# Geology of the Tabakoto gold deposit,
Kédougou-Kéniéba Inlier, West African Craton, Mali

Mahamadou Diallo, Stefano Salvi, Lenka Baratoux, Didier Béziat, Olivier

Vanderhaeghe, Ibrahima Labou, David Baratoux, Saïdou Ly

► To cite this version:

Mahamadou Diallo, Stefano Salvi, Lenka Baratoux, Didier Béziat, Olivier Vanderhaeghe, et al.. Geology of the Tabakoto gold deposit, Kédougou-Kéniéba Inlier, West African Craton, Mali. Bulletin de la Société Géologique de France, 2024, 195, pp.24. 10.1051/bsgf/2024024. hal-04982055

HAL Id: hal-04982055

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# Geology of the Tabakoto gold deposit, Kédougou-Kéniéba Inlier, West African Craton, Mali

Mahamadou Diallo<fnref n="1" />*, Stefano Salvi<fnref n="2" />, Lenka Baratoux<fnref n="2" />,<fnref n="3" />, Didier Béziat<fnref n="2" />, Olivier Vanderhaeghe<fnref n="2" />, Ibrahima Labou<fnref n="4" />, David Baratoux<fnref n="2" />,<fnref n="3" /> and Saïdou Ly<fnref n="1" />

Received: 10 January 2024 / Accepted: 21 October 2024 / Publishing online: 17 December 2024

**Abstract** – The Tabakoto gold deposit is part of the highly endowed west-Malian gold belt, which hosts several world-class deposits. Located in the Paleoproterozoic Kédougou-Kéniéba Inlier (KKI) of the West African Craton (WAC), the deposit is contained in Birimian metasedimentary rocks of the Kofi series that are intruded by magmatic dikes. The Tabakoto deposit is characterized by multiple overlapping stages of magmatic dike intrusions and hydrothermal alteration during distinct regional-scale deformation events. The metasedimentary rocks are characterized by a S<sub>0/1</sub> foliation delineated by the alternation of metagreywacke and meta-argillite. This foliation is affected by upright folds with N-S trending axial planes marked by a S<sub>2</sub> schistosity and cross-cut by conjugate steep-dipping dextral NE-SW and sinistral NW-SE trending faults. These structures record regional-scale E-W shortening, first associated with crustal thickening and then with N-S stretching, evolving from ductile to brittle deformation. The gold-bearing V<sub>2</sub> quartz-pyrite veins developed in dikes and metasedimentary units, are parallel to the S<sub>2</sub> schistosity of the D<sub>T2</sub> deformation event. These veins contained reduced phases such as pyrrhotite, loellingite and scheelite as well as bismuth. The second gold-bearing V<sub>3a</sub> quartz-carbonate veins are located in the NE-SW and NW-SE trending faults of the D<sub>T3</sub> deformation event. The latter veins are marked by sodic alteration followed by carbonate (i.e., dolomite-ankerite ± calcite ± siderite) and phyllic (i.e., chlorite-muscovite-sericite) alterations. In the quartz-pyrite veins, gold occurs as inclusions in pyrite, arsenopyrite, and pyrrhotite, whereas in quartz-carbonate shear veins, gold is present in microfractures cross-cutting pyrite and arsenopyrite and at the contact between grains of pyrite and arsenopyrite, and of arsenopyrite and pyrrhotite. The litho-structural and mineralogical features of the Tabakoto deposit portray a polyphase mineralization with (i) an intrusion-hosted orogenic gold system, where the metabasaltic to metarhyolitic dikes acted as suitable emplacement for fluids flow and (ii) a hydrothermal fluid circulation under ductile to brittle conditions.

**Keywords:** Mali / Kédougou-Kéniéba Inlier / Tabakoto gold deposit / Hydrothermal alteration / Mineralization / Orogenic gold

**Résumé** – Géologie du gisement d'or de Tabakoto, Boutonnière de Kédougou-Kéniéba, Craton Ouest Africain, Mali. Le gisement d'or de Tabakoto fait partie de la ceinture aurifère ouest-malienne très riche, qui abrite plusieurs gisements de classe mondiale. Situé au sein de la boutonnière paléoprotérozoïque de Kédougou-Kéniéba (BKK) du Craton Ouest Africain (COA), le gisement est encaissé dans les roches métasédimentaires birimiennes de la série de Kofi qui est intrudée par des dykes magmatiques. Le gisement de Tabakoto est caractérisé par de multiples étapes superposées d'intrusions de dykes magmatiques et d'altération hydrothermale au cours d'événements de déformation distincts à l'échelle régionale. Les roches métasédimentaires sont caractérisées par une foliation S<sub>0/1</sub> délimitée par l'alternance de métagreywacke et de méta-argillite. Cette foliation est affectée par des plis droits avec des plans axiaux orientés N-S marqués

<footnote n="1">¹ Laboratoire de Cartographie et Ressources Minérales, Département de Géologie et Mines, ENI-ABT, 410 Avenue Van Vollenhoven, BP 242, Bamako, Mali</footnote>

<footnote n="2">² Géosciences Environnement Toulouse, Université de Toulouse III, CNRS & IRD, 14 Avenue Edouard Belin,31400 Toulouse, France</footnote>

<footnote n="3">³ UFR Sciences de la Terre et des Ressources Minières (STRM), Université Félix Houphouët-Boigny, Abidjan-Cocody, 22 B.P. 582 Abidjan 22, Côte d'Ivoire</footnote>

<footnote n="4">⁴ Ecole nationale supérieure des Mines et de la Géologie (ENSMG) Ex_IST, Université Cheikh Anta DIOP de Dakar, Senegal</footnote>

<footnote>*Corresponding author: mahamadou.diallo@mesrs.ml</footnote>
par une schistosité S₂ et recoupés par des failles cassantes conjuguées dextres à fort pendage orientées NE-SW et senestres orientées NW-SE. Ces structures témoignent d'un raccourcissement E-W à l'échelle régionale, d'abord associé à un épaississement crustal, puis à un étirement N-S, passant d'une déformation ductile à une déformation cassante. Les veines aurifères de quartz-pyrite V₂ sont développées dans les dykes et les unités métasédimentaires et sont parallèles à la schistosité S₂ de la phase de déformation Dₜ₂. Ces veines contiennent des phases réduites telles que la pyrrhotite, la loellingite et la scheelite ainsi que du bismuth. Les deuxièmes veines aurifères de quartz-carbonate sont situées dans les failles orientées NE-SW et NW-SE de la phase de déformation Dₜ₃. Ces dernières veines sont marquées par une altération sodique suivie d'une altération carbonatée (c.-à-d. dolomite-ankérite ± calcite ± sidérite) et phyllitique (c.-à-d. chlorite-muscovite-séricite). Dans les veines de quartz-pyrite, l'or est présent sous forme d'inclusions dans la pyrite, l'arsénopyrite et la pyrrhotite, tandis que dans les veines de quartz-carbonate, l'or est présent dans les microfractures recoupant la pyrite et l'arsénopyrite et au contact entre la pyrite et l'arsénopyrite ou entre l'arsénopyrite et la pyrrhotite. Les caractéristiques litho-structurales et minéralogiques du gisement de Tabakoto montrent une minéralisation polyphasée avec (i) un système aurifère orogénique à intrusion, où les dykes métabasaltiques à métarhyolitiques ont servi d'emplacement approprié pour l'écoulement des fluides et (ii) une circulation de fluides hydrothermaux dans des conditions ductiles à cassantes.

**Mots clés :** Mali / Boutonnière de Kédougou-Kéniéba / Gisement d'or de Tabakoto / Altération hydrothermale / Minéralisation / Or orogénique

# 1 Introduction

In the past decade, numerous studies conducted on the southern part of the West African Craton (WAC) have led to its recognition as the foremost Paleoproterozoic gold province globally (Goldfarb et al., 2017). Paleoproterozoic gold deposits in the WAC (Fig. 1a) is marked by the three major tectonic episodes (Masurel et al., 2021): the Eoeburnean (2200 to 2135 Ma), the early Eburnean (2110 to 2095 Ma), and the late Eburnean (2095 to 2060 Ma), each characterized by complex deformation, metamorphic, and magmatic processes. Although several styles of gold mineralization developed during these episodes (Goldfarb et al., 2017; Masurel et al., 2021), by far the majority correspond to the orogenic deposit type (e.g., Milési et al., 1992; Béziat et al., 2008; Feybesse et al., 2006; Lawrence et al., 2013a; Baratoux et al., 2015; Markwitz et al., 2016; Goldfarb et al., 2017; Masurel et al., 2017a; Thébaud et al., 2020; Masurel et al., 2021). Deposits that have not been classified as orogenic include (i) paleo-placers associated with quartz pebble conglomerates in the Ashanti belt in Ghana (e.g., Milési et al., 1989; Hirdes and Nunoo, 1994), (ii) the porphyry-Cu type deposit at Gaoua in Burkina Faso (Le Mignot et al., 2017), (iii) the skarn type deposits of Ity and Tongon in Côte d'Ivoire (e.g., Béziat et al., 2016; Lawrence et al., 2017), and (iv) the intrusion-related gold systems at Morila in Mali (e.g., McFarlane et al., 2011) and Bonikro in Côte d'Ivoire (Masurel et al., 2019). This last type of deposit is distinguished by the metal association of Au-Bi-Te-As and is spatially and temporally related to moderately-reduced granitic plutonism (Sillitoe and Thompson, 1998; Thompson and Newberry, 2000; Hart, 2007).

