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Géoscience Sciences De La Planète Comptes Rendus Géoscience Sciences de la Planète Jérémie Aubineau, Olabode M. Bankole, Fabien Baron, Brian Grégoire and Abderrazak El Albani Authigenic kaolinite and sudoite in sandstones from the Paleoproterozoic Franceville sub-basin (Gabon) Volume 353, issue 1 (2021), p. 209-226 <https://doi.org/10.5802/crgeos.62> © Académie des sciences, Paris and the authors, 2021. Some rights reserved. This article is licensed under the Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/ Les Comptes Rendus. Géoscience — Sciences de la Planète sont membres du Centre Mersenne pour l’édition scientifique ouverte www.centre-mersenne.org Comptes Rendus Géoscience — Sciences de la Planète 2021, 353, nO 1, p. 209-226 https://doi.org/10.5802/crgeos.62 Review Article — Petrology, Geochemistry Authigenic kaolinite and sudoite in sandstones from the Paleoproterozoic Franceville sub-basin (Gabon) , a, b b b b Jérémie Aubineau ¤ , Olabode M. Bankole , Fabien Baron , Brian Grégoire and Abderrazak El Albanib a Géosciences Montpellier, UMR 5243, CC 60 – University of Montpellier, Montpellier, France b IC2MP UMR 7285 CNRS, University of Poitiers, Poitiers, France E-mails: [email protected] (J. Aubineau), [email protected] (O. M. Bankole), [email protected] (F.Baron), [email protected] (B. Grégoire), [email protected] (A. El Albani) Abstract. The mineral paragenetic sequence of the 2.1-billion-year-old (Ga) Francevillian basin is im- portant for understanding the diagenetic fluid history that allowed the preservation of the oldest ecosystem, including bacterial and more advanced forms of life in the FB2 Member. However, a full characterization of the clay mineralogy of the FB2 microbial mat-related structures (MRS) and associ- ated 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 su- doite 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 pro- moted by fault-controlled acidic and oxidized brines that might have migrated during the Francevil- lian basin inversion. These results imply that the porosity and permeability of sedimentary rocks dom- inantly control the mineralogical assemblage of the FB2 Member. Keywords. Sudoite, Kaolinite, Diagenesi, Paragenesis, Sandstones, Franceville sub-basin. Manuscript received 21st March 2021, revised and accepted 1st June 2021. 1. Introduction Mg- and Fe-rich countertype, sudoite usually occurs in low-temperature environments [ 300 °C; Beaufort Ç Sudoite is characterized by a dioctahedral 2:1 layer et al., 2015, Lanari et al., 2014], including diagenetic and a trioctahedral interlayer sheet and belongs and hydrothermal systems [e.g., Beaufort et al., 2005, to the di-trioctahedral chlorite mineral group with Daniels and Altaner, 1990, Hayashi and Oinuma, an ideal composition of (Al3Mg2)(Si3Al)O10(OH)8 1964, Hillier et al., 2006, Percival and Kodama, 1989, [Bailey, 1980]. Although this type of Al- and Mg-rich Rodríguez-Ruiz et al., 2019, Ruiz Cruz and Sanz de chlorite is far less common than its tri-trioctahedral Galdeano, 2005, Schultz, 1963, Truche et al., 2018]. In such geological settings, sudoite likely forms from modifications of kaolin minerals (i.e., kaolinite, dick- ¤ Corresponding author. ISSN (electronic) : 1778-7025 https://comptes-rendus.academie-sciences.fr/geoscience/ 210 Jérémie Aubineau et al. ite, and nacrite) through a stepwise reaction under life [El Albani et al., 2014, 2010] capable of move- the influence of Mg-rich fluids. Sudoite also crys- ment [El Albani et al., 2019] and has provided a con- tallizes at the expense of diagenetic beidellite when siderable interest in the complex life’s early evolu- kaolins are lacking [Biernacka, 2014]. These min- tion. Thus, deciphering the post-depositional mod- eral transformations are generally controlled by a ification processes in these unmetamorphosed sed- dissolution–crystallization mechanism since con- imentary rocks may provide a glimpse of long-term vincing evidence of kaolinite–sudoite or smectite– eVects of diagenetic history on the preservation of sudoite mixed-layer minerals (MLMs) are lacking the Francevillian biota. This paper investigates the [see review in Beaufort et al., 2015]. A large amount di-trioctahedral chlorite and kaolin minerals in sand- of sudoite has been reported in the Proterozoic ura- stone and black shale facies of the Francevillian FB2 nium (U)-bearing sandstones that unconformably Member. Here we integrated X-ray diVraction (XRD), overlie Archean basements [Beaufort et al., 2005, infrared spectroscopy, and optical and electron mi- Billault et al., 