THÈSE
Pour l'obtention du grade de
DOCTEUR DE L'UNIVERSITÉ DE POITIERS UFR des sciences fondamentales et appliquées
Institut de chimie des milieux et matériaux de Poitiers - IC2MP (Diplôme National - Arrêté du 7 août 2006)
École doctorale : Sciences pour l'environnement - Gay Lussac (La Rochelle) Secteur de recherche : Terre solide et enveloppes superficielles
Présentée par :
Olabode Modupe Bankole
Transformations diagénétiques des dépôts silicoclastiques FA du bassin de Franceville au Gabon (2.2-2.0 Ga) par l'invasion
de solutions oxydo-réductrices
Directeur(s) de Thèse : Abderrazak El Albani
Soutenue le 16 décembre 2015 devant le jury Jury :
Président Alain Meunier Professeur, IC2MP, Université de Poitiers Rapporteur Jean-Luc Potdevin Professeur, Université de Lille
Rapporteur Michel Cuney Directeur de recherche, CREGU, CNRS, Vandœuvre les Nancy Membre Abderrazak El Albani Professeur, IC2MP, Université de Poitiers
Membre François Gauthier-Lafaye Directeur de recherche émérite, CNRS, Université de Strasbourg
Membre Olivier Rouxel Chercheur associé, IFREMER, Brest
Membre Andrey Bekker Associate Professor, University of California, Riverside
Pour citer cette thèse :
Olabode Modupe Bankole. Transformations diagénétiques des dépôts silicoclastiques FA du bassin de Franceville
au Gabon (2.2-2.0 Ga) par l'invasion de solutions oxydo-réductrices [En ligne]. Thèse Terre solide et enveloppes
THÈSE
Pour l'obtention du grade de
DOCTEUR DE L'UNIVERSITÉ DE POITIERS
UFR des sciences fondamentales et appliquées
Institut de chimie des milieux et matériaux de Poitiers - IC2MP (Diplôme National - Arrêté du 7 août 2006)
École doctorale : Sciences pour l'environnement - Gay Lussac
Secteur de recherche : Géosciences - Terre solide et enveloppes superficielles
Présentée par :
Olabode Modupe BANKOLE
**********************************************************
TRANSFORMATION DIAGENETIQUES DES DEPOTS SILICOCLASTIQUES FA
DU BASSIN DE FRANCEVILLE AU GABON (2.2-2.0 GA) PAR L’INVASION DE
SOLUTION OXYDO-REDUCTRICES
********************************************************** Directeur de Thèse : Abderrazak EL ALBANI
********************************************************** Soutenue le 16 décembre 2015 devant le jury
Membres du jury :
Rapporteurs :
Jean-Luc POTDEVIN Professeur, Université de Lille 1
Michel CUNEY Directeur de Recherche Emérite, CNRS, Université de Nancy 1
Examinateurs :
Alain MEUNIER Professeur, Université de Poitier
Olivier ROUXEL Chercher associé, HDR, IFREMER, Brest
François GAUTHIER-LAFAYE Directeur de Recherche Emérite, CNRS, Université de Strasbourg
Andrey BEKKER Professeur, Université de Californie, Riverside, USA Abderrazak EL ALBANI Professeur, Université de Poitiers (Directeur de Thèse)
2
DIAGENETIC TRANSFORMATIONS AND REDOX FLUID
INVASION INTO SILICICLASTIC FA FORMATION,
iii
Résume
Dans le bassin de Franceville (Gabon), les sédiments détritiques non-métamorphosés d’âge Paléoprotérozoïque (2,15 Ga) des formations FA et FB ont fait l’objet d’une étude faciologique, pétrographique et géochimique. L’objectif était de déterminer l’origine de ces matériaux clastiques et des minéralisations uranifères associées, mais aussi d’en retracer l’histoire diagénétique à travers l’évolution des paléoconditions rédox et des fluides qui les ont percolés.
La pétrographie des faciès montre que la minéralogie et la texture initiales de ces sédiments ont été fortement modifiées au cours de la diagenèse précoce. Au toît de la formation FA, les quartz-arénites faiblement granoclassées ont été très tôt cimentées par du quartz, s’opposant ainsi à l’action ultérieure de la diagenèse d’enfouissement. Dans les arènes arkosiques, la séquence paragénétique liée aux ciments authigènes suggère que les interactions fluides-roches ont été polyphasées et que les éléments chimiques nécessaires à leur précipitation trouvent leur origine dans l’altération de minéraux détritiques. Pour un ensemble lithologique donné, les effets de la diagenèse varient peu d’un faciès à l’autre, ce qui indique un fort contrôle des paramètres initiaux tels que la minéralogie, la texture et la composition du fluide poral. Par conséquent, ces faciès sédimentaires nous informent directement sur les propriétés hydrologiques qui régnaient dans le bassin sédimentaire, et sur la nature de la diagenèse précoce qui les a affectés.
Les données pétrographiques et géochimiques montrent que la précipitation de l’hématite en lits ferrugineux a débuté juste après le dépôt des sédiments, dont l’altération des minéraux primaires a libéré le fer, plus tard redistribué durant la diagenèse. Ainsi, la mise en évidence d’une corrélation positive entre les valeurs de 56Fe et les rapports Fe/Mg mesurés sur échantillon total suggère que
le fer se répartit entre deux pôles que sont l’hématite authigène et les silicates porteur du fer. En revanche, l’absence de relation entre les rapports Fe3+/Fe
T et les compositions isotopiques du fer
démontre que des oxydes riches en isotopes lourds du fer préexistaient dans les sédiments lorsque la diagenèse précoce a débuté. Le fer présent initialement à l’état réduit dans des faciès sédimentaires verts s’est progressivement oxydé au cours de la diagenèse, processus à l’origine de la formation des lits ferrugineux.Dans la formation FA du Bassin de Franceville, les gisements uranifères résultent de la libération de l’uranium contenu à l’origine dans les grès oxydés des dépôts fluviatiles inférieurs puis de sa migration et de son piégeage dans les grès réduits et les mudstones silteux des formations de deltas de marée supérieures, où il s’est concentré par un mécanisme de type roll-front.
Les signatures géochimiques (Distribution des Terres Rares, LaN/YbN, Th/Sc, Zr/Sc, GdN/YbN, Cr/Th, La/Sc) des formations FA et FB confirment que ces sédiments dérivent exclusivement de roches ignées felsiques, qu’elles résultent d’un mélange de produits issus du socle archéen et post-archéen. Enfin des diagrammes discrimants et des rapports en éléments majeurs et traces montrent que, si la plupart des grès de la formation FA se sont déposés sur une marge passive, les mudstones et les grès fins des formations FA et FB ont sédimenté sur une marge continentale active en relation avec la tectonique paléoprotérozoïque de la Ceinture orogénique du Centre-Ouest de l’Afrique.
iv
Abstract
The FA and FB Formations clastic sediments have been subjected to detailed facies, petrographic, and geochemical analyses in relation to diagenesis, fluid flow, paleo-redox conditions, provenance, and uranium mineralization during the evolution of the unmetamorphosed Paleoproterozoic (ca. 2.15 Ga) Franceville Basin, Gabon.
Lithofacies analyses in combination with petrographic studies indicate that the original mineralogical and textural properties of the sediments have been greatly modified during diagenesis. The moderately sorted quartz arenite at the top of FA underwent early quartz cementation; thus preventing it from subsequent burial diagenetic processes. The inferred paragenetic sequence of authigenic cements in the arkosic arenites suggest multi fluid-rock interactions with most of the ions needed for their precipitations likely sourced during alteration of detrital precursors. The observed slight variations in the diagenetic pathways in different lithofacies associations are closely related to primary mineralogy, texture, and nature of pore fluid. This relationship suggests that depositional facies might provide an insight into the diagenetic pathways and hydrologic properties of sediments in sedimentary basins.
Petrographic features coupled with whole rock geochemical and iron isotope analyses suggest that hematite precipitation in the red beds started after sediment deposition with the iron internally derived by alteration of iron-bearing minerals and redistributed during late diagenesis. Positive correlation between Fe/Mg ratio and δ56Fe values of bulk samples suggests mixing relationship with end members being authigenic hematite and iron-bearing silicates. The lack of relationship between Fe3+/Fe
T ratios and iron isotope compositions suggest that the isotopically heavy iron oxide was already present in the sediments during early diagenesis, and was incorporated into green (reduced) facies that likely replaced red facies during diagenesis or burial. Uranium released from the lower, fluvial oxidized sandstones and added to the reduced sandstones and silty mudstones in the upper tidal-deltaic sediments potentially resulted in a uranium mineralization of a roll-front type in the FA Formation of the Franceville Basin.