The Paleoproterozoic Kédougou-Kéniéba Inlier (KKI; Fig. 1b) represents the second-largest gold endowment in the West African Craton after the Ashanti belt in Ghana (e.g., Goldfarb et al., 2017). Similarly to the rest of the WAC, most of the Paleoproterozoic gold deposits in the KKI are of the orogenic type. Examples include the Sabodala deposit (Senegal) (Fig. 1b) hosted in mafic and ultramafic rocks (Sylla et al., 2016), and the Massawa deposit (Fig. 1b) localized in shear zones within greenschist facies volcanoclastic rocks and

metagreywacke, both in the Mako belt (Treloar et al., 2014). In the eastern reaches of the KKI, within the Kofi series (essentially in Mali), a number of well-studied deposits (Fig. 1b) occur in the vicinity of granitoid bodies, which has prompted researchers to inquire about the connection between magmatic activity and mineralization. These include the Gara deposit (Loulo district), hosted in tourmaline-rich folded metaturbidites (Dommanget et al., 1993; Lawrence et al., 2013b); the Sadiola deposit, hosted in detrital metasediments and impure marbles (Masurel et al., 2017a, 2017b); and the Gounkoto deposit, hosted in metagreywacke with sodic or phyllic alteration (Lambert-Smith et al., 2016a). However, most authors conclude that these deposits are also of orogenic type (e.g., Lambert-Smith et al., 2016b; Masurel et al., 2017b, 2017c; Allibone et al., 2020). Nevertheless, deposits of types other than orogenic are described in the KKI. These include the Alamoutala skarn deposit (Masurel et al., 2017b) and the Yatela deposit (Fig. 1b), latter been a residuum type deposit hosted in marble and diorite altered to saprolite (Hein et al., 2015; Masurel et al., 2016).

The Tabakoto gold deposit, located within the Kofi metasedimentary series of the KKI (Fig. 1b), has not been subject to scientific studies yet. Prospects have shown evidence for multiple mineralization events related to polyphased deformation, as well as a close association with several generations of intersecting magmatic dikes, which encouraged previous exploration campaigns (e.g. Nevsun Resources Ltd., Avion Resources Corporation, Endeavour Mining Corporation) to question the role of magmatism on the mineralization (e.g., Nielsen, 2002; Hein, 2008; De Hert et al., 2014).

In this paper, we present new structural, petrological, mineralogical and geochemical data from both mineralized ore and barren host rocks of the Tabakoto deposit, and establish the spatial and temporal relationships between deformation, hydrothermal alteration and gold mineralization phases. These data provide new insights to discuss the chronology and mechanisms of gold mobilization and deposition in the framework of the different tectonic-magmatic-hydrothermal phases, and put it into perspective at the scale of the KKI.
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** (a) Simplified geological map of the Leo-Man Shield showing the location of the most gold deposits across the WAC (modified after Thiéblemont *et al.*, 2016). (b) Zoom of the Kédougou-Kéniéba Inlier from (a). The red box locates the Tabakoto district. *Abbreviation: MB:* Mako belt; *DDS:* Dialé-Daléma series; *FB:* Falémé belt; *KS:* Kofi series; *TGD:* Tabakoto gold district; *Sa:* Saraya pluton; *Mo:* Moussala granodiorite; *Ga:* Gamaye monzogranite; *Yt:* Yatea monzogranite; *Sek:* Sekekoto granodiorite; *Al:* Alamoutala granodiorite.</figcaption></figure>

# 2 Regional geological setting

## 2.1 Geological background

The Tabakoto gold deposit belongs to the western Malian gold district located in the eastern part of the Paleoproterozoic Kédougou-Kéniéba Inlier (Fig. 1b). The KKI consists of Birimian volcano-plutonic belts known as the Mako and Falémé greenstone belts, alternating with metasedimentary Dialé-Daléma and Kofi series (Fig. 1b), which were deformed and metamorphosed during the Eburnean orogeny (Bonhomme, 1962; Ledru et al., 1991). The NNE-SSW to N-S trending Mako belt (MB, Fig. 1b) is composed of basalt, locally displaying pillow structures, gabbro and ultramafic rocks of tholeiitic affinity interlayered with immature detrital sediments and a more or less differentiated sequence with calc-alkaline affinity (gabbro, basalt, andesite, dacite, and rhyolite) occurring as veins cross-cutting the tholeiitic rocks (Hirdes and Davis, 2002; Dioh et al., 2006; Labou et al., 2020). The Falémé belt (FB, Fig. 1b) is made of metamorphosed pyroclastic rocks, subordinate rhyodacite lavas, and andesite flows with preserved pillow structures, interbedded with metavolcanoclastics, metagreywackes and metacarbonates (Bassot, 1987; Hirdes and Davis, 2002). The protoliths of the Dialé-Daléma (DDS) and Kofi (KS) series (Fig. 1b) are dominated by sediments, largely derived from erosion of the Mako belt (Bassot, 1966; Koné et al., 2020). The metasedimentary rocks are intercalated with volcanoclastic rocks and rhyolite flows (Bassot, 1987; Hirdes and Davis, 2002). These features evoke an epicontinental paleoenvironment close to a magmatic arc.

The different belts and series of the inlier are intruded by numerous granitic to diorite plutons (Dioh et al., 2006; Gueye et al., 2008; Masurel et al., 2017c) and by post-Eburnean mafic dikes (Baratoux et al., 2019). The Kofi series is intruded by deformed syntectonic monzogranite plutons such as the Gamaye (Ga) and Yatea (Yt) plutons, the latter being located about 4 km to the North of the Tabakoto deposit (Fig. 1b).

## 2.2 Tectonic framework

The KKI displays a complex superposition of structures that has been attributed to three deformation phases with an early period of crustal thickening ($D_1$) followed by a period of transcurrent tectonics ($D_2-D_3$) characterized by ductile to brittle structures (Milési et al., 1989; Ledru et al., 1991; Diene et al., 2012; Diatta et al., 2017; Masurel et al., 2017c).

The $D_1$ phase was first proposed to be represented by inclined NNW-SSE to N-S trending folds with associated reverse faults accommodating ENE-WSW to E-W horizontal shortening best documented in the magmatic series of the Mako belt and the volcano-sedimentary series of the Dialé-Daléma (Milési et al., 1989; Gueye et al., 2008; Diene et al., 2012; Masurel et al., 2017c; Allibone et al., 2020). However, more recently, Diallo et al. (2020) identified a WNW-ESE to NW-SE trending $S_{0/1}$ foliation in the northern part of the Mako belt marked in regional magnetic data by magnetic lineaments and that locally correlates with isoclinal folds identified in the field.
The D₂ is manifested by regional-scale upright NNE-SSW trending folds, N-S to NNE-SSW trending thrusts and shear zones (Milési et al., 1989; Ledru et al., 1991, Dabo and Aifa, 2010;Diene et al., 2012; Lawrence et al., 2013a; Masurel et al., 2017c; Diallo et al., 2024). The D₂ is characterized by the transposition of earlier structures as identified in magnetic data by a regional deflection of WNW-ESE trending lineaments into NNE-SSW trending lineaments indicating an apparent dextral sense of shear (Diallo et al., 2020). Accordingly to previous works, D₂ marks a switch from a dominantly coaxial to a non-coaxial deformation regime (Diene et al., 2015; Lambert-Smith et al., 2016a; Masurel et al., 2017c).

The D₃ phase is marked by a transposition of N-S to NNE-SSW trending D₂ structures and localization of deformation along major shear zones such as the Senegalo-Malian Shear Zone (SMSZ; Bassot and Dommanget, 1986) and the Main Transcurrent Zone (MTZ; Ledru et al., 1991). D₃ is also accompanied by voluminous calc-alkaline magmatism (Gueye et al., 2008), circulation of hydrothermal fluids and gold mineralization along secondary structure of regional-scale shear zones and/or faults (Lawrence et al., 2013a; Treloar et al., 2014; Diene et al., 2015; Dabo et al., 2016; Lambert-Smith et al., 2016a; Masurel et al., 2017c).

As an alternative, based on the analysis of regional scale structures, Chardon et al. (2020) propose a deformation continuum characterized by anastomosed shear zones that accommodated distributed shortening and lateral flow of the orogenic lithosphere between the converging Kénéma-Man and Congo Archean provinces. In this work we will use the framework of deformation events discretized into several phases for practical purposes even if the Eburnean orogeny may also be considered as a continuum of deformation varying in intensity with time, as proposed by the latter author.

# 3 Methods and materials

## 3.1 Mapping and sampling

Systematic face mapping of the pit workings (Fig. 2) for Tabakoto North Pit (TNP) and underground galleries for TNP and Tabakoto Main Pit (TMP) was completed, in order to emphasize the relationship between deformation, emplacement of various dikes generations, hydrothermal alteration and gold mineralization. Logging of drill core sections was also undertaken. Samples investigated in this paper come from underground workings and drill core, and include country rocks and all types of ores. Structural data were reported as strike/dip/quadrant for the planar structures and plunge/azimuth for linear structures.

## 3.2 Petrographic study

Polished thin sections were prepared from 18 representative samples at the laboratory of Geoscience Environment Toulouse (GET) and were used for petrographic observation, SEM (Scanning Electron Microscope) and EPMA (Electron Probe Micro-Analyser) investigations.

SEM analyses were performed with a JEOL JSM6360LV Microscope for SEM imaging. X-ray microanalysis (EDS: Energy Dispersive Spectrometry) were obtained with an

X-Ray Microanalysis BRUKER System. This setup provided semi-quantitative chemical analyses. EPMA was used for *in situ* quantitative microanalysis of the chemical composition of sulfides, oxides and gold. These analyses were carried out at the Raimond Castaing micro-characterization center in Toulouse (University Toulouse III-Paul Sabatier) using a Cameca SX Five electronic microprobe composed of five wavelength dispersion spectrometers (WDS). The analysis conditions were 15 kV for the acceleration voltage, for currents of 10 or 20 nA depending on the resistance of the minerals to the electron beam. The acquisition times were 10 s at the peak and 5 s at either side of the peak for the continuous background.