2002, Kister et al., 2005, Percival and croscopy to examine the clay mineralogy and pet- Kodama, 1989, Riegler et al., 2014, Truche et al., 2018], rography of authigenic minerals and their textural where the origin of sudoite was believed to have re- relationships to provide additional insights into the sulted from interactions of oxidized and acidic fluids diagenetic fluid history and mineral paragenetic se- with the underlying crystalline rocks within active quence in the Franceville sub-basin of the Francevil- fault zones. Therefore, sudoite can sometimes be a lian Group. potential proxy for tracing U mineralization. Despite numerous studies on the diagenetic min- 2. Geological setting eral phases of the Paleoproterozoic Franceville sub- basin of Gabon [Aubineau et al., 2019, Bankole et al., The Francevillian basin is composed of four intracra- 2018, 2016, 2015, Bros et al., 1992, Gauthier-Lafaye, tonic sub-basins—Booué, Lastourville, Okondja, and 1986, Ngombi-Pemba et al., 2014, Ossa Ossa et al., Franceville—in southeastern Gabon [Bouton et al., 2013, Weber, 1968], the presence of kaolin min- 2009]. The basin fill consists of 1.0 to 2.5-km-thick erals and di-trioctahedral chlorite have not been Paleoproterozoic siliciclastic sedimentary succession well-documented or constrained in the Francevillian commonly referred to as the Francevillian Group rock sequence, especially in sandstones [cf. Ngombi- [Figure1a; Weber, 1968]. Importantly, the Francevil- Pemba et al., 2014]. Synthetically, assemblages of lian deposits were not aVected by metamorphic detrital-shaped (including mica and ferromagnesian processes and have only been subjected to diage- chlorite) and diagenetic (illite, tri-trioctahedral chlo- netic and low-temperature hydrothermal processes rite, and smectite-rich and illite-rich illite–smectite, [Aubineau et al., 2020, Bankole et al., 2016, 2015, I-S MLMs) clay minerals dominantly characterize the Gauthier-Lafaye and Weber, 1989]. Specifically, the Francevillian Group sedimentary rocks. Most of the Franceville sub-basin records a maximum burial diagenetic clay minerals in the Francevillian sedi- depth and temperature of about 2500 m and 80 °C, re- ments result from the alteration of aluminosilicate spectively [Ngombi-Pemba et al., 2014, Weber, 1968], minerals [Gauthier-Lafaye, 1986]. The surprising sur- which corresponds to burial conditions of a common vival of smectite-rich (R0) I-S MLMs in localized geothermal gradient. stratigraphic levels (from the FB1b, to the FB2b units) These sediments rest unconformably on the of the Franceville sub-basin has been attributed to Archean crystalline basement within the west potassium deficiency in the system [Aubineau et al., Congolese craton. The basement rocks are mainly 2019, Bankole et al., 2015, Ngombi-Pemba et al., composed of Mesoarchean (3200–2800 Ma), TTG 2014]. Indeed the variable proportion of K-feldspar (tonalite–trondhjemite–granodiorite), and green- was recognized in other rock units of the Franceville stone belts [Mouélé et al., 2014, Thiéblemont et al., sub-basin [Bankole et al., 2015, Ngombi-Pemba et al., 2009]. The synthesis of the geodynamic evolution 2014], suggesting mixing of multiple source rocks of the Francevillian basin was recently detailed by [Bankole et al., 2020]. Weber et al.[2016]. The FB2 Member of the Franceville sub-basin is The Francevillian Group has been divided into believed to host the earliest potential multicellular five lithostratigraphic formations, labeled FA to FE C. R. Géoscience — 2021, 353, nO 1, 209-226 Jérémie Aubineau et al. 211 Figure 1. (a) Geological map of the Paleoproterozoic Franceville sub-basin showing the study area— Moulendé Quarry [modified from Bouton et al., 2009]. (b) Lithostratigraphic column of the FB2 Member in the Moulendé Quarry displaying sampling positions of both microbial mat structures and host sedi- ments (red arrows). Images in (a) and (b) are modified from Aubineau et al.[2020] and Aubineau et al. [2019], respectively. from the oldest to the youngest. The Lower FA for- (FB1b unit), and manganese-rich black shales [FB1c mation is dominated by braided fluvial conglom- unit; Azziley Azzibrouck, 1986, Gauthier-Lafaye and erates and sandstones, while the Upper FA for- Weber, 2003, Pombo, 2004, Reynaud et al., 2017]. mation is composed of fluvio-deltaic, fine-grained Following an episode of sea-level fall, massive sand- mudstones and sandstones [Bankole et al., 2015, stones frequently intercalated by dm-thick black Feybesse et al., 1998, Gauthier-Lafaye and Weber, shale layers
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