Geochemical proxies (REE pattern, LaN/YbN, Th/Sc, Zr/Sc, GdN/YbN, Cr/Th, La/Sc) for the FA and FB formation are consistent with sediments derived exclusively from felsic igneous source andfavour sediments derived from mixture of Archean and Post-Archean felsic sources. Discrimination diagrams and elemental ratios of major and trace elements suggest that deposition of most of the FA sandstone in a passive margin, while the mudstones and fine-grained sandstones of FA and FB were deposited in an active continental margin during Paleoproterozoic tectonic regimes of the West Central African Belt.
v
Dedication
To the memory of my beloved parent: Mr. Olakulehin and Mrs. Fausat BANKOLE. Thanks dad and mom for teaching me discipline, hardwork and honesty early in life. It is painful that death took you early.
vi
Acknowledgements
It is necessary for me to express my profound gratitude and sincere appreciation to all the people who contributed towards the completion of my PhD program. Firstly, I am grateful to the
Université de Poitiers and the Ministry of Education Poitou-Charentes Region, for providing
the financial support for my PhD program. Many thanks to the directors in IC2MP-HYDRASA laboratory for providing conducive and supportive environment for my stay and laboratory analyses, without which completion of this program would not have been a reality.
Secondly, I’d like to specially appreciate my supervisor, Prof. El Albani Abderrazak, for his continuous motivations, guidance, understanding, and directions during the course of this research. I thank you for the freedom to express myself and work freely with you. Also, I won’t forget the countless cakes and chocolates you usually drop in my office in a long time; those candies do keep me on from sleeping at work. I would also like to thank Prof. Alain Meunier, my “pseudo co-supervisor”, for always being there for countless discussions and knowledge sharing with me. I also thank you for proof-reading my writings.
Special thanks to Dr. Phillipe Boulvais and Dr. Olivier Rouxel for the time spent in your laboratories for carbonate and iron isotope analyses at Université Rennes 1 and IFREMER, respectively. Also worthy of mentioning is Dr. Herve Bellon of Université de Brest, who assisted in analyzing the K-Ar isotopes. You all introduced me to isotopes, and I must say you are the best! I am also grateful to Dr. François Gauthier-Lafaye for several discussions on the geology of the Franceville Basin during the field trip to Gabon and off the field; thanks for the knowledge sharing. I won’t forget to acknowledge Dr. Andrey Bekker for your time in discussion whenever you are in Poitiers, numerous paper exchanges, and thought provoking questions on red beds and the Franceville Basin.
Splendid thanks to Claude Laforest (my adopted papa), Claude Fontaine, and Nathalie Dauger for their unquantifiable technical assistance, sample preparations, and understanding during my laboratory work. I also appreciate Claude Fontaine for the French translation. I also wish to appreciate all staff of Hydrasa laboratory; Erasmus Mundus team - Sophie Levesque, Hubert Marie-France, and Patricia Patrier (IMACS programme coordinator). My co-PhD colleagues in no particular order: Nathaelle, Valentine, Jean-Christoph, Liva, Genia, Charlie, Fabien, Stelina, Bennoit, and others at Hydrasa laboratory for their cooperation and supports.
I am also indebted to my siblings for their understanding and supports through this period of being away from home. Huge thanks to my wife, Janet, and son, Joshua, for their understanding and supporting patience during this period.
Finally, I appreciate all the people of Poitiers and France for making my stay a home away from home.
vii
Table of Content
Résume ... iii Abstract ... iv Dedication ... v Acknowledgements ... viList of Figures ... xii
List of Tables ... xviii
INTRODUCTION GÉNÉRALE ... I OBJECTIFS ... III STRUCTURE DE LA THESE ... IV PART I ... 1
1.0 GENERAL INTRODUCTION ... 2
1.1 AIMS AND OBJECTIVES ... 4
1.2 THESIS STRUCTURE ... 5
2.0 GEOLOGICAL BACKGROUND, STUDY AREA, AND METHODS ... 8
2.1 GEOLOGICAL SETTINGS OF THE FRANCEVILLE BASIN ... 8
2.1.1 Tectonics and Sedimentation ... 9
2.1.2 Lithostratigraphy ... 9
2.1.2.1 FA Formation ... 11
2.1.2.2 FB Formation ... 12
2.1.2.3 FC Formation ... 14
2.1.2.4 FD Formation ... 14
2.1.3 Post-depositional Deformation, Diagenesis, and Organic Matter ... 15
2.1.4 Uranium Mineralization in the Franceville Basin ... 16
2.1.5 Paleo-fluids and Fluid Inclusions ... 17
2.1.6 Age Constraints ... 18
2.2 STUDY AREA AND SAMPLING ... 19
2.3 METHODOLOGY ... 22
2.3.1 Optical Microscopy ... 22
2.3.2 Scanning Electron Microscopy and Energy Dispersive Spectrometry (SEM/EDS) 22 2.3.3 Cathodoluminescence Microscopy (CL) ... 22
2.3.4 X-ray Diffraction Analysis (XRD)... 23
2.3.5 Whole-rock Geochemical Analyses ... 24
2.3.6 K-Ar Isotope Analyses ... 24
viii
2.3.8 Iron Isotope Systematics and Analyses ... 26
PART II ... 29
3.0 LITHOFACIES ANALYSIS AND DEPOSITIONAL ENVIRONMENT ... 30
3.1 LITHOSTRATIGRAPHY OF THE STUDIED AREAS ... 30
3.1.1 Basin edge (Mabinga drill hole: NW) ... 31
3.1.2 Intermediate (Bangombé drill holes) ... 32
3.1.3 Basin centre (Kaya-Kaya/Kiene-Otobo drill holes: SE)... 33
3.1.4 Lithostratigraphic correlation of FA Formation ... 34
3.2 LITHOFACIES DESCRIPTIONS ... 35
3.2.1 FB1 Formation ... 35
3.2.2 FA Formation ... 35
3.2.2.1 Lithofacies 1 - conglomerate and sandstone (LF1) ... 35
3.2.2.2 Lithofacies 2 - heterolithics (LF2) ... 38
3.2.2.3 Lithofacies 3 - Fine-grained (LF3) ... 39
3.3 INTERPRETATION AND DISCUSSION ... 41
3.3.1 Fluvial Lithofacies Association ... 41
3.3.2 Deltaic lithofacies Association ... 41
3.3.3 Tidal-deltaic Lithofacies Association ... 42
3.4 CONCLUSIONS ... 42
4.0 PETROGRAPHY AND DIAGENESIS ... 44
4.1 PETROGRAPHY ... 45
4.1.1 FB1 Formation ... 45
4.1.2 FA Formation ... 46
4.1.2.1 Texture and Mineralogy ... 46
4.1.2.1.1 Mudstones ... 49
4.1.2.1.2 Sandstones ... 49
4.2 DIAGENESIS AND DIAGENETIC MINERALS ... 52
4.2.1 Authigenic Quartz ... 52
4.2.2 Carbonate Cements ... 54
4.2.3 Sulphates ... 57
4.2.4 Fe-oxides (Hematite) ... 59
4.2.5 Pyrite ... 60
4.2.6 Titanium Oxides/Titanium-iron Oxides ... 60
4.2.7 Authigenic Feldspar ... 62
4.2.8 Clay Minerals ... 63
ix
4.2.8.2 Chlorite ... 63
4.3 CARBON AND OXYGEN ISOTOPES OF CARBONATE CEMENTS ... 64
4.3.1 FB1 Formation ... 65
4.3.2 FA Formation ... 65
4.4 CLAY MINERALOGY AND CHEMICAL COMPOSITIONS ... 72
4.4.1 Illite ... 72
4.4.2 Chlorite ... 75
4.4.3 Illite-smectite Mixed-layer ... 81
4.5 STRATIGRAPHIC MINERAL DISTRIBUTION ACROSS FB1 AND FA FORMATIONS ... 82
4.6 DIAGENETIC SEQUENCE (PARAGENESIS) ... 84
4.7 CONCLUSIONS ... 86
5.0 K-AR ILLITE DATING ... 88
5.1 ILLITE POLYTYPES ... 88
5.2 K-AR DATA ... 88
5.3 DISCUSSION AND CONCLUSIONS ... 92
PUBLISHED ARTICLE: ... 94
PART III ... 115
6.0 BULK ROCK GEOCHEMISTRY ... 116
6.1 MAJOR ELEMENTS (MEs) ... 118
6.1.1 Controls on the Major Elements ... 118
6.1.2 Geochemical Sediment Classification ... 119
6.2 TRACE ELEMENTS (TEs) AND RARE EARTH ELEMENTS (REEs) ... 121
6.2.1 Large Ion Lithophile Elements (LILE): Rb, Ba, Sr, Th, and U ... 123
6.2.2 High Field Strength Elements (HFSE): Zr, Y, Hf, Ta, and Nb ... 123
6.2.3 Transition Trace Elements (TTEs): Sc, V, Cr, Co, and Ni ... 123
6.2.4 Rare Earth Elements (REEs) ... 125
6.2.4.1 REE Mobility ... 129
6.3 WEATHERING AND SEDIMENTARY RECYCLING ... 130
6.4 PROVENANCE... 132
6.5 TECTONICS ... 134
6. 6 DISCUSSION AND CONCLUSIONS ... 137
7.0 ORIGIN OF URANIUM MINERALIZATION AND RED BEDS IN THE PALEOPROTEROZOIC FRANCEVILLE BASIN, GABON ... 142
ABSTRACT ... 142
x
7.2 GEOLOGICAL BACKGROUND ... 146
7.3 SAMPLES AND METHODS ... 149
7.3.1 Samples and Study Area ... 149
7.3.2 Methods ... 149
7.3.2.1 Core observation ... 149
7.3.2.2 Petrography ... 149