## 3.3 Geochemistry

Several samples of magmatic rocks (dikes) were selected for whole-rock geochemical analyses, to obtain major and trace element data (Tab. 1). These were carried out at the Central Analytical Facilities, Stellenbosch University (South Africa). Major element concentrations were obtained by X-ray fluorescence spectrometry (XRF) while trace element concentrations were obtained by ICP-MS (Inductively Coupled Plasma Mass Spectrometer) with the following analytical conditions: Agilent 8800, Carrier gas: 0.8 L/min Ar + 0.004 L/min Nitrogen. Four geostandards (BHVO & BCR glass and BHVO & BCR powder) were used to check the reliability of the data, correct the measurements for instrumental drift and recalculate concentrations.

# 4 Petrography and geochemistry of the host rocks

## 4.1 Petrography

The most abundant lithology in the Tabakoto area are metasedimentary rocks of the Kofi series, mostly metagreywacke alternating with meta-argillite. These are intruded by mafic to felsic dikes and are affected to various extent by hydrothermal alteration (Fig. 2b).

### 4.1.1 Metasedimentary rocks

The metasedimentary rocks consist of metagreywacke intercalated with meta-argillite, ranging from decameters to meters in thickness, which are interpreted to represent a turbiditic sequence (Figs. 3a, 3b). Transposition of the sedimentary bedding into a composite foliation is accompanied by greenschist facies metamorphism.

Metagreywacke consists essentially of arkose composed of angular to subrounded quartz and feldspar floating in a matrix (40–70%), which consists of carbonate, mica (muscovite or sericite, biotite), quartz, albite and chlorite (Figs. 3c, 3d). Locally, K-feldspar is more abundant, whereby the rocks are classified as fedspathic metagreywacke. Veins and breccia are localized in metagreywacke layers (Fig. 3a) and at the contact with magmatic dikes of the first generation. These features are interpreted to reflect heterogeneous deformation and strain
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** (a) Schematic map (not to scale) of the Tabakoto north pit (TNP) directly contiguous to the Tabakoto main pit (TMP). (b) Geological map of the Tabakoto North open pit (TNP) showing the architectural relationship between lithologies, structures and mineralizations. A-Á in b: Cross section interpretation through the Tabakoto deposit highlighting the litho-structural architecture shown in figure 9.</figcaption></figure>
<table><caption>**Table 1.** Whole rock major and trace element concentrations for the magmatic intrusions at Tabakoto deposit.</caption><thead><tr><th rowspan="2">Lithology<br>Sample</th><th colspan="3">Lamprophyre</th><th colspan="3">Metabasalt</th><th colspan="3">Meta-andesite</th><th colspan="3">Metadacite</th><th>Metarhyolite</th></tr><tr><th>T14</th><th>T5</th><th>T2</th><th>T8</th><th>T12</th><th>T3</th><th>T1</th><th>T6</th><th>T4</th><th>T13</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>45.58</td><td>49.4</td><td>52.6</td><td>62.29</td><td>61.87</td><td>61.4</td><td>65.4</td><td>69.7</td><td>69.14</td><td>73.2</td></tr><tr><td>TiO<sub>2</sub></td><td>0.71</td><td>0.47</td><td>0.68</td><td>0.38</td><td>0.54</td><td>0.45</td><td>0.29</td><td>0.21</td><td>0.23</td><td>0.22</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>12.22</td><td>13.73</td><td>18.55</td><td>15.25</td><td>14.8</td><td>12.25</td><td>12.95</td><td>13.1</td><td>15.08</td><td>13.15</td></tr><tr><td>CaO</td><td>7.35</td><td>3.45</td><td>3.91</td><td>3.05</td><td>4.25</td><td>2.91</td><td>3.77</td><td>3.1</td><td>2.57</td><td>1.63</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.07</td><td>0.02</td><td>0.021</td><td>0.04</td><td>0.04</td><td>0.02</td><td>0.01</td><td>0.01</td><td>0.03</td><td>0.01</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>8.74</td><td>16.63</td><td>7.09</td><td>3.85</td><td>4.99</td><td>5.71</td><td>5.76</td><td>4.59</td><td>1.79</td><td>2.99</td></tr><tr><td>K<sub>2</sub>O</td><td>2.64</td><td>0.06</td><td>3.41</td><td>1.99</td><td>2.46</td><td>6.03</td><td>0.36</td><td>0.14</td><td>2.19</td><td>0.18</td></tr><tr><td>MgO</td><td>7.88</td><td>1.56</td><td>2.83</td><td>1.8</td><td>2.62</td><td>1.71</td><td>0.87</td><td>0.41</td><td>0.44</td><td>0.72</td></tr><tr><td>MnO</td><td>0.14</td><td>0.04</td><td>0.07</td><td>0.06</td><td>0.07</td><td>0.05</td><td>0.07</td><td>0.04</td><td>0.02</td><td>0.04</td></tr><tr><td>Na<sub>2</sub>O</td><td>3.08</td><td>6.65</td><td>5</td><td>5.47</td><td>4.17</td><td>2.87</td><td>6.76</td><td>7.21</td><td>6.43</td><td>7.48</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>0.26</td><td>0.1</td><td>0.18</td><td>0.14</td><td>0.18</td><td>&lt;0.01</td><td>0.08</td><td>0.06</td><td>0.06</td><td>0.08</td></tr><tr><td>L.O.I.</td><td>11.43</td><td>6.91</td><td>6.57</td><td>5.84</td><td>3.20</td><td>4.7</td><td>4.07</td><td>2.76</td><td>1.80</td><td>2.01</td></tr><tr><td><strong>Total</strong></td><td><strong>100.1</strong></td><td><strong>99</strong></td><td><strong>101</strong></td><td><strong>100.2</strong></td><td><strong>99.2</strong></td><td><strong>98.3</strong></td><td><strong>100.4</strong></td><td><strong>101.3</strong></td><td><strong>99.8</strong></td><td><strong>101.8</strong></td></tr><tr><td>Ni</td><td>115.4</td><td>44.3</td><td>64</td><td>60.8</td><td>73.8</td><td>50</td><td>54</td><td>6</td><td>68.4</td><td>9</td></tr><tr><td>Cr</td><td>509.1</td><td>201.1</td><td>150</td><td>300.7</td><td>306.5</td><td>110</td><td>60</td><td>30</td><td>277</td><td>40</td></tr><tr><td>V</td><td>188.4</td><td>62</td><td>183</td><td>74.5</td><td>105.2</td><td>61</td><td>66</td><td>28</td><td>40.6</td><td>26</td></tr><tr><td>Co</td><td>35.47</td><td>86.33</td><td>18</td><td>9.65</td><td>13.98</td><td>15</td><td>32</td><td>25</td><td>4.5</td><td>5</td></tr><tr><td>Cs</td><td>22.02</td><td>0.34</td><td>5.46</td><td>1.91</td><td>1.27</td><td>1.43</td><td>0.58</td><td>0.24</td><td>0.70</td><td>0.13</td></tr><tr><td>Rb</td><td>128.9</td><td>1.116</td><td>143</td><td>58.9</td><td>46.3</td><td>99.2</td><td>11.2</td><td>2.5</td><td>49.1</td><td>3.8</td></tr><tr><td>Ba</td><td>673.1</td><td>15.75</td><td>1085</td><td>1349</td><td>1173</td><td>864</td><td>119.5</td><td>46.2</td><td>787</td><td>204</td></tr><tr><td>Th</td><td>4.4</td><td>4.36</td><td>8.05</td><td>5.6</td><td>6.85</td><td>3.52</td><td>5.69</td><td>6.29</td><td>6.24</td><td>5.5</td></tr><tr><td>U</td><td>1.386</td><td>1.175</td><td>2.98</td><td>2.15</td><td>2.16</td><td>1.07</td><td>2.9</td><td>3.03</td><td>3.3</td><td>2.7</td></tr><tr><td>Nb</td><td>7.92</td><td>5.96</td><td>10.6</td><td>10.05</td><td>9.24</td><td>5.1</td><td>4</td><td>3.5</td><td>3.47</td><td>3.2</td></tr><tr><td>La</td><td>27.13</td><td>22.99</td><td>46.2</td><td>31.61</td><td>36.15</td><td>17</td><td>19</td><td>16.7</td><td>30.4</td><td>16.6</td></tr><tr><td>Ce</td><td>57.91</td><td>43.7</td><td>91.7</td><td>63.8</td><td>74.1</td><td>36.2</td><td>36.1</td><td>29.3</td><td>62.3</td><td>31.7</td></tr><tr><td>Pb</td><td>8.1</td><td>28.17</td><td>8</td><td>6.42</td><td>10.66</td><td>321</td><td>703</td><td>16</td><td>6.78</td><td>373</td></tr><tr><td>Pr</td><td>6.92</td><td>5.08</td><td>10.9</td><td>7.27</td><td>8.44</td><td>4.1</td><td>4.09</td><td>3.2</td><td>5.93</td><td>3.38</td></tr><tr><td>Sr</td><td>792.2</td><td>88.56</td><td>745</td><td>870</td><td>707</td><td>874</td><td>541</td><td>293</td><td>585</td><td>684</td></tr><tr><td>Nd</td><td>30.47</td><td>21.78</td><td>44.9</td><td>29.1</td><td>34.71</td><td>16.7</td><td>16.6</td><td>13.4</td><td>21.7</td><td>13.4</td></tr><tr><td>Zr</td><td>120.7</td><td>124.9</td><td>189</td><td>152.5</td><td>147.8</td><td>81</td><td>114</td><td>117</td><td>110.1</td><td>104</td></tr><tr><td>Sm</td><td>5.89</td><td>4.7</td><td>7.19</td><td>5.2</td><td>6.19</td><td>2.79</td><td>3.15</td><td>2.39</td><td>2.77</td><td>2.17</td></tr><tr><td>Eu</td><td>1.35</td><td>2.42</td><td>2.15</td><td>1.17</td><td>1.41</td><td>0.76</td><td>1.08</td><td>1.18</td><td>0.96</td><td>0.62</td></tr><tr><td>Gd</td><td>4.49</td><td>3.97</td><td>5.23</td><td>3.57</td><td>4.33</td><td>2.77</td><td>2.46</td><td>2.26</td><td>1.97</td><td>1.47</td></tr><tr><td>Dy</td><td>3.5</td><td>3.16</td><td>3.74</td><td>2.51</td><td>3.18</td><td>1.92</td><td>1.57</td><td>1.37</td><td>0.98</td><td>0.7</td></tr><tr><td>Ho</td><td>0.65</td><td>0.55</td><td>0.67</td><td>0.43</td><td>0.57</td><td>0.44</td><td>0.26</td><td>0.29</td><td>0.16</td><td>0.12</td></tr><tr><td>Er</td><td>1.68</td><td>1.512</td><td>2.18</td><td>1.26</td><td>1.5</td><td>1.41</td><td>0.78</td><td>0.63</td><td>0.43</td><td>0.39</td></tr><tr><td>Y</td><td>16.21</td><td>14.02</td><td>18.9</td><td>12.14</td><td>15.07</td><td>10.5</td><td>8.1</td><td>7.5</td><td>5.07</td><td>3.8</td></tr><tr><td>Yb</td><td>1.64</td><td>1.35</td><td>1.75</td><td>1.24</td><td>1.42</td><td>0.98</td><td>0.62</td><td>0.43</td><td>0.205</td><td>0.26</td></tr><tr><td>Lu</td><td>0.24</td><td>0.16</td><td>0.25</td><td>0.20</td><td>0.22</td><td>0.14</td><td>0.1</td><td>0.05</td><td>0.05</td><td>0.03</td></tr><tr><td>Tb</td><td>0.55</td><td>0.51</td><td>0.68</td><td>0.454</td><td>0.53</td><td>0.34</td><td>0.29</td><td>0.28</td><td>0.18</td><td>0.14</td></tr><tr><td>Ta</td><td>0.43</td><td>0.47</td><td>0.8</td><td>0.702</td><td>0.68</td><td>0.5</td><td>0.6</td><td>0.3</td><td>0.28</td><td>0.3</td></tr><tr><td>Mo</td><td>9.73</td><td>10.65</td><td>1</td><td>21.1</td><td>20.5</td><td>1</td><td>4</td><td>2</td><td>21.4</td><td>2</td></tr><tr><td>Hf</td><td>3.14</td><td>3.33</td><td>5.2</td><td>3.95</td><td>3.95</td><td>2.6</td><td>3.7</td><td>3.2</td><td>3.04</td><td>2.8</td></tr><tr><td>ΣREE</td><td>638</td><td>534</td><td>949</td><td>641</td><td>750</td><td>377</td><td>385</td><td>324</td><td>518</td><td>301</td></tr><tr><td>La<sub>N</sub>/Yb<sub>N</sub></td><td>15.60</td><td>16.06</td><td>24.90</td><td>24.04</td><td>24.01</td><td>16.36</td><td>28.90</td><td>36.62</td><td>139.85</td><td>60.21</td></tr><tr><td>La<sub>N</sub>/Sm<sub>N</sub></td><td>3.29</td><td>3.50</td><td>4.59</td><td>4.35</td><td>4.18</td><td>4.36</td><td>4.31</td><td>5</td><td>7.85</td><td>5.47</td></tr><tr><td>Gd<sub>N</sub>/Yb<sub>N</sub></td><td>2.61</td><td>2.81</td><td>2.85</td><td>2.75</td><td>2.91</td><td>2.70</td><td>3.79</td><td>5.02</td><td>9.18</td><td>5.40</td></tr><tr><td>La<sub>N</sub>/Nd<sub>N</sub></td><td>1.94</td><td>2.30</td><td>2.24</td><td>2.36</td><td>2.27</td><td>2.22</td><td>2.49</td><td>2.71</td><td>3.05</td><td>2.70</td></tr><tr><td>Sm<sub>N</sub>/Gd<sub>N</sub></td><td>1.81</td><td>1.63</td><td>1.90</td><td>2.01</td><td>1.97</td><td>1.39</td><td>1.77</td><td>1.46</td><td>1.94</td><td>2.04</td></tr><tr><td>Dy<sub>N</sub>/Lu<sub>N</sub></td><td>1.76</td><td>2.30</td><td>1.78</td><td>1.53</td><td>1.74</td><td>1.63</td><td>1.87</td><td>3.26</td><td>2.20</td><td>2.77</td></tr><tr><td>Eu/Eu*</td><td>0.68</td><td>1.47</td><td>0.90</td><td>0.69</td><td>0.70</td><td>0.73</td><td>1.01</td><td>1.35</td><td>1.05</td><td>0.88</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Characteristic features of metasedimentary units in the Tabakoto deposit. (a) Drill core interval consisting of metagreywacke intersected by multiple hydrothermal V₂ vein events. (b) Drill core interval consisting of meta-argillite showing a folded V₁ veins with sub-vertical V₂ veins. (c) and (d) Photomicrographs of representative metagreywacke in thin section (c, cross-polarized light and d, plane-polarized light). (e) and (f) Representative photograph showing the precipitation of veins in more competent units (dikes) than less competent metasedimentary unit.
*Abbreviation: Ab: albite; Bt: biotite; Cb: carbonate; Chl: chlorite; Ma: matrix; Mt: metasediments; Qz: quartz; Ser: sericite; Sul: sulfide; V: vein.</figcaption></figure>

