Abstract
The mineral paragenetic sequence of the 2.1-billion-year-old (Ga) Francevillian basin is important for understanding the diagenetic fluid history that allowed the preservation of the oldest ecosystem, including bacterial and more advanced forms of life in the FB 2 Member. However, a full characterization of the clay mineralogy of the FB 2 microbial mat-related structures (MRS) and associated host sediments (sandstones and black shales) is yet to be determined. Petrographic, microscopic, and mineralogical analyses reveal the concurrent presence of authigenic vermiform kaolinite and sudoite in the MRS and host sediments. Kaolinite formed along cleavages of altered muscovite and as pore-filling during early diagenesis, while sudoite likely precipitated at the expense of kaolinite that undergone secondary dissolution later in the diagenetic sequence. The formation of sudoite was promoted by fault-controlled acidic and oxidized brines that might have migrated during the Francevillian basin inversion. These results imply that the porosity and permeability of sedimentary rocks dominantly control the mineralogical assemblage of the FB 2 Member.
Highlights
Sudoite is characterized by a dioctahedral 2:1 layer and a trioctahedral interlayer sheet and belongs to the di-trioctahedral chlorite mineral group with an ideal composition of (Al3Mg2)(Si3Al)O10(OH)8 [Bailey, 1980]
A large amount of sudoite has been reported in the Proterozoic uranium (U)-bearing sandstones that unconformably overlie Archean basements [Beaufort et al, 2005, Billault et al, 2002, Kister et al, 2005, Percival and Kodama, 1989, Riegler et al, 2014, Truche et al, 2018], where the origin of sudoite was believed to have resulted from interactions of oxidized and acidic fluids with the underlying crystalline rocks within active fault zones
The illitization process was microbially enhanced in the FB2 Member [Aubineau et al, 2019], this study reveals that the biological activity of mat-related structures (MRS) did not influence kaolinite and sudoite formations
Summary
Sudoite is characterized by a dioctahedral 2:1 layer and a trioctahedral interlayer sheet and belongs to the di-trioctahedral chlorite mineral group with an ideal composition of (Al3Mg2)(Si3Al)O10(OH)8 [Bailey, 1980]. Mg- and Fe-rich countertype, sudoite usually occurs in low-temperature environments [
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