7.3.2.3 X-ray diffraction (XRD) ... 149
7.3.2.3 Whole Rock Analyses ... 151
7.3.2.4 Fe Isotope Analysis ... 152
7.4 RESULTS ... 152
7.4.1 Core Observations and Lithofacies Description ... 152
7.4.2 Petrography and Diagenetic Facies... 153
7.4.2.1 Texture and Detrital Mineralogy ... 153
7.4.2.2 Diagenetic Facies ... 153
7.4.2.3 Red Facies ... 154
7.4.2.4 Drab Facies (green, gray, and black) ... 155
7.4.2.5 White Facies ... 155
7.4.3 Clay Mineralogy and Chemistry ... 157
7.4.4 Geochemical Data ... 158
7.4.4.1 Whole-rock geochemistry ... 158
7.4.4.2 Fe Content and Fe Isotope Composition ... 164
7.5 DISCUSSION ... 166
7.5.1 Origin and Time of Hematite Formation in FA Formation Red beds ... 167
7.5.2 Geochemical constraints ... 169
7.5.2.1 Paleo-redox conditions recorded by FA Formation sediments ... 169
7.5.2.2 Controls on REE distribution ... 172
7.5.3 Implications for Origin of the Uranium Deposit ... 175
7.6 CONCLUSIONS ... 175
PART IV ... 177
8.0 GENERAL CONCLUSIONS AND FUTURE WORKS ... 178
8.1 GENERAL CONCLUSIONS ... 178
8.2 AREAS OF FUTURE RESEARCH ... 180
REFERENCES ... 183
APPENDIX ... 200
APPENDIX A Descriptions of representative samples from the studied drill holes ... 202
xi
APPENDIX C Calculated compositional formula for representative detrital and authigenic
feldspars based on 8 oxygen ... 209 APPENDIX D Calculated compositional formula for representative authigenic carbonates based on 3 oxygen ... 210 APPENDIX E Half-unit compositional formula for representative illite based on 11 oxygen ... 211 APPENDIX F Half-unit compositional formula for representative “true chlorite” based on 14 oxygen ... 212 APPENDIX G Half-unit compositional formula for representative “true chlorite” based on 14 oxygen in red beds ... 214 APPENDIX H Representative XRD diffractograms ... 215 APPENDIX I Geochemical composition of the studied samples Basin ... 219
xii
List of Figures
Chapter 2Figure 2. 1 General geological map of Gabon showing the different intracratonic sub-basins that makes
up the Paleoproterozoic Franceville series (black square inset) and the study basin (red square inset) (Thiéblemont et al., 2014). ... 10 Figure 2. 2 Regional geological and structural map of Franceville Basin (Ossa Ossa, 2010) showing the location of the study area (inset) and general geology of the surrounding area. ... 10 Figure 2. 3 Tectonics and sedimentation of the basin showing the basic stratigraphic units of the Franceville Series (modified from Préat et al., 2011). ... 11 Figure 2. 4 Simplified lithostratigraphic column of the Franceville Series in the Franceville Basin (modified from Gauthier-Lafaye and Weber, 2003). ... 13 Figure 2. 5 Paleogeography of FA Formation in the Franceville Basin (modified from Gauthier-Lafaye and Weber, 1989). ... 14 Figure 2. 6 Geological Map of the study area showing locations of study drill holes. ... 20 Figure 2. 7 Studied drill holes and sampling positions (not drawn to scale) ... 21
Chapter 3
Figure 3. 1 Lithofacies description of Mabinga drill hole, Franceville Basin. ... 31 Figure 3. 2 Cross-sections of studied drill holes in Bangombé ... 32 Figure 3. 3 Lithofacies description of representative drill hole in Bangombé, BA 2, Franceville Basin.
... 33
Figure 3. 4 Lithofacies descriptions of representative drill core in Kaya-Kaya/Kiéne-Otobo, GR15, Franceville Basin. ... 34 Figure 3. 5 Stratigraphic sections showing the vertical succession of lithofacies along NW-SE transect. The drill cores are based on inferred FA/FB boundary after core logging. ... 36 Figure 3. 6 Core images of representative lithofacies in lower FB1 Formation: (A) convolute (1) and planar laminations (2) in a mudstone; (B) load cast structure (arrow) of fine-grained sandstone submerged into a laminated shale; (C) Structureless fine- to coarse-grained sandstone; and (D) parallel laminations with organic materials (arrow) concentrated along the beddings. ... 37 Figure 3. 7 Core images of conglomerate and sandstone lithofacies: (A-C) massive, structureless, matrix-supported conglomerates with larger clasts floating in sandy matrix; (D) massive structureless coarse-grained sandstone; (E) oblique cross-beddings in medium- to coarse-grained sandstone; and (F) inclined parallel laminations in medium- to coarse-grained sandstone. ... 38 Figure 3. 8 Core images of representative heterolithics lithofacies: (A) fine-grained sandstone with interbedded coarse-grained sandstone; (B) inclined planer-laminations in fine- to medium-grained sandstone with sharp contact; (C) wavy heterolithic silty mudstone; (D) lenticular bedding in silty-mudstone; (E) inclined planar-laminations above cross- and flaser beddings in silty silty-mudstone; and (F) wavy laminations in mudstone.... 40 Figure 3. 9 Core images of representative fine-grained lithofacies: (A and B) fine-grained with ripples and cross-laminations; (C) plane-laminations in mudstone. ... 40
xiii
Chapter 4
Figure 4. 1 X-ray diffraction patterns of the bulk powdered of representative lower FB1 Formation samples (Ch: chlorite; I/M: illite/mica; Q: quartz; Ab: albite; Py: pyrite; Do: dolomite; FMS: Fine-grained sandstone + mudstones). ... 46 Figure 4. 2 Petrographic characteristics of representative FB1 Formation samples: (A) fine laminations in FB1 Formation mudstone. Note the concentration of organic materials (OM) in the thick darker layers (PL); (B) sparsely dispersed detrital grains of quartz and mica floating in dolomite (Dol) and organic rich matrix cements (PL); (C-D) pressure solution (stylolite) and long contact (LC) in clast dominated sandstone. Note the quartz cement (Qc) between grains in figure D (XL); (E) unaltered albite and pore-filling chlorite (BSE); (F) slight albitization of albite around grain edges (BSE).[Bio-biotite; Q-quartz; Py-pyrite; Ill-illite; Ch-chlorite; PL-plane polarized light; XL-crossed-polarized light; BSE-backscattered electron]. ... 47 Figure 4. 3 Photomicrographs of various textural characteristics in FA sandstones: (A-B) poorly sorted medium- to coarse-grained arkosic arenites. Note the cross-hatch twinning of microcline (KF) (XL); (C) stylolite contact (styl) in clast dominated sandstone; (D) concavo-convex (conv) contact between quartz grains. Note the pore-filling illite and syntaxial quartz overgrowth (Q2) (XL); (E) poorly sorted, angular quartz wacke with clasts floating in clay rich matrix (XL); (F) moderately sorted, sub-rounded silicified quartz arenite (XL); (G) moderately sorted, rounded silicified quartz arenite. Note the pore-filling quartz cement (Q1) between quartz grains (PL); (H) poorly sorted and angular detrital grains in clay rich matrix with altered and deformed biotites (Bio) (XL). [Qm-monocrystalline quartz; Qp-polycrystalline quartz; Ill-illite; Bio-biotite; Q-quartz; Or-orthoclase; Mu-muscovite; PL-plane polarized light; XL-crossed-polarized light]. ... 48 Figure 4. 4 Photomicrographs of microfabrics in mudstones (A) replacement of biotite by hematite along the biotite cleavage (hematization) in red coloured mudstone (BSE); (B) Dolomite (Do) rich mudstone (XL). [XL-crossed-polarized light; BSE-backscattered electron]. ... 49 Figure 4. 5 Photomicrographs of feldspars in FA sandstone showing the effects of dissolution and replacements along cleavage planes: (A) partly altered microcline (XL); (B) unaltered plagioclase (XL); (C) chloritized K-feldspar (PL); (D) alteration of feldspar by calcite (XL). [XL-crossed-polarized light; PL-plane polarized light]. ... 51 Figure 4. 6 Chemical compositions of feldspars within the studied samples ... 51 Figure 4. 7 Photomicrographs of the quartz cements and syntaxial overgrowths on quartz grains (A) forming early quartz cementation (Q1) in quartz arenite (PL); (B) early pore-filling quartz cements (Q1) (XL). Note Q1 appears to predate significant grain-to-grain compaction; (C-D) syntaxial quartz overgrowth (Q2) in arkosic sandstone (XL). [XL-crossed-polarized light; PL-plane polarized light]. 53 Figure 4. 8 Photomicrographs of carbonate cements: (A-D) detrital grains of medium- to coarse-grained sandstones enveloped by poikilotopic carbonate cements (A-XL; B-CL; C-BSE; D-CL). Note the crack opening of quartz and incipient replacement of the feldspar grain along cleavage plane in D. The presence of floating grains likely indicates early carbonate precipitation during burial; (E) replacement of albite along the grain margin by calcite (BSE); (F) zoning of dolomite (CL). The differences in luminescent colour result from variation in Fe and Mn contents (CL); (G) barite postdates pore-filling calcite (BSE); (H) pore-lining and grain coating hematite predating pore-filling calcite or likely due to later remobilization of Fe-oxide (BSE). [XL-crossed-polarized light; PL-plane polarized light; CL-cathodoluminescence]. ... 56 Figure 4. 9 Chemical composition of authigenic carbonates in FA Formation sediments ... 58