partitioning of the metasedimentary series coeval with the circulation of a hydrothermal fluid marked by ductile deformation of the less competent metasedimentary unit and by brittle deformation of the more competent metagreywacke and dikes localizing the precipitation of veins (Figs. 3e, 3f).

Meta-argillite facies are finely layered and display a slaty texture (Fig. 3b). They are composed of quartz, feldspars, micas (muscovite, sericite), and carbonates within a finely recrystallized matrix.

### 4.1.2 Magmatic rocks

A variety of dikes, up to several meters in thickness, intrude the metasedimentary rocks at Tabakoto. Cross-cutting relationships and chemical composition of the dikes allow the distinction of (i) a first generation of dikes with a composition ranging from basaltic to rhyolitic that are affected by deformation and metamorphism; (ii) a second generation of lamprophyre dikes, and (iii) a third generation of dolerite dikes (Figs. 2–6).
4.1.2.1 First dike generation: metabasalt to metarhyolite

Metabasaltic to metarhyolitic dikes are relatively abundant
throughout the deposit (Figs. 2b–6). They are steeply dipping
and are transposed in the dominant N-S to NNE-SSW strike
regional foliation.

The metabasalt dikes (samples T2 and T5) have a microlithic porphyritic texture with subhedral to euhedral phenocrysts of plagioclase strongly altered to albite and sericite (60–85%) (Figs. 4a, 4b). The matrix comprises a secondary paragenesis of albitized plagioclase, amphibole microliths and recrystallized quartz and carbonate, and very small amounts of rutile and hematite (≤1%).

The meta-andesite (samples T3, T8 and T12), metadacite (samples T1, T4 and T6), and metarhyolite (sample T13) dikes also have a microlithic porphyritic textures. Meta-andesite dikes (Figs. 4c, 4d) are characterized by quartz as microcrystals and isolated phenocrysts, plagioclases in microliths and phenocrysts destabilized into albite, sericite and calcite. Biotite occurs as microliths and euhedral phenocrysts, deformed and arranged parallel to the foliation. It is altered to calcite, chlorite, muscovite and Fe oxy-hydroxides. The matrix consists of albitized plagioclase and biotite microliths, recrystallized quartz, sericite, epidote, calcite, and chlorite (Fig. 4d).

Metadacite and metarhyolite dikes contain quartz, plagio-
clase and biotite phenocrysts in variable proportions included
in fine-grained matrix composed of albite, quartz, sericite, and
biotite (Figs. 4e, 4f). In the pit workings, the metarhyolite dikes
are located in the southwestern part of the deposit. The
metadacite dikes are more widespread, thinner and, are N-S to
NE-SW striking with a steep dip (Fig. 2b). The metadacite and
metarhyolite contain accessory minerals such as apatite,
monazite, zircon, and rutile.

4.1.2.2 Second dike generation: lamprophyres

The lamprophyre dikes (sample T14; Fig. 6b) are up to 3 m
wide. They are dark greyish color and are characterized by an
aphanitic texture with small phenocrysts of biotite, amphibole
and plagioclase. The matrix mostly consists of albite, calcite,
ankerite, quartz, epidote, sericite and minor amounts of
chlorite as well as tiny calcite veins, which indicates
chloritization of the primary magmatic assemblage
(Figs. 5a-5c). The dikes generally display a dominant
NE-SW to E-W strike with a steep dip, and systematically
intersect the metabasalt to metarhyolite dikes (Fig. 2b). These
features indicate that they postdate the first generation dike and
they were affected by metamorphism.

4.1.2.3 Third dike generation: dolerite

Dolerite dikes have an E-W strike and are steeply dipping. They were observed only in underground workings (Fig. 5). These dolerite dikes do not show any evidence of deformation, regional metamorphism, nor hydrothermal alteration. These dikes intersect all other lithologies in the area and clearly represent the last magmatic event in the Tabakoto deposit (Fig. 5d). It is possible that they correspond to the N090° dikes of the Sambarabougou swarm in the western part of the KKI dated at 1521 ± 3 Ma (Baratoux et al., 2019).

4.2 Geochemistry

The geochemical analyses carried out on the magmatic rocks
of the Tabakoto deposit are reported in Table 1. Metabasalts
(samples T2 and T5) and lamprophyre (sample T14) show an
alkaline affinity respectively in the SiO₂ vs. Zr/TiO₂ diagram
(Fig. 6a) of Winchester and Floyd (1977) and K₂O-SiO₂ diagram
(Fig. 7b) of Raeisi et al. (2019), while meta-andesites,
metadacites and metarhyolites show a calc-alkaline affinity in
the AFM diagram (Fig. 7a) of Irvine and Baragar (1971).

