4.5 Article

Microbial metallogenesis of Cryogenian manganese ore deposits in South China

Journal

PRECAMBRIAN RESEARCH
Volume 322, Issue -, Pages 122-135

Publisher

ELSEVIER SCIENCE BV
DOI: 10.1016/j.precamres.2019.01.004

Keywords

Geomicrobiology; Post-Sturtian; Datangpo; Guizhou

Funding

  1. National Natural Science Foundation of China [U1812402]
  2. Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) [CUG170684]
  3. China Geological Survey (CGS) Project [DD20160346]
  4. Guizhou Science Innovation Team Project [2018-5618]
  5. Research Project of Guizhou Bureau of Geology and Mineral Exploration and Development (2016) [30]
  6. National Research, Development and Innovation Office
  7. National Scientific Research Fund Hungary [125060]
  8. Support of Excellence of Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences

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The Datangpo Formation manganese deposits (DFMnD) in South China formed during the interglacial stage between the Sturtian and Marinoan glaciations of the Cryogenian period. These black shale-hosted deposits are composed of massive Mn-carbonates with microscopic laminae/laminations and cherty veins. To date, it has been thought that the DFMnD formed through inorganic processes, which were controlled by redox changes in the post-Sturtian Nanhua Rift Basin, South China. However, in this study, systematic petrographic, mineralogical, and geochemical analyses indicate a microbially mediated origin of the Mn ore deposits. Mineralized microbial woven micro-textures (observed at the pm scale) and microbial fossils are common in the laminated Mn-carbonate ores. We infer that microbial enzyme activity formed poorly crystallized Mn oxide/hydroxides and carbonaceous material, which transformed to rhodochrosite, kutnohorite, ankerite/dolomite, framboidal pyrite, and apatite via diagenesis. Some micro-scale quartz and K-feldspar may be detrital but most appears to have formed during diagenesis or through hydrothermal activity. A micro-mineralogical profile determined by 2500 spectra via high-resolution in situ micro-Raman spectroscopy also revealed cyclic laminations of Ca-rhodochrosite as microbialite (ankerite/dolomite) and quartz, indicating a mineralized biomat system. Ca-rhodochrosite transformed to kutnohorite under elevated temperatures, as indicated by the maturation level of organic matter (determined via Raman spectroscopy). Alternating micro-laminae denote cyclic changes in microbial groups (Mn- and Fe-oxidizing microbes versus cyanobacteria) during the formation of the Mn ore deposits. Our proposed model for the microbially mediated metallogenesis of Mn-carbonate deposits begins with enzymatic multi-copper oxidase processes associated with autotrophic microbial activity under obligatory oxic conditions, which results in the precipitation of Mn bio-oxides. Following their burial in organic-rich sediments, the Mn(IV) oxides and hydroxides are reduced, producing soluble Mn(II) via processes mediated by heterotrophic microbes under suboxic conditions, which in turn form the Mn-carbonates. This microbial metallogenesis model for the Cryogenian DFMnD in South China is similar to that proposed for the Jurassic Urkut Mn deposit in Hungary, indicating that a two-step microbially mediated process of Mn ore formation might be common throughout geological history.

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