xiv
Figure 4. 10 Photomicrographs of authigenic sulphates: (A) extensive pore-filling anhydrites with poikilotopic texture (An) (XL); (B) anhydrite and barite appearing to postdate pore-filling illite (BSE); (C) hematite appears to postdate pore-filling anhydrite (BSE); (D) anhydrite replacing K-feldspar along margins and filling secondary dissolution pore spaces (BSE). [XL-crossed-polarized light; BSE-backscattered electron]. ... 59 Figure 4. 11 Photomicrographs of hematite occurrences: (A) macroscopic view of red coloured sandstone (XL); (B) hematite coatings around quartz grains. Note the absence of hematite at grain contacts (PL); (C) finely dispersed and pore-filling hematite (XL); (D) hematite stains on pore-filling calcite (XL). [XL-crossed-polarized light; PL-plane polarized light]. ... 60 Figure 4. 12 Photomicrographs of pyrite and titanium oxides: (A-B) euhedral pore filling pyrite and gypsum after quartz dissolution (XP and PL); (C) cubic crystal of pyrite within anhydrite cement (XL); (D) pyrite in association with pore-filling chlorite (BSE); (E) authigenic titanium oxide on postdating pore-filling illite (BSE); (F) postdating Ti-Fe oxide on illite cement (BSE). [XL-crossed-polarized light; PL-plane polarized light; BSE-backscattered electron]. ... 61 Figure 4. 13 Backscattered electron photomicrographs of albitization of feldspars: (A) replacement of K-feldspar by albite and calcite; (B) replacement of K-feldspar by barite and albite; (C) replacement of K-feldspar by Sr-barite along the cleavage plane; (D) anhydrite and barite replacement of albite. ... 62 Figure 4. 14 Photomicrographs of clay minerals: (A) chloritization of K-feldspar with illite filling the secondary pore space (BSE); (B) pore-filling chlorite (PL); (C) aggregate of coarse chlorite within the matrix with associated pyrite (XL); (D) pore-filling illite and chlorite (BSE). [XL-crossed-polarized light; PL-plane polarized light; BSE-backscattered electron]. ... 64 Figure 4. 15 Plots of: (A) δ18O and (B) δ13C isotope composition as a function of depth ... 68
Figure 4. 16 Plot of C and O isotope compositions analyzed from bulk samples ... 69 Figure 4. 17 XRD patterns of representative oriented <2 µm fractions in FA Formation. [AD-air-dried; EG-ethylene glycol; Ch-chlorite; I-illite; Q-quartz; MS-mudstone; CS-coarse-grained sandstone; FMS-fine- to medium grained sandstone; MCS-medium- to coarse-grained sandstone]. ... 73 Figure 4. 18 Chemographic projection of illite and chlorite chemical composition from FA sediments on 4Si-M+-R2+ ternary plot [4Si = Si4+/4; M+= K++ Na++ 2Ca2+; R2+= Fe2++ Mg2+]. (Modified from Meunier and Velde, 2004). ... 74 Figure 4. 19 XRD patterns of representative random <2 µm powder mounts for illite polytypes. [Q-quartz; Alb-albite; KF-K-feldspar; MGS-medium-grained sandstone; FGS-fine-grained sandstone; CGS-coarse grained sandstone].... 74 Figure 4. 20 XRD patterns of representative oriented <2 µm fractions at different treatments in FA Formation. [AD-air-dried; EG-ethylene glycol; Ch-chlorite; I-illite; Q-quartz] ... 77 Figure 4. 21 XRD patterns of representative random <2 µm powder mounts for chlorite polytypes. [Q-quartz; MS-mudstone; CS-coarse grained sandstone]. ... 78 Figure 4. 22 Structural formula of chlorite from FA and lower FB1 Formations plotted in the trioctahedral region of the vector representation of chlorite compositions (Wiewiora and Weiss, 1990) with Si and R2+as orthogonal axes and octahedral occupancy and total Al shown by contours. ... 78
Figure 4. 23 Chlorite compositions from lower FB1 and FA Formation sediments. Compositional fields are modified from Velde (1985). ... 79
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Figure 4. 24 XRD patterns of representative oriented <2 µm fractions in FB1 Formation. [AD-air-dried; EG-ethylene glycol; Ch-chlorite; I-illite; I/S (R1)-ordered illite/smectite mixed layer; I/S (R0)-random illite/smectite mixed layer Q-quartz; A-albite; MS-mudstone; MCS-medium- to coarse-grained sandstone] ... 83 Figure 4. 25 Mineral distributions across FA/FB1 boundary and FA Formation. [Q.A. - quartz arenite]
... 84
Figure 4. 26 Paragenetic sequences of principal authigenic minerals in FA Formation (modified from Bankole et al., 2015). ... 86
Chapter 5
Figure 5. 1 XRD patterns of representative oriented <1 µm fractions in FA Formation. [Black coloured pattern-air-dried; Red coloured pattern-ethylene glycol; Ch-chlorite; I-illite; KF-K-feldspar] ... 89 Figure 5. 2 Air-dried XRD patterns of representative sample at different size fractions in FA Formation. [Q-quartz; Ch-chlorite; I-illite; KF-K-feldspar, Alb-albite] ... 89 Figure 5. 3 XRD patterns of representative random powder mounts for illite polytypes at different size fractions. ... 90 Figure 5. 4 Plot of K-Ar ages against K2O contents ... 92
Chapter 6
Figure 6. 1 Bivariate plots of selected major oxides in the analysed FA and FB samples: (A-H) Major elements versus Al2O3; (I) CaO versus P2O5. ... 120
Figure 6. 2 Geochemical classification and compositional diagrams of representative rocks in FA and FB Formations (A) log (Fe2O3T/K2O) versus log (SiO2/Al2O3) (Herron, 1988); (B) log (SiO2/Al2O3)
versus log (K2O/Al2O3) (Pettijohn et al., 1972). ... 121
Figure 6. 3 Multi-element upper continental crust (UCC) normalized comparison diagrams for average composition of FA and FB clastic sediments: (A) Large ion lithophile elements; (B) High field strength elements; (C) Transitional trace elements. [UCC values from Rudnick and Gao, 2004]. ... 124 Figure 6. 4 Bivariate variation diagrams of Al2O3 versus trace elements: (A-B) Rb and Ba (Large ion
lithophile elements); (C) Rb versus Ba; (D-F) Zr, Y, Nb (High field strength elements); (G-I) Sc, V, Cr (Transitional trace elements). ... 125 Figure 6. 5 Chondrite-normalized REE patterns for clastic sediments from FA and FB Formation: (A) FB FMS; (B) FA FMS; (C) FA FMS. Data for C1-chondrite are compiled from Taylor and McLennan (1985). ... 127 Figure 6. 6 Chondrite-normalized REE patterns for clastic sediments from FA and FB Formation: (A) FB FMS; (B) FA FMS; (C) FA FMS. Data for C1-chondrite are compiled from Taylor and McLennan (1985). ... 128 Figure 6. 7 Bivariate variation diagram of REET (Total REE) with respect to: (A) Al2O3; (B) TiO2; (C)
Zr; and (D) Nb. ... 129 Figure 6. 8 Plot of Th against La to discriminate altered and unaltered monazite in the studied samples (Mathieu et al, 2001) [inserted dotted red line = altered monazites]. ... 130 Figure 6. 9 Discriminant diagrams illustrating weathering and recycling of studied FA and FB sediments: (A) Th/U vs Th; (B) Th/Sc vs Zr/Sc (McLennan et al., 1993). [UCC=upper continental crust].