Alkaline metabasalts show highly fractionated patterns (LaN/YbN=24.9–16.1) marked by significant LREE (Light Rare Earth Elements) enrichment from 292 to 145 times the chondrites and clear HREE (Heavy Rare Earth Elements) depletion from 10 to 7 times the chondrites with positive (Eu/Eu*=1.5) and negative (Eu/Eu*=0.9) europium anomalies (Fig. 7c). The positive europium anomaly reflects the observed plagioclase accumulation. Lamprophyre also shows a highly fractionated pattern (LaN/YbN=15.6) similar to those of alkaline metabasalts, with a pronounced negative europium anomaly (Eu/Eu*=0.68). In the expanded diagram of trace elements normalized to the primitive mantle (Fig. 7d), all samples display a dispersion of LILEs (Cs, Rb and Ba), probably due to the various impacts of post-magmatic alteration, as well as positive anomalies in U and negative anomalies in Nb, Sr and Zr. The lamprophyre is an exception, showing a positive Sr anomaly.

With respect to chondrites, calc-alkaline rocks are highly
enriched in LREE (229 to 105 times), and very depleted in HREE
(9 to 1 times) with very steep patterns (LaN/YbN = 139.8–16.4)
(Fig. 7c). Europium shows negative anomalies in meta-andesites
(Eu/Eu* = 0.73–0.69) and metarhyolite (Eu/Eu* = 0.88), and
positive anomalies in metadacites (Eu/Eu* = 1.35–1.01). The
calc-alkaline rocks at Tabakoto also show a gradient of REE
enrichment from metarhyolite through metadacite to meta-
andesite, suggesting that these different rocks derive from the
same parental magma that evolved by fractional crystallization.
In the expanded diagram of trace elements normalized to the
primitive mantle (Fig. 7d), calc-alkaline rocks show a dispersion
of LILEs, positive U and Sr anomalies and very pronounced
negative Nb anomalies.

5 Deformation events and veins formation

5.1 Deformation events

The Tabakoto deposit displays a complex superposition
of structures, characterized by a succession of three
deformation phases. The structural position of the different
vein populations of the deposit allowed us to establish the
timing of their formation relative to the deformation events
(Figs. 8–14). The nomenclature used in this paper refers to
the local deformation of the Tabakoto deposit denominated
D<sub>Tn</sub>, where T stands for Tabakoto, and n is the phase of
deformation identified at the deposit scale. Structures such
as foliations (S), folds (F) and veins (V) are denoted by
numbers (i.e., S<sub>n</sub>, F<sub>n</sub> and V<sub>n</sub>). Note that we are agnostic at
this stage of the relationship between the first phase recorded
at the Tabakoto deposit (D<sub>Tn</sub>) and the regional-scale
deformation event D<sub>n</sub>.
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** (a) Slabbed hand sample of brecciated metabasalt filled with abundant sulfides, quartz and carbonate. (b) Representative photomicrograph of the matrix of metabasalt in thin section (cross-polarized light). (c) Slabbed hand sample of meta-andesite. (d) Representative photomicrograph of meta-andesite in thin section showing a V₂ vein with carbonate developed on its edges as evidence of multi-stage veins infilling (cross-polarized light). (e) Slabbed hand sample showing metarhyolite containing V₁ and V₂ veins. (f) Photomicrograph of representative metarhyolite cut by a V₂ vein in thin section (plane-polarized light). *Abbreviation:* Ab: albite; Cb: carbonate; Chl: chlorite; Fr: fracture; Ma: matrix; Pl: plagioclase; Qz: quartz; Ser: sericite; V: vein.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** (a) Slabbed hand sample of lamprophyre. (b) and (c) Representative photomicrographs of a lamprophyre in thin section (b, plane-polarized light and c, cross-polarized light). (d) Photograph of underground workings showing late dolerite dike cross-cutting early dike and metasediments. *Abbreviation:* Amp: amphibole; Bt: biotite; Cb: carbonate; Chl: chlorite; Fsp: feldspar; Ma: matrix.</figcaption></figure>

### 5.1.1 $D_{T1}$ and $D_{T2}$ ductile deformation phases

The metasedimentary rocks of the Tabakoto deposit display a composite foliation marked by the alternation of quartz-rich and mica-rich layers that corresponds to transposition of bedding $S_0$ into a $S_1$ schistosity during $F_1$ folding (Fig. 2b). This structure is also marked by the preferred orientation of micas and the deformation of quartz grains. The $S_1$ schistosity is steeply dipping (70° to 88°) generally towards the southeast but locally to the northwest (Figs. 8a and 9). Rootless $F_1$ isoclinal fold hinges are locally preserved (Fig. 9). This foliation is cross-cut by dikes of the first generation (Figs. 2b and 11c). The $F_1$ folding event, as well as the development of the $S_{0/1}$ foliation are classified as the $D_{T1}$ deformation event.

The $S_{0/1}$ and the first dike generation are affected by upright folds ($F_2$) with an NNE-SSW to NE-SW trending subvertical axial planar $S_2$ schistosity (Figs. 2b, 8–10) oriented NNE-SSW with an average dip of 78° (Fig. 8a). The $S_2$ schistosity is pronounced in non-competent rocks but refracts in competent rock units (i.e., in metagreywacke and dikes, Fig. 11c) or resolves as a spaced-fracture schistosity. High strain zones parallel to the $S_2$ schistosity marked by

tight folding of the $S_1$ schistosity (Fig. 11a, c) are localized in meta-argilites. The first dike generation locally transposed into the $S_2$ schistosity. The $F_2$ folds are also identified at the regional scale and affect the Kofi metasedimentary series. The superposition of $F_1$ and $F_2$ folds results in hook-type interference features (Figs. 9, 11a). The $F_2$ folding event, as well as the development of the $S_2$ schistosity, record an E-W shortening accommodated by a vertical stretching and is classified as the $D_{T2}$ deformation event (Fig. 14).

### 5.1.2 $D_{T3}$ ductile-brittle deformation phase

Vertical NE-SW to E-W trending lamprophyre dikes cross-cut the $S_2$ schistosity and thus postdate $D_{T2}$ deformation. A set of mutually cross-cutting conjugate subvertical dextral NE-SW and sinistral NW-SE trending ductile-brittle faults affect the metasedimentary rocks and the first and second generations of dikes (Figs. 2b, 8b and 12). The NE-SW faults strike from N55° to N70° with dips ranging from 70° to 88° and the NW-SE faults strike from N115° to N158° with dips ranging from 55° to 87° (Figs. 8b and 12).
<figure><img src="image_7.png" /><figcaption>**Fig. 6.** (a) SiO₂ versus Zr/TiO₂ diagram of Winchester and Floyd (1977) classifying the first magmatic dike generation of the Tabakoto deposit. (b) Plot of lamprophyre dike from the Tabakoto deposit on the MgO-K₂O-Al₂O₃ classification diagram of Bergman (1987).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 7.** (a) AFM diagram (Irvine and Baragar, 1971) showing the magmatic affinity of the first dike generations. (b) showing alkaline affinity of the lamprophyre dike on the K₂O vs. SiO₂ wt.% diagram of Raeisi *et al.* (2019). (c) Diagram showing REE patterns normalized to chondrites (Barrat *et al.*, 2014). (d) Expanded incompatible element diagram normalized to the Primitive Mantle (Sun and McDonough, 1989) for the magmatic dikes at Tabakoto.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Representation of structural data plotted on equal-area stereonets, using the lower hemisphere convention. (a) represent S₀/₁ and S₂ in detrital metasedimentary rocks as well as the N-trending fault/shear zones measurements. It is represented the data acquired during mapping of the TNP and database obtained from mine. (b) shows the NE and NW-trending faults of Dₜ₃ deformation event of the deposit, and the late fracture of Dₜ₃ deformation event.</figcaption></figure>

A set of minor E-W normal faults and/or fractures (<10 cm in length) with dips ranging from 73° to 88° (Fig. 8b), cross-cut the metasedimentary rocks. These structures are attributed to D<sub>T3</sub> deformation and mark a change from ductile-brittle to brittle structures, which is consistent with an E-W shortening direction and N-S stretching (Figs. 12 and 14).

## 5.2 Veins

Based on orientation, structural features and mineral paragenesis, we classify four type of veins at Tabakoto (Figs. 10–14): (i) barren folded milky quartz veins (V₁); (ii) gold-bearing sheared smoky quartz-pyrite veins (V₂); (iii) S-shaped fault-fill lenses of gold-bearing quartz – carbonate veins (V₃a), and (iv) barren fracture-fill quartz veinlets (V₃b).

The V₁ veins are characterized by coarse-grained milky quartz. They are localized in metasedimentary rocks, and do not show alteration halos (Fig. 10d). Quartz crystals show undulose extinction, and serrated grain boundaries, pointing to intracrystalline deformation and dynamic recrystallization (Fig. 15a). In zones of limited D<sub>T2</sub> deformation, V₁ veins cross-cut the S₀/₁ foliation and display a distribution as conjugate sets of shallow-dipping tension gashes with a NE-SW and WNW-ESE direction (Fig. 10d). The V₁ veins commonly affected by F₂ folds, are boudinaged and transposed into the S₂ schistosity (Figs. 10b, 10d). These features indicate that they formed after the onset of D<sub>T1</sub> but before D<sub>T2</sub> (Figs. 10d and 14). They are interpreted to record an hydrothermal event that predated the ore stage.

The V₂ veins are generally parallel to the N-S to NNE-SSW trending S₂ schistosity (Figs. 2b and 11c) and occur in the metasediments, the first dike generation, and along their margins. In the metasediments, they are generally localized in zone of high strain, where they are defined as shear veins (Fig. 10b). These veins are thus interpreted as shear veins synchronous to Dₜ₂ event.

