Authigenic kaolinite and sudoite in sandstones from the Paleoproterozoic Franceville sub-basin (Gabon)

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 $\mathrm{FB}_{2}$ Member. However, a full characterization of the clay mineralogy of the $\mathrm{FB}_{2}$ 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 $\mathrm{FB}_{2}$ Member.

Keywords

• Sudoite
• Kaolinite
• Diagenesi
• Paragenesis
• Sandstones
• Franceville sub-basin