xvi
Figure 6. 10 Major elements geochemical discriminantion function diagram for provenance signatures (Roser and Korsh, 1988). [Discrimination function, DF1 = -1.773TiO2 + 0.607Al2O3 + 0.76Fe2O3T –
1.5MgO + 0.616CaO + 0.509Na2O – 1.224K2O – 9.09; DF2 = 0.445TiO2 + 0.07Al2O3 – 0.25Fe2O3T –
1.142MgO + 0.438CaO + 1.475Na2O + 1.426K2O – 6.861]. ... 133
Figure 6. 11 Trace elements geochemical discriminant function diagram for provenance signatures: (A) TiO2 vs Ni (Floyd et al., 1989); (B) La/Th vs Hf (Floyd and Leveridge, 1987); (C) Th/Sc vs La/Sc
(Sugitani et al., 2006); (D) Cr/Th vs Th/Sc (Armstrong-Altrin et al., 2004). [UCC: Upper Continental Crust; A: andesite; B: basalt; F: felsic volcanic rock; G: granite; T: tonalite-trondhjemite-granodiorite). ... 135 Figure 6. 12 Major element composition of FB and FA formations samples on geochemical tectonic setting discrimination diagrams: (A) K2O/Na2O versus SiO2 (Roser and Korsh, 1986); (B) TiO2 versus
Fe2O3+MgO (Bhatia, 1983). [ACM= Active Continental Margin; PM = Passive Continental Margin;
CIA = Continental Island Arc; OIA = Oceanic Island Arc). ... 136 Figure 6. 13 Tectonic discrimination plots for based on immobile trace elements (Bhatia and Crook, 1986): (A) La-Th-Sc; (B) Th-Sc-Zr/10. [ACM= Active Continental Margin; PM = Passive Continental Margin; CIA = Continental Island Arc; OIA = Oceanic Island Arc). ... 137 Figure 6. 14Plot of chondrite-normalized Eu anomaly (Eu/EN*) against fractionation in HREE,
(GdN/YbN) for clastic sediments from FA and FB formations (Taylor and McLennan, 1985). ... 141
Chapter 7
Figure 7. 1 (A) Geological map of the Franceville Basin showing the location of the studied area and the map of Gabon in insert; (B) stratigraphic column of the Paleoproterozoic strata in the Franceville Basin (modified after Gauthier-Lafaye and Weber, 2003). ... 148 Figure 7. 2 N to S cross-section across the central part of the Franceville Basin near Kiené-Otobo (see fig. 1), showing variations in sediment color within the basin (modified from Haubensack, 1981; Gauthier-Lafaye, 1986). ... 148 Figure 7. 3 Characteristic red beds of the FA Formation, Franceville Basin ... 150 Figure 7. 4 Representative lithostratigraphic column of the FA Formation in the GR 15 borehole in the central part of the basin. ... 151 Figure 7. 5 Photomicrographs of representative samples of the FA Formation: (A) deformed, hematized biotite in fine-grained, red sediment (XP); (B) pore-filling hematitic pigment within illite matrix and hematized mica grain (BSE); (C) pseudomorph of hematite after trellis ilmenite texture (BSE); (D) pore-filling hematite (PL); (E) pore-lining and grain-coating hematite predating pore-pore-filling calcite (BSE); (F) pore-filling and grain-coating hematite. Note hematite between detrital grain contacts and predating quartz overgrowths (PL); (G) crystals of hematite and pore-filling anhydrite (BSE) (XP); (H) cubic, euhedral pyrite in the secondary pores of anhydrite (QO: quartz overgrowth; XP: cross-polarized; BSE: back-scattered electron). ... 156 Figure 7. 6 Representative XRD patterns of clay-size fractions in the studied samples of the FA Formation. (AD: air-dried; EG: ethylene glycol; R-FG: Red fine-grained sandstone; G-MS: Green mudstone; R-MCG: Red medium- to coarse-grained sandstone). ... 157 Figure 7. 7 Plot of tetrahedral Al against octahedral Fe/(Fe+Mg) cations in chlorite (after Curtis and others, 1985). Chlorite structural formula is calculated based on 14 oxygens. [RFG: red, fine-grained sandstone; RMCG: red, medium- to grained sandstone; GMCG = green, medium- to coarse-grained sandstone). ... 158
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Figure 7. 8 Scatter diagrams of major elements: (A-G) Binary diagrams with selected major elements. (R-FG: Red fine-grained sandstone; G-FG: Green fine-grained sandstone; GB-MS: Gray-black mudstone; G-MS: Green mudstone; R-MS: Red mudstone; R-MCG: Red medium- to coarse-grained sandstone; GB-MCG: Gray-black medium- to coarse-grained sandstone) ... 161 Figure 7. 9 Shale-normalized (PAAS) REE patterns for studied samples of the FA Formation: (A) mudstone-siltstone (MS); (B) fine-grained sandstones (FG); (C) medium- to coarse-grained sandstones (MCG) ... 162 Figure 7. 10 Plot of CeN and PrN anomalies for the FA Formation samples. Most of the samples do not
display significant Ce anomalies except sample 942.2 with a true positive Ce anomaly and GR15-993.3 with a true negative Ce anomaly. ... 163 Figure 7. 11 Cross-plot of U/Th ratios versus total Fe content (FeT) (FG+MS: fine-grained sandstone
+ mudstone-siltstone; GB: gray-black; MCG: medium- to coarse-grained sandstone). ... 163 Figure 7. 12 Cross plot of total Fe content (FeT) versus Fe isotope composition (δ56FeT) (FGM:
Fine-grained sandstone + mudstone; MCG: Medium- to coarse-Fine-grained sandstone). ... 166 Figure 7. 13 Apparent paragenetic relationships of diagenetic minerals in the FA Formation sediments.