The V₃ veins display various sizes and comprise: (i) S-shaped fault-fill lenses of gold-bearing quartz-carbonate veins (V₃a) and (ii) fracture-fill quartz veinlets (V₃b). The V₃a are hosted in NE-SW and NW-SE oriented Dₜ₃ faults (Figs. 12a–12e) and commonly display quartz and albite crystals with a fibrous and/or crack-seal texture (Figs. 16a, 16b). The V₃a veins present quartz with undulose extinction and subgrains that attest for dynamic recrystallization (Fig. 16a). The V₃a veins result from a succession of brittle deformation events alternating with precipitation during the movement of the Dₜ₃ faults (Figs. 12–14).

Millimetric to centimetric V₃b barren veinlets cross-cut the V₁ and V₂ veins. (Fig. 12f). V₃b veinlets are E-W trending and subvertical, which corresponds to the plane orientation of stress and are interpreted as tension gashes emplaced during Dₜ₃ event (Figs. 13 and 12).

# 6 Orebody characteristics

## 6.1 Structural geometry and controls on mineralization

Two main ore trends are identified at the scale of the Tabakoto deposit. The first corresponds to gold included in
<figure><img src="image_10.png" /><figcaption>**Fig. 9.** (a) View of the north wall of the Tabakoto North pit workings (TNP), showing the first mineralized magmatic dikes intruding the metasedimentary units. (b) Interpretation of the structural context of the north wall shown in (a).</figcaption></figure>

sulfides disseminated in the first dike generation and in V₂ veins, hosted by the metasedimentary units and the first dike generation (Figs. 13, 17a, b, d, e). The ore trend cuts across structural features of the Dₜ₁ deformation event. In this ore trend, grades range between 3 and 10 g/t Au.

The second ore trend is hosted in the V₃a veins localized in the NE-SW and NW-SE trending and steeply dipping Dₜ₃ faults (Figs. 12 and 13). Average grades are estimated between 0.8 to 3 g/t. The intersection between the V₂ and V₃a veins displays grades up to 10 g/t.

## 6.2 Ore mineralogy and alteration paragenesis

V₂ veins of first gold mineralization are marked by an alteration halo that is more pervasive in the first dike generation than the metasediments. Mineralogically, V₂ veins are characterized by quartz, albite, carbonate (dolomite and ankerite), and a relatively high proportion of Fe-As sulfides. Metasomatic alteration consists of (i) albitization, manifested as the nearly complete replacement of plagioclase in the matrix by albite (Fig. 15b); (ii) K-feldspar alteration are developed in
<figure><img src="image_11.png" /></figure>

**Fig. 10.** Field photographs illustrating structural features of D<sub>T2</sub> deformation events at the Tabakoto deposit. (a) Zoom from white box in Fig. 9a showing a first dike generation and its deformed contact in the pit workings. (b) Photograph of underground workings showing shear zone displaying textures defining the D<sub>T2</sub> deformation event with S<sub>2</sub>-parallel quartz-pyrite veins (V<sub>2</sub>) and folded V<sub>1</sub> vein in metasedimentary units. (c) Photograph of underground workings showing a dike containing numerous V<sub>2</sub> veins. (d) Photograph of underground workings showing a first dike generation and related mineralized V<sub>2</sub> veins cutting the conjugate tension gashes V<sub>1</sub> quartz veins in metasedimentary rocks. A post-Paleoproterozoic dolerite dike (bottom) intersects both the felsic dike and metasedimentary units. (e) Zoom from the photograph in (d) highlighting the cross-cutting relationship between V<sub>1</sub> and V<sub>2</sub>.

the dikes as a replacement of the albite minerals (Fig. 15c);
(iii) carbonate alteration (dolomite and ankerite), variably
developed (5-60%), and, like the potassic alteration, super-
imposed on albitization with distinct episodes of carbonation
(Fig. 15d) also evidenced by multi-stage vein infilling (Fig. 4d).

The foremost sulfide minerals of the V₂ veins consists of
pyrite, followed by arsenopyrite and pyrrhotite with lesser
amounts of chalcopyrite, sphalerite and traces of galena and
loellingite (Fig. 17). Pyrite displays various morphologies. It
can form single crystals which commonly contain abundant

inclusions of gangue minerals as well as other sulfides (i.e.,
chalcopyrite, galena, pyrrhotite), plus gold (Fig. 17a). Pyrite
also forms aggregates where it is intermingled with euhedral
to subhedral arsenopyrite, and less commonly pyrrhotite
(Figs. 17a, 17b). Two main types of pyrite are identified
namely (Tab. 2) (i) pyrite with no or very little As and
(ii) pyrite with As content up to 1.7 wt%. Arsenopyrite forms
either anhedral to subhedral inclusions in pyrite or occurs as
euhedral crystals again associated with pyrite (Fig. 17b).
Pyrrhotite mostly occurs as subhedral to anhedral inclusions
<figure><img src="image_12.png" /><figcaption>**Fig. 11.** (a) Photograph of the S₂ schistosity cross-cutting the metasedimentary units and filled by mineralized quartz-pyrite veins (V₂) as well as fold interference. (b) Zoom from the white box in (a) illustrating mineralization into the S₂ schistosity. (c) Multiple relationships between the first dike generation, mineralized quartz V₂ veins, and high strain zones (HSZ). (d) Zoom from the white box in (c) showing the felsic dike contact, illustrating the reactivation of the contact leading to the formation of a shear zone filled by V₂ quartz-pyrite veins. *Abbreviation: F*: fold; *Marg*: meta-argillite; *Mgw*: metagreywacke; *Sul*: sulfide; *HSZ*: high strain zone; *V*: vein.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 12.** Photographs, taken in the North pit workings and underground gallery, illustrating macroscopic features of the D<sub>T3</sub> ductile-brittle to brittle deformation. (a) A NW-SE trending fault zone (NW fault) cross-cuts a NE-SW trending fault zone (NE fault). (b) Another occurrence where a NE-SW trending fault cross-cuts a NW-SE trending fault. (c) A NW-SE trending fault-fill vein surface. (d) Underground wall showing a mineralized fault-fill vein mirror of the D<sub>T3</sub> deformation event (e) Underground wall showing a cross-cutting relationship between V<sub>2</sub> veins and a D<sub>T3</sub> fault containing deformed V<sub>3a</sub> veins. (f) Underground wall showing V<sub>3b</sub> veinlets (oriented N105) related to the late faulting of the D<sub>T3</sub> event cross-cutting earlier vein (oriented N30). *Abbreviation:* Mt: metasedimentary unit; Qz: quartz; Sul: sulfide.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 13.** Geometrical model of the veins distribution in Tabakoto deposit (modified from Roux *et al.*, 2016).</figcaption></figure>

in pyrite and arsenopyrite. Locally, euhedral arsenopyrite is surrounded by pyrrhotite (Fig. 17c). Chalcopyrite may be observed intergrown with pyrrhotite and as inclusions within pyrite and arsenopyrite. Galena is found in arsenopyrite and pyrite as fine-grained inclusions or filling fractures in arsenopyrite. Small grains of loellingite (As > 60 wt%; Tab. 2) occur locally as inclusions in pyrrhotite or in the matrix close to pyrrhotite (Fig. 17d). Some sphalerite, gersdorffite, pentlandite and scheelite were also found in the first dike generation and V₂ veins. Small grains of native bismuth and silver were detected within arsenopyrite (Fig. 17e). In the V₂ veins, gold grains are typically <15 µm and occur mostly as (i) inclusions in the sulfides (pyrite and arsenopyrite, more rarely pyrrhotite) and (ii) at the contact between two different sulfides (Figs. 17a, 17b, 17f). Microprobe analyses of these gold grains indicate Ag contents between 18 and 22 wt% (Tab. 3).

In the V₃a veins, in addition to quartz and carbonate, albite is common, and, with muscovite, occurs in strain shadows around sulfides (Figs. 16c, 16e). K-feldspar occurs along the edges or in fractured sulfides (Figs. 18b, 18c). Carbonates consist of dolomite and ankerite, with rare calcite and siderite (Figs. 16d). Phyllic alteration is broadly expressed around these V₃a veins and is represented by chlorite, muscovite and sericite. These alteration zones are pervasive, in which case sericite predominates. Chlorite is found in zones with intense hydrothermal alteration and in fractures of sulfides with muscovite (Figs. 18a, c, d). Chlorite is also developed in the strain shadows around pyrite and arsenopyrite, in which case it is associated with ankerite and/or albite (Fig. 16e). Muscovite is frequently found in fractured sulfides associated with gold (Fig. 18c). Muscovite is also developed as a rosette structure associated with chlorite and carbonate (Fig. 16f).

Pyrite and arsenopyrite are the most common sulfide minerals in the V₃a veins. They form massive anhedral crystals, which are commonly broken up and fractures are filled by albite, muscovite and sericite. Other sulfides such as pyrrhotite, galena, sphalerite and chalcopyrite complement the paragenesis. In the mineralization associated with the V₃a veins, gold occurs as (Fig. 18): (i) late infill or discrete gold veinlets filling cracks in pyrite and arsenopyrite in association with K-feldspar, chlorite and muscovite and (ii) as free native gold grains in the gangue closely associated with arsenopyrite and/or pyrite and chlorite alteration. It forms grains of size up to 30 µm and contains silver values between 11 and 13 wt% (Tab. 3). Locally, gold grains were also observed at the contact between the veins and the host rock.

# 7 Discussion

## 7.1 Features of the Tabakoto deposit within its regional geological and tectonic framework

New mapping of the north pit workings (Fig. 2b) and structural analysis of the whole Tabakoto deposit (TNP and TMP, Fig. 2a) reveals that it is dominated by a metasedimentary unit cross-cut by different generations of dikes with an alkaline to calc-alkaline signature (Figs. 7a, 7b). The pronounced negative Nb anomalies and the enrichment and fractionation in the REE spectra of the calc-alkaline rocks are consistent with magmas emplaced in subduction zones. Alkaline rocks with a negative Nb anomaly can be generated when a lithospheric mantle is stimulated by a hot spot. Synchronous emplacement of alkaline basalts and calc-alkaline andesites has been described in Baja, California, México (Benoit *et al.*, 2002), where tearing of the slab opened an asthenopsheric window and lets through lower mantle melts with adakitic signatures.

