautotrophic cycle I under obligatory oxic conditions, which in the case of manganese precipitates metal ions from the aquatic system to solid form, and a heterotrophic cycle II, under suboxic conditions forming metal carbonates (rhodochrosite and siderite) under suboxic conditions (Polgári et al, 2012b). The basic principle of the method is that the microbial species mineralize only at a certain stage of the population growth periods (Polgári et al, 2012a)
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