... 170
Figure 7. 14 Cross plot of: (A) (K2O+Na2O)/Al2O3 versus Fe2O3T/(Fe2O3T+MgO); (B) FeT/Mg molar
ratio versus Fe isotope compositions for the FA Formation sandstones and mudstones. (FGM: Fine-grained sandstone + mudstone; MCG: Medium- to coarse-Fine-grained sandstone; Ch: Chlorite; H: hematite; I: Illite; F: Feldspar; B: Biotite). ... 173 Figure 7. 15 Cross plot of: (A) 1000*Cr/Fe versus bulk Fe isotope composition, δ56Fe
T; (B) Fe3+/FeT
molar ratio versus Fe isotope composition, δ56Fe
T, for the FA Formation sandstones and mudstones
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List of Tables
Table 2. 1 Summary of geochronological successions of events in the Franceville Series ... 19 Table 4. 1 Carbon and oxygen isotope values of carbonate cements in FA and FB1 Formations ... 67 Table 4. 2 Diagnostic reflections for chlorite polytypes (Brindley and Brown, 1980) ... 77 Table 5. 1 XRD data of the size fractions of the study samples of FA Formation from the central part of Franceville Basin ... 91 Table 5. 2 K-Ar illite data of clay size-fractions from one mudstone and two sandstones in FA Formation from the central part of Franceville Basin ... 91 Table 7. 1 Fe content and iron isotope composition of the FA Formation sediments. ... 164
I
INTRODUCTION GÉNÉRALE
Le Paléoproterozoïque (2,5 – 1,6 Ga) est une ère très longue couvrant 20% de l’ensemble des temps géologiques, depuis la naissance de la Terre. Elle a vu la vie primitive évoluer vers les formes plus complexes que sont les eukaryotes, la teneur en oxygène augmenter notablement au cours du Great Oxygen Event (« GOE ») et corrélativement la teneur en dioxyde de carbone (CO2) chuter mais, plus que tout, elle aura connu la formation et la stabilisation des supercontinents. Cette période critique de la Géologie est caractérisée par plusieurs orogenèses liées à la tectonique des plaques et plusieurs évènements volcaniques concomitants, dont l’interaction a conduit à la formation des bassins sédimentaires. Par la suite, la plupart des roches sédimentaires paléoprotérozoïques ont été soumises à divers degrés de métamorphisme et/ou intrudés de roches ignées. Malgré cela, d’importantes étendues de sédiments paléoprotérozoïques situées en domaine intracratonique, et reposant en discordance sur les roches archéennes, ont échappé jusqu’à ce jour au métamorphisme (i.e. Bassins de Thelon, d’Athabasca, du Transvaal, de McArthur et de Franceville).
Les bassins sédimentaires d’âge Paléoprotérozoïque constituent donc une grande part des archives de l’histoire de la Terre et contiennent 75% de toutes les ressources métalliques mondiales, telles que Au, Pt, Fe, Mn, Zn, U et Cr (Alterman et Corcoran, 2003). De même, le potentiel pétrolier des bassins protérozoïques (2.5 – 0,54 Ga) est connu depuis longtemps, ce qui a conduit à y étendre la prospection pétrolière et gazière, comme par exemple dans le Bassin de McArthur (Australie), daté du Protérozoïque moyen. Contrairement à celle de leurs homologues du Phanérozoïque, qui ont focalisé l’attention des chercheurs en raison de leurs fortes accumulations en hydrocarbures, l’étude des bassins paléoprotérozoïques nécessitent de relever de nombreux défis, liés à des changements dynamiques majeurs de l’environnement planétaire (atmosphère, océans et biosphère) mais également à la tectonique des plaques. Il en appert que l’on ne peut interpréter les processus et environnements sédimentaires du Précambrien (> 0,54 Ga) comme ceux du Phanérozoïque. De plus, les processus d’altération postérieurs aux dépôts et la surimpression des activités orogéniques, métamorphiques et volcaniques, ayant affecté les roches détritiques précambriennes, ont altéré les éléments de diagnostique que l’on peut tirer des structures sédimentaires, des fossiles et de leurs traces (environnement marin) ou de la lithologie, des variations texturales et/ou des mudstones (niveaux repères), comme l’on souligné Eriksson et al., (1998) et Catuneanu et al. (2005).
II
Dans le Sud-Est du Gabon, le Bassin paléoprotéroïque non-métamorphique de Franceville (2,1 Ga) est l’un des mieux préservés de cette période au monde et constitue de ce fait une zone privilégiée pour l’étude des premières étapes de la vie et de la sédimentation dans les environnements primitifs. A ce jour, il est aussi connu comme l’un des plus vieux systèmes pétroliers et compte parmi les grandes zones d’accumulation de matière organique du Précambrien (Mossman et al., 2005). C’est également dans ses strates que furent découvertes les concentrations en uranium les plus élevées jamais rencontrées dans un bassin sédimentaire, la première pile nucléaire naturelle (ca 1.8-2.0 Ga; Gauthier-Lafaye and Weber, 1989) et les plus anciens macrofossiles multicellulaires (El Albani et al., 2010, 2014). Par ailleurs ce bassin recèle également la troisième réserve mondiale de manganèse (Gauthier-Lafaye and Weber, 2003; Gauthier-Lafaye, 2006). Exceptionnel à plus d’un titre, ce bassin a fait l’objet de très nombreuses études sédimentologiques, stratigraphiques, gîtologiques et paléobio-logiques…(e.g., Weber, 1968; Haubensack, 1981; Gauthier-Lafaye, 1986; Gauthier-Lafaye and Weber, 1989 and 2003; Mossman et al., 2005; El Albani et al., 2010, 2014; Mathieu et al., 2000; Ossa Ossa et al., 2013, 2014; Canfield et al., 2013).
Les roches des formations FA et FB qui comblent le bassin de Franceville n’ont été affectées que par des processus de basse température (Bros et al., 1992; Mathieu et al., 2000; Gauthier-Lafaye and Weber, 1989, 2003; Ossa Ossa et al., 2013). Aussi, ces sédiments sont-ils des témoignages capitaux pour l’étude des effets à long terme des processus d’altération survenus pendant et après leur dépôt. Ils permettent par conséquent de comparer les archives sédimentaires du Paléoprotérozoïques et leurs environnements associés à celles des temps plus récents. C’est en outre dans ces formations FA et FB que s’observent les concentrations pétrolières et uranifères, dont elles constituent à la fois la source et le réservoir.
La plupart des travaux antérieurs portant sur la sédimentologie et la minéralogie de la formation FA et de la zone de transition FA/FB1 sont focalisés sur les roches de la partie nord-ouest du bassin (domaine proximal), c’est-à-dire au voisinage des réacteurs nucléaires d’Oklo et dans la région de Bangombe (e.g. Cortial et al., 1990; Cuney and Mathieu, 2000; Gauthier-Lafaye and Weber, 1989; Gauthier-Lafaye et al., 1996; Hidaka and Gauthier-Lafaye, 2000; Mathieu et al., 2000, 2001). En revanche, seules quelques études à caractère local ont été menées sur les roches sédimentaires de la partie sud-est de ce bassin (e.g., Haubensack, 1981; Gauthier-Lafaye, 1986; Gauthier-Lafaye and Weber, 1989; Ossa Ossa, 2010, 2014). En dépit des données minéralogiques et pétrographiques disponibles, aucune synthèse régionale de ce domaine n’a
III
été réalisée. De même, aucun essai de classification géochimique de ces roches aux fins de détermination de leur origine n’a été tenté à ce jour.
Par conséquent, notre premier objectif a été l’élaboration d’une synthèse des travaux précédents (e.g., Haubensack, 1981; Gauthier-Lafaye, 1986; Mathieu, 1999; Ossa Ossa, 2010, etc.), afin de mieux cerner, à l’échelle du Bassin Francevillien, les paramètres ayant contrôlés la sédimentation et la distribution des minéraux, ainsi que ceux liés aux processus d’altération post-dépôt, aux fluides diagénétiques et à leur migration. Puis, dans une seconde étape, la caractérisation géochimique fine de ces roches a visé à cibler leurs sources et le contexte tectonique qui a vu leur mise en place. Dans ces conditions, l’étude détaillée des sédiments rubéfiés visible dans la partie distale du bassin est apparue essentielle. Les processus de rubéfaction et leur durée sont des éléments fondamentaux à la compréhension des changements paléo-environnementaux qui se sont produits au cours de cette période critique de l’évolution de notre Planète, notamment l’augmentation de la teneur en oxygène atmosphérique.
OBJECTIFS
Le but de ce travail est l’étude des processus d’altération syn- et post-sédimentaires liés aux changements de couleurs de dépôts observables à l’échelle régionale, afin d’évaluer l’influence des fluides diagénétiques ayant percolés la formation FA, depuis les environ-nments proximaux jusqu’au domaine distal du Bassin de Franceville.
En considérant que les processus de diagenèse précoce et l’histoire des fluides associés sont le reflet des paramètres ayant contrôlé le fonctionnement de ce bassin au cours de son évolution, une analyse multi-variable a été réalisée pour reconstruire l’histoire diagénétique de ce bassin. Les variables intégrées à notre méthodologie ont été les données, stratigraphiques, lithologiques et faciologiques collectées sur le terrain ainsi que les données pétrographiques et géochimiques (élémentaires et isotopiques) acquises au laboratoire :
(1) Les lithofaciès et leurs séquences sont directement liés aux paléo-conditions de dépôts ;
(2) La minéralogie qualitative des minéraux primaires et diagénétiques permet d’établir des séquences minérales paragénétiques nécessaires à la détermination des paramètres syn- et post-dépôts qui ont contrôlés à la fois la circulation des fluides et la cristallisation des phases secondaires.