The structural record of the metasedimentary units and dikes portrays a polyphased tectonic-metamorphic evolution characterized by deformation under ductile to brittle regimes (Figs. 14 and 19). Folding of the metasedimentary series and associated bedding-parallel foliation S₁ are consistent with the early phase of crustal thickening. In the western part of the KKI, the Dialé-Daléma metasedimentary series yield detrital zircon grains with U-Pb (LA-ICPMS) ages ranging from 2200 to 2100 Ma, which provide a maximum depositional age for the sediments (Hirdes and Davis, 2002; Koné *et al.*, 2020).
<table><caption>**Fig. 14.** Structural evolution of the Tabakoto deposit and its relationships with tectonic ore stage during different mineralizing events. Line thickness in the paragenesis section indicates mineral abundance.</caption><thead><tr><th rowspan="3">Structural regime</th><th rowspan="3">Regional scale</th><th>D1</th><th>D2</th><th>D3/D4</th></tr><tr><th>DT1</th><th>DT2<br>E-W shortening</th><th>DT3<br>E-W shortening</th></tr><tr><th>Ductile regime<br>Folding of S0 bedding<br>Intrusion of first generation of dikes<br>Tight to isoclinal folding (F1)<br>N to NNE-trending schistosity (S1)</th><th>Ductile dominantly coaxial regime<br>Vertical stretching<br>N-S high strain zones<br>F2 Folding of S0/1, V1 and dikes<br>Penetrative S2 schistosity<br>Locally folded V1 transposed into S2</th><th>Intrusion of second generation of dikes<br>Ductile-brtle to brittle regime<br>Horizontal N-S stretching<br>Subvertical NE-SW and NW-SE ductile- brittle fault<br>Brittle E-W normal faults and/or fractures</th></tr></thead><tbody><tr><th rowspan="2">Hydrothermal event</th><th rowspan="2"></th><th>V1</th><th>V2</th><th>V3</th></tr><tr><th>Vein opening at the DT1/DT2 transition<br>Milky quartz veins</th><th>N to NNE striking shear veins<br>Smokey-quartz-pyrite veins</th><th>Fault filled quartz-carbonate veins (V3a)<br>Localized fracture filled quartz veinets (V3b)<br>Internal quartz fibers and crack-seal texture</th></tr><tr><th>Mineralizing event</th><th></th><th>Quartz with no sulfide<br>Barren with respect to gold</th><th>Quartz + Fe-As sulfides<br>Disseminated and including gold- bearing sulfides</th><th>Quartz-carbonate + Fe-As sulfides<br>Visible and sulfide fracture-bound gold (V3a)</th></tr><tr><th rowspan="10">Paragenesis</th><th>Quartz</th><td></td><td></td><td></td></tr><tr><th>Biotite</th><td></td><td></td><td></td></tr><tr><th>Dolomite/Ankerite</th><td></td><td></td><td></td></tr><tr><th>Calcite/Siderite</th><td></td><td></td><td></td></tr><tr><th>Albite</th><td></td><td></td><td></td></tr><tr><th>Chlorite</th><td></td><td></td><td></td></tr><tr><th>Muscovite/Sericite</th><td></td><td></td><td></td></tr><tr><th>Pyrite</th><td></td><td></td><td></td></tr><tr><th>Arsenopyrite</th><td></td><td></td><td></td></tr><tr><th>Pyrrhotite</th><td></td><td></td><td></td></tr><tr><th>Chalcopyrite</th><td></td><td></td><td></td></tr><tr><th>Sphalerite</th><td></td><td></td><td></td></tr><tr><th>Galena</th><td></td><td></td><td></td></tr><tr><th>Sheelite</th><td></td><td></td><td></td></tr><tr><th>Loellingite</th><td></td><td></td><td></td></tr><tr><th>Gold</th><td></td><td></td><td></td></tr><tr><th>Schematic representation of structures in cross section</th><th></th><th>D1</th><th>V1</th><th>V2</th><th>V3</th></tr></tbody></table>

$V_1$ veins, emplaced and then deformed during crustal thickening $D_{T1}$ event, are poor in sulfides and barren of mineralization. At the regional scale, the $D_{T1}$ deformation event of the Tabakoto deposit correlates with the regional $D_1$ deformation event (Gueye et al., 2008; Diene et al., 2012; Masurel et al., 2017c).

The first dike generation is not cross-cut by $V_1$ veins. During subsequent $D_{T2}$ deformation, these dikes were affected by localized ductile to brittle shearing along their contacts (Fig. 10a). The first generation dikes are folded by $F_2$ but do not show $F_1/F_2$ interference patterns. These features are consistent with an emplacement of the first dike generation between $D_{T1}$ and $D_{T2}$. The N-S to NNE-SSW-trending subvertical $S_2$ penetrative schistosity in the axial plane of upright $F_2$ folds records thickening of the metasedimentary unit during E-W shortening attributed to $D_{T2}$ deformation (Figs. 8a and 14). The $V_2$ veins were emplaced within the metasediments, along the contacts between the metasediments and the first dike generation, and also cross-cut the dikes of first generation (Figs. 10, 11 and 19). These veins are interpreted to be the result of hydrothermal fluids circulating during the syn-$D_{T2}$ deformation. At the regional scale, the $D_{T2}$ deformation event

of the Tabakoto deposit correlates with the regional $D_2$ transpressive tectonic event defined elsewhere in the West African Craton (e.g., in KK1, Milési et al., 1989; Ledru et al., 1991; Dabo and Aifa, 2010; Diene et al., 2012; Masurel et al., 2017a; in southern Mali, McFarlane et al., 2011; in other regions of the WAC, Pouclet et al., 2006; Hein, 2010).

Structures of $D_{T1}$ and $D_{T2}$ are cross-cut by NE-SW and NW-SE-striking faults and gold-bearing S-shaped quartz – carbonate veins ($V_{3a}$), which indicate emplacement of the latter during the $D_{T3}$ deformation event (Figs. 12, 14 and 19). These structures are consistent with E-W shortening and N-S stretching. Similar faults to those of the $D_{T3}$ are also described elsewhere in the KK1 (e.g., Dabo et al., 2016; Masurel et al., 2017a; Diallo et al., 2024). Hence, $D_{T3}$ faults and fractures and associated veins ($V_{3a}$, $V_{3b}$) at Tabakoto may be correlated with regional $D_3$ and/or $D_4$ deformation events described in Senegal (Dabo et al., 2016; Diatta et al., 2017) and in Dialafara area in Mali (Diallo et al., 2024).

The Tabakoto deposit, like typical orogenic gold deposits in the WAC and elsewhere, is not clearly associated with a shear zone, as is the case for instance for the Sadiola and Loulo district deposits. Rather, the Tabakoto deposit is characterized
<figure><img src="image_15.png" /><figcaption>**Fig. 15.** Photomicrographs showing thin sections (a, b, c; cross-polarized light) and SEM-BSE images (d) illustrating representative textural and mineralogical features of rocks. (a) Metagreywacke displaying subgrain deformation in quartz, indicating dynamic recrystallization. (b) Albitization in metadacite marked by albitized plagioclase and the development of albite microlites in the groundmass. (c) Replacement of albite by K-feldspar with ankerite in metabasalt. (d) Carbonate alteration in metarhyolite showing ankerite and dolomite superimposed on albite with inclusions of zircon, monazite and apatite. *Abbreviation:* Ab: albite; Ap: apatite; Ank: ankerite; Cb: carbonate; Dol: dolomite; Kfs: K-feldspar; Mnz: monazite; Pl: plagioclase; Zrn: zircon.</figcaption></figure>

by multiple overlapping stages of magmatic intrusions and hydrothermal alteration during distinct regional-scale deformation events. To better constrain the relationships between mineralization and magmatism, an estimate age of the absolute ages of the dikes in the Tabakoto deposit can be obtained from the literature. Directly to the west, in the Loulo district, and to the north at Sadiola, compositionally and texturally similar dikes to those of the first generation at Tabakoto provided ages within an interval between 2090 and 2060 Ma (Masurel et al., 2017a,b,c; Allibone et al., 2020; Lambert-Smith et al., 2020). This age span is also reported by Allibone et al. (2020) as the age of the discontinuous, small-displacement, sinistral shear zones that host the neighboring Yaléa and Gounkoto deposits. We thus suggest that the magmatic and possibly related hydrothermal events involved at Tabakoto probably occurred during the same age interval between 2090 and 2060 Ma. Thus, the Tabakoto deposit can be described as formed during the Eburnean orogeny marked by E-W shortening first associated with crustal thickening accommodated by ductile deformation

(D<sub>T2</sub> event) followed by N-S horizontal stretching (D<sub>T3</sub> event) marked by ductile to brittle deformation.

## 7.2 Controls on ore deposition

The first mineralization phase at the Tabakoto deposit occurred as disseminated gold-bearing sulfides in the first dike generation and during progressive deformation related to the emplacement of an array of quartz-sulfide hydrothermal veins (V<sub>2</sub>), preferentially located in the first dike generation (metabasalt, meta-andesite, metadacite and metarhyolite) and in the metasedimentary units (Fig. 19).