IV
(3) La composition chimique globale permet de suivre l’évolution et la distribution des éléments au sein de la formation FA (considérée comme un réservoir) et à la transition FA/FB1. Les signatures obtenues permettent de déterminer la source de ces éléments et fournissent des informations sur le contexte tectonique ;
(4) La stratigraphie et la minéralogie des lits rubéfiés rencontrés dans la formation FA à l’échelle du bassin renseignent sur la durée et la nature des mécanismes responsables du changement de couleur général et de leur relation avec la minéralisation uranifère dans une période d’augmentation considérable de la teneur en oxygène atmosphérique.
Cette étude fait partie du programme permanent de recherches mené par l’Université de Poitiers (France) sur le réexamen et la reconstruction du Bassin de Franceville (Gabon). Les résultats obtenus et les conclusions qui en ont été tirées doivent beaucoup aux auteurs des travaux antérieurs notamment Haubensack (1981), Gauthier-Lafaye (1986) et Ossa Ossa (2010).
STRUCTURE DE LA THESE
La présentation de cette étude est divisée en quatre grandes parties, elles-mêmes découpées en plusieurs chapitres.
Partie I groupe l’introduction générale et les objectifs du travail (chapitre 1). Suit une revue exhaustive sur la géologie du Bassin de Franceville et des zones d’étude (chapitre 2). Ce chapitre donne également une brève description des différentes approches mise en œuvre au cours de cette thèse.
Partie II expose la première partie des résultats et leur discussion relative à la sédimentologie et la pétrographie des roches étudiées. Cette partie a été subdivisée en trois chapitres. Le
chapitre 3 porte sur l’étude de plusieurs carottage ayant permis de décrire les lithofaciès
régionaux et de tracer leur distribution spatiale, étapes incontournables à la reconstitution des environnements de dépôt qui leurs sont liés, depuis le domaine proximal jusqu’au domaine distal du Bassin de Franceville. Le chapitre 4 expose les caractéristiques pétrographiques et diagénétiques des minéraux primaires et authigènes rencontrés dans ces lithofaciès afin de mieux cerner la durée des différents évènements paragénétiques. C’est pourquoi ce chapitre contient des données détaillées sur la minéralogie des argiles, leur chimie et leurs polytypes,
V
mais également les résultats isotopiques du carbone et de l’oxygène des carbonates qui ont été utilisés pour découvrir l’origine des fluides diagénétiques. Enfin, le chapitre 5 discute des nouvelles datations obtenues par la méthode K-Ar sur de l’illite authigène.
Cette partie est la version exhaustive d’un article paru dans la revue Precambrian Research et intitulé :
Bankole, O.M., El Albani, A., Meunier, A., Gauthier-Lafaye, F., 2015. Textural and paleo-fluid flow control on diagenesis in the Paleoproterozoic Franceville Basin, South Eastern, Gabon, Precambrian Research 268, 115-134.
Cet article est une étude systématique des assemblages de lithofaciès régionaux, rencontrés dans les formations FA et FB1 inférieure, de leur pétrographie et des processus diagénétiques associés, qui a conduit à l’établissement de séquences minérales paragénétiques et à leur interprétation quant à la migration des plaéo-fluides dans la Formation FA du Bassin de Franceville.
Partie III constitue le second grand volet de ce travail où l’on trouvera les résultats de l’étude géochimique menées sur les sédiments. La composition des roches sources est encore aujourd’hui le trait principal reflété dans la composition des sédiments. Les classifications géochimiques et les distributions élémentaires des Formations FA et FB1 sont commentées au
chapitre 6. La discussion porte sur l’interprétation de la géochimie des sédiments quant à leur
origine, aux facteurs tectoniques et aux processus d’altération ayant affectés leurs aires sources. Enfin, le chapitre 7 s’attache spécifiquement à dresser un tableau de la temporalité et des conditions qui ont prévalu à la minéralisation de l’uranium et à la formation des lits rubéfiés d’extension régionale dans la Formation FA. Dans ce contexte la précipitation de l’hématite est un phénomène fondamental qui a fait l’objet d’investigations pétrographiques, géochimiques et isotopiques, notamment sur des échantillons en position distal dans le Bassin de Franceville. Les résultats obtenus mettent en évidence le jeu complexe de conditions géochimiques et oxydo-réductrices sur le développement généralisé de la rubéfaction dans le bassin et la minéralisation de l’uranium est situation plus proximale.
Ce chapitre a donné lieu à une publication dans la revue American Journal of Science, sous la référence :
Bankole, O.M., El Albani, A., Meunier, A., Rouxel, O., Gauthier-Lafaye, F., Bekker, A., (submitted: October, 2015). Origin of uranium mineralization and red beds in the Paleoproterozoic Franceville Basin, Gabon, American Journal of Science
VI
Partie IV met en relief les conclusions générales de ce travail et les perspectives futures qu’elles sous-tendent (chapter 8).
1
PART I
INTRODUCTION, GEOLOGIC BACKGROUND, SAMPLE
AREA AND METHODOLOGY
2
1.0 GENERAL INTRODUCTION
The Paleoproterozoic (2.5-1.6 Ga) is the longest era of geologic time and spans approximately 20% of the entire geological time. The Archean-Proterozoic transition period (2.5-2.0 Ga) occupies of 500 Ma of the Earth history with substantial changes in several geological and geochemical events. This era saw the evolution of life and geological events such as development of the earliest eukaryotes, rise in atmospheric oxygen coined the Great Oxygenation Event ("GOE"), reduction in atmospheric carbon dioxide (CO2), and most importantly, formation and stabilization of supercontinents. It was a critical interval in the geological history and characterized by several orogenic events, plate tectonics and concomitant volcanic activity that led to the formation of sedimentary basins. As such, many Paleoproterozoic sedimentary rocks have undergone variable degree of metamorphism and igneous overprints. However, vast exposures of unmetamorphosed Paleoproterozoic clastic sediments are preserved in many areas and are separated from Archean rocks by an unconformity (e.g., Thelon Basin, Athabasca Basin, Transvaal Basin, McArthur Basin, and Franceville Basin).
The Precambrian (>0.54 Ga) sedimentary record accounts for about 85% of entire geological time, and the Precambrian rocks serve as a major source of almost 75% of Earth’s global mineral resources such as Au, Pt, Fe, Mn, Zn, U, and Cr (Alterman and Corcoran, 2002). Also, the existence of petroleum in Proterozoic (2.5-0.54 Ga) basins is no longer a surprise as hydrocarbon exploration activities have been extended to Proterozoic basins due to discovery of excellent source rocks and good reservoirs for oil and gas prospections (e.g., Middle Proterozoic McArthur Basin, Australia). In addition, Precambrian rocks not only provide records on the Earths evolution (crustal, geochemical, continental, and paleo-atmospheric) but also contain evidences on the origin and developments of early life (e.g., El Albani et al., 2010, 2014). Due to successive post-depositional alteration events and overprints (orogenies and metamorphisms), Precambrian clastic rocks commonly reflect deteriorated diagnostic features of modern sediments such as sedimentary structures, fossils and trace fossils (for marine settings), lithological and textural variations (Eriksson et al., 1998; Catuneanu et al., 2005) that makes establishment of well defined depositional environment a fundamental problem and difficult. In comparison, the Phanerozoic (<0.54 Ga) counterparts are well defined and has received more and wider attention among researchers due to its better quality preservation, good fossil records, and higher volumes of hydrocarbon accumulations. Nevertheless, available
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records of Precambrian sedimentation patterns, lithologies, structures, and basin evolution mostly within glacial, aeolian, deltaic, and possibly lacustrine exhibit many similarities to modern-Phanerozoic counterparts, (Eriksson et al., 1998, 2001, 2005; Catuneanu et al., 2005; Bose et al., 2012). However, slight distinctiveness have been noted to exist especially in shallow marine environments, which must be treated with cautions due to its preservational bias towards epicontinental sea deposits (Bose et al., 2012). Similarly, the evolution of both Precambrian and Phanerozoic sedimentary basins are believed to be controlled by same interaction of plate tectonic and magmatic thermal processes, which were further modified by of paleoclimate and eustasy (e.g., Erikson et al., 2001). Therefore, controls on sedimentation and stratigraphic on the Precambrian basins, which are basic factors in sequence stratigraphy, have also been thought to be unifporm with the younger Phanerozoic basins (e.g., Catuneanu et al., 2005).
Apart from the issues related to shallow marine sediments, the challenges in the analysis of Precambrian sedimentary rocks/basins are related to those factors controlling the rates and intensities of weathering, erosion, transport, deposition, lithification, and diagenetic processes (e.g., Erikson et al., 2001). These enigmas include seculiar changes in magmatic processes and plate tectonics, dynamic changes in the atmosphere-ocean and the biosphere (Paleoclimate and eustasy), degree of preservation, and absence of datable fossils (geochronology) (Eriksson et al., 1998, 2001, 2005).
The unmetamorphosed Paleoproterozoic Franceville Basin (ca 2.15 Ga), in the southeastern Gabon, is one of the best preserved sedimentary basin of this age in the world and it is an important area for the study of the early Earth history and sedimentary system. It is one of the oldest petroleum systems in the world and ranked as a major Precambrian accumulation of organic matter (Mossman et al., 2005). The basin is well known for being the host to the first known natural fossil nuclear-fission reactors (ca 1.8-2.0 Ga; Gauthier-Lafaye and Weber, 1989), discovery of oldest multicellular macrofossils (El Albani et al., 2010, 2014), oldest high-grade uranium ore in sedimentary basin, and host to the third known manganese reserve in the world (Gauthier-Lafaye and Weber, 2003; Gauthier-Lafaye, 2006). Therefore, the basin has been extensively studied for sedimentology, stratigraphy, uranium and manganese exploration, and evolution of life (e.g., Weber, 1968; Haubensack, 1981; Lafaye, 1986; Gauthier-Lafaye and Weber, 1989 and 2003; Mossman et al., 2005; El Albani et al., 2010, 2014; Mathieu et al., 2000; Ossa Ossa et al., 2013, 2014; Canfield et al., 2013).
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The rocks that make up the basin-filling FA and FB formations of Franceville basin have only been affected by low-temperature processes (Bros et al., 1992; Mathieu et al., 2000; Gauthier-Lafaye and Weber, 1989, 2003; Ossa Ossa et al., 2013). Thus, these rocks provide an excellent opportunity to study the long-term effects of depositional and post-depositional alteration processes in a Paleoproterozoic sedimentary basin in comparison to modern sedimentary environments. The FA and FB formations are of particular interest from the point of petroleum, uranium, and biogeochemistry because they represent reservoir and source rocks, respectively.
Most of the previous works on the sedimentology and mineralogy of FA Formation and FA/FB1 transition have focused on the rocks in the north-western part of the basin (proximal) around the natural nuclear fission reactors at Oklo and parts of Bangombe (e.g. Cortial et al., 1990; Cuney and Mathieu, 2000; Gauthier-Lafaye and Weber, 1989; Gauthier-Lafaye et al., 1996; Hidaka and Gauthier-Lafaye, 2000; Mathieu et al., 2000, 2001). On the other hand, only few studies have been carried out on the rocks in the south-eastern part of the basin (e.g., Haubensack, 1981; Gauthier-Lafaye, 1986; Gauthier-Lafaye and Weber, 1989; Ossa Ossa, 2010, 2014). Despite the vast available data on the mineralogy and petrography of these sediments, detailed integrated studies on a regional scale are very few as most of the studies are done on type localities. Geochemical classification of sediments and provenance data for the Franceville Basin sediments appear to be non-existent to the best of my knowledge during this present study.
Therefore, the motivation behind this study is to build and advance on previous work (e.g., Haubensack, 1981; Gauthier-Lafaye, 1986; Mathieu, 1999; Ossa Ossa, 2010) in terms of depositional controls, mineral distributions, post-depositional alterations, diagenetic fluids and fluid migration on a regional scale. In addition, to provide, for the first time, detailed geochemical characteristics of the sediment and infer the provenance and tectonic settings of their parent materials. Also, to provide a detailed characterization of the red-coloured sediments in the distal part of the basin and the relative timing and mechanisms of reddening, which are all poorly understood and constrained.
1.1 AIMS AND OBJECTIVES
The aim of this thesis is to investigate the depositional and post-depositional alteration processes associated with the basin-wide colour changes and evaluate the influence of
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diagenetic fluids on the FA Formation rocks from the distal to proximal environments of Franceville Basin. Considering that post-depositional processes (diagenesis) and fluid flow histories in sedimentary basins reflect control of basin-scale processes that occur throughout the basin evolution; a combined basin analysis tools are employed to reconstruct the diagenetic and fluid history of the basin. This study combined an integrated methodology involving field observations, facies analysis, petrography, geochemistry and isotopic analysis to:
1. characterize the lithofacies, sequences and infer paleo-depositional processes of the FA Formation sediments;
2. qualitatively investigate the mineralogy, diagenetic processes and over-printings, and establish mineral paragenetic sequences;
3. constrain the impact of mineralogical, pre- and post-depositional controls on paleo-fluid flow and mineralization processes;
4. determine bulk elemental composition and their distributions between the FA reservoir and FB1 transition boundary to infer their provenance and tectonic settings;
5. constrain the timing and mechanisms of reddening of the red beds, their relative sequence of colouring, and their relationship with uranium mineralization.
This study is part of the on-going research program on the investigation and reconstruction of the Franceville Basin, Gabon at the Université de Poitiers, France. It builds on previous work on the basin, most notably by Haubensack (1981), Gauthier-Lafaye (1986), and Ossa Ossa (2010).
1.2 THESIS STRUCTURE
The layout of this study is divided into four different parts which are sub-divided into 8 chapters.
Part I starts with the general introduction and objectives of the study (chapters 1). This is followed by chapter 2 that presents an in-depth review of the geological background of Franceville Basin and the study areas. The various methodological approaches used in this study are briefly outlined in the later part of chapter 2.
Part II provides the first part of the results and discusses the sedimentology and petrology aspects of this research. This part consists of three chapters: chapter 3 examines the lithofacies analysis and depositional environment of the Franceville Basin from the proximal to distal part
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of the basin using drill cores to understand the various lithofacies associations and their distributions. Chapter 4 then discusses the petrographic characterization and diagenesis of associated detrital and authigenic minerals in these lithofacies to understand the relative timing of various paragenetic events. Stable isotopes of C and O in diagenetic carbonates are used to infer the origin of diagenetic fluids. This chapter also detailed the clay mineralogy, chemistry, and polytypes. Chapter 5 discusses the K-Ar isotope dating of the authigenic illite.
Part II is an expanded version of manuscript already published in Precambrian Research journal:
Bankole, O.M., El Albani, A., Meunier, A., Gauthier-Lafaye, F., 2015. Textural and paleo-fluid flow control on diagenesis in the Paleoproterozoic Franceville Basin, South Eastern, Gabon, Precambrian Research 268, 115-134.
The manuscript takes a systematic approach to studying the regional lithofacies assemblages, petrography and diagenesis of the FA and lower FB1 formation rocks, by describing and establishing mineral paragenetic sequences and their impact on paleofluid migration in FA Formation of Franceville Basin.
Part III presents the second major division of the results and investigates the geochemical composition of the sediments. The composition of the source rocks has a significant influence on the final composition of the sediments. The geochemical classifications and elemental distributions in the sediments of FA and FB1 formations are critically addressed in chapter 6. This chapter deals with the geochemistry of the sediments in order to infer the tectonic settings of the source rocks, understand the chemical weathering in the source area, and the provenance of the sediments.
Chapter 7 focuses on the petrographic, geochemical, and isotopic constraints on the origin of
uranium mineralization and the red beds, especially the timing and mechanisms of hematite precipitation, in the distal part of the Franceville Basin. This chapter addresses the complex geochemical and isotopic constraints of the FA Formation red beds in the distal part of the basin and the sequence of sediment colouration in relation to paleo-redox condition and uranium mineralization in the Franceville Basin. This chapter reproduces a manuscript submitted for publication in the American Journal of Science:
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Bankole, O.M., El Albani, A., Meunier, A., Rouxel, O., Gauthier-Lafaye, F., Bekker, A., (submitted: October, 2015). Origin of uranium mineralization and red beds in the Paleoproterozoic Franceville Basin, Gabon, American Journal of Science.