In addition to sulfides commonly present in gold deposits in the WAC, these veins contain reduced phases such as pyrrhotite and loellingite as well as bismuth and scheelite. Loellingite formation has been described to generally occur in two different settings (Barnicoat et al., 1991; Neumayr et al., 1993; Tomkins and Mavrogenes, 2001 and references therein) depending on the metamorphic degree: (i) during prograde metamorphism associated with the desulfurization of arseno-
<figure><img src="image_16.png" /><figcaption>**Fig. 16.** (a) and (b) Thin section photomicrographs showing quartz fibers development and crack-seal texture in a V₃ quartz vein as well as dynamic quartz recrystallization by intracrystalline and subgrain deformation in (a). (c) Carbonate alteration with arsenopyrite showing albite in pressure shadows. (d) BSE image of skeletal pyrite in carbonate (siderite and calcite) in metadacite. *Abbreviation*: Ab: albite; Apy: arsenopyrite; Cal: calcite; Cb: carbonate; Ma: matrix; Ms: muscovite; Py: pyrite; Qz: quartz; Sd: siderite.</figcaption></figure>

pyrite with the formation of loellingite and pyrrhotite (Barnicoat et al., 1991); and (ii) during retrograde metamorphism associated with the sulfurization and destabilization of loellingite and pyrrhotite, leading to the formation of arsenopyrite and, locally, a new generation of pyrrhotite (Neumayr et al., 1993). The association of these minerals (po-lo-bi-sch) associated with gold is not common in typical

orogenic settings. and in other gold deposits of the WAC, the presence of loellingite linked with gold mineralization has only been reported at the Morila deposit, in southern Mali, where mineralization was interpreted to be intrusion related (McFarlane et al., 2011).

At Tabakoto, loellingite is found as inclusions in pyrrhotite or in contact with it. Pyrrhotite also includes gold
<figure><img src="image_17.png" /><figcaption>**Fig. 17.** Photomicrographs taken from thin sections (a, b, c; reflected light), and SEM-BSE (d, e, f), illustrating representative textural and mineralogical features of first ore trend minerals at Tabakoto. (a) A pyrite grain in metarhyolite, displaying a subhedral texture and showing evidence of partial alteration, next to euhedral arsenopyrite. Pyrrhotite and visible gold occur as inclusions within the pyrite. (b) Textural relationships between pyrite and arsenopyrite grains in a metarhyolite dike. Note the visible gold grain at their contact. (c) Arsenopyrite rods (euhedral, hence likely retrograde) intergrown with pyrrhotite. (d) Loellingite and gold grain included in pyrrhotite in a metadacite. (e) Arsenopyrite in meta-andesite, with an inclusion containing native silver, bismuth, pyrrhotite and chalcopyrite. (f) Native gold particles included in pyrite. *Abbreviation: Ag*: silver; *Apy*: arsenopyrite; *Au*: gold; *Bi*: bismuth; *Ccp*: chalcopyrite; *Gn*: Galena; *Lo*: loellingite; *Po*: pyrrhotite; *Py*: pyrite.</figcaption></figure>
<table><caption>**Table 2.** EDS microprobe analyses of sulfides from the Tabakoto deposit (wt%).</caption><thead><tr><th>Sample</th><th>Phase</th><th>S</th><th>As</th><th>Fe</th><th>Cu</th><th>Zn</th><th>Ni</th><th>Co</th><th>Total</th></tr></thead><tbody><tr><td>T1</td><td>Pyrite</td><td>52.93</td><td>0.53</td><td>47.28</td><td>0.02</td><td></td><td>0.09</td><td>0.06</td><td>100.91</td></tr><tr><td>T13</td><td>Pyrite</td><td>52.65</td><td>1.66</td><td>46.61</td><td>0.02</td><td></td><td>0.3</td><td>0.2</td><td>101.44</td></tr><tr><td>T14</td><td>Pyrite</td><td>53.14</td><td></td><td>45.52</td><td>0.01</td><td></td><td>1.45</td><td>0.5</td><td>100.62</td></tr><tr><td>T15</td><td>Arsenopyrite</td><td>21.26</td><td>43.26</td><td>35.67</td><td></td><td></td><td></td><td></td><td>100.19</td></tr><tr><td>T1</td><td>Arsenopyrite</td><td>21.50</td><td>42.95</td><td>36.05</td><td></td><td></td><td></td><td></td><td>100.5</td></tr><tr><td>T13</td><td>Pyrrhotite</td><td>39.29</td><td>0.08</td><td>60.95</td><td></td><td>0.03</td><td>0.09</td><td>0.03</td><td>100.47</td></tr><tr><td>T2</td><td>Pyrrhotite</td><td>38.71</td><td>0.01</td><td>61.22</td><td></td><td>0.02</td><td>0.1</td><td>0.05</td><td>100.11</td></tr><tr><td>T4</td><td>Loellingite</td><td>2.63</td><td>67.9</td><td>29.4</td><td></td><td></td><td></td><td></td><td>99.93</td></tr><tr><td>T4</td><td>Loellingite</td><td>6.02</td><td>63.05</td><td>30.76</td><td></td><td></td><td></td><td></td><td>99.83</td></tr><tr><td>T10</td><td>Chalcopyrite</td><td>35.15</td><td></td><td>30.95</td><td>34.6</td><td></td><td></td><td>0.04</td><td>100.74</td></tr><tr><td>T2</td><td>Chalcopyrite</td><td>34.74</td><td></td><td>30.86</td><td>34.37</td><td></td><td></td><td>0.03</td><td>100.00</td></tr><tr><td>T2</td><td>Sphalerite</td><td>33.63</td><td></td><td>7.17</td><td></td><td>58.06</td><td>0.01</td><td></td><td>98.87</td></tr><tr><td>T14</td><td>Gersdorffite</td><td>20.30</td><td>44.29</td><td>6.79</td><td>0.03</td><td>0.02</td><td>7.92</td><td>20.84</td><td>100.19</td></tr></tbody></table>

<table><caption>**Table 3.** EDS microprobe analyses of gold grains at Tabakoto deposit (wt%).</caption><thead><tr><th>Sample</th><th>Phase</th><th>Ag</th><th>Au</th><th>Bi</th><th>Total</th></tr></thead><tbody><tr><td>T13</td><td>Au (inclusion)</td><td>18.77</td><td>81.66</td><td>0.09</td><td>100.52</td></tr><tr><td>T13</td><td>Au (inclusion)</td><td>19.08</td><td>80.86</td><td>0.02</td><td>99.96</td></tr><tr><td>T1</td><td>Au (fracture)</td><td>12.08</td><td>87.4</td><td>0.15</td><td>99.63</td></tr><tr><td>T1</td><td>Au (fracture)</td><td>11.1</td><td>88.79</td><td>0.13</td><td>100.02</td></tr><tr><td>T1</td><td>Au (Matrix)</td><td>12.6</td><td>87.15</td><td>0.08</td><td>99.83</td></tr><tr><td>T1</td><td>Au (Matrix)</td><td>11.42</td><td>87.99</td><td>0.15</td><td>99.56</td></tr><tr><td>T4</td><td>Au (Loellingite)</td><td>21.25</td><td>78.53</td><td>0.17</td><td>99.95</td></tr><tr><td>T4</td><td>Au (Loellingite)</td><td>21.12</td><td>78.39</td><td>0.14</td><td>99.56</td></tr></tbody></table>

(Fig. 17d) and arsenopyrite at equilibrium (Figs. 18e, 18f). This assemblage seems to correspond to retrograde reactions (e.g., Neumayr et al., 1993) suggesting that, at Tabakoto, relatively hot conditions (upper greenschist facies) may have been attained locally during D<sub>T2</sub>. The atomic-percent-of-As-in-arsenopyrite geothermometer of Sharp et al. (1985), applied to the stable arsenopyrite-pyrite-pyrrhotite assemblage of the V<sub>2</sub> alteration halo gives a temperature of ~430–450 °C, which is consistent with the presence of biotite in the metasediments. Chlorite overgrowths on biotite records cooling following this metasomatic event. This cooling would also represent favorable condition to trigger the precipitation of As-rich pyrite and arsenopyrite as well as gold (e.g., Perfetti et al., 2008), which would have been easily trapped in these sulfides.

Another characteristic of the Tabakoto deposit is the presence of lamprophyre dikes, which have not yet been described in the KKl. Similar dikes are also reported in the Syama deposit, southern Mali (Traoré et al., 2016), a deposit mostly hosted in metabasalt and meta-andesite. However, the lamprophyres cross-cut the S<sub>2</sub> shistosity and related mineralized V<sub>2</sub> veins and V<sub>3a</sub> veins. These would imply that they were emplaced after the major mineralization events, and are considered here as post-mineralization.

The second mineralization phase is related to the formation of the NE-SW and NW-SE trending faults that formed during the D<sub>T3</sub> deformation event. Field relations suggest that these faults acted as channel driving the fluid flow which formed a large number of quartz – carbonate veins (V<sub>3a</sub>). This episode seems to have occurred at lower temperature conditions (greenschist facies), more typical of other orogenic deposits in the WAC, as suggested by brittle structures, common sulfides such as pyrite and arsenopyrite, and alteration dominated by albite, sericite and carbonate. In these veins, gold occurs mostly as fracture filling, suggesting that it precipitated at a later stage than the sulfides, possibly during successive reactivation of these veins, during progressive deformation.

Data reveal that gold mineralization of the Tabakoto deposit is associated with both an episode of dike intrusion and a later phase of hydrothermal event. The association between gold mineralisation and a range of calc-alkaline magmatism have been documented in the WAC at Morila deposit in Mali (McFarlane et al., 2011) and at Bonikro in Ivory Coast (Masurel et al., 2019), which are defined as intrusion-related gold system. However, at Tabakoto deposit, the bulk of the gold mineralization is associated with two sets of quartz veining developed in high strain zones (D<sub>T2</sub>) and faults (D<sub>T3</sub>) affecting both the dikes of different nature (metabasalt to
<figure><img src="image_18.png" /><figcaption>**Fig. 18.** Textural features, shown in SEM-BSE (a, b, c, d) and reflected light thin sections (e, f) images of second ore trend minerals at Tabakoto. (a) Gold particles along deformation cracks in arsenopyrite in metadacite. (b) Gold particles along deformation cracks in pyrite in meta-andesite. (c) Gold particles along deformation cracks in arsenopyrite in metadacite. (d) Free gold grains within the gangue close to arsenopyrite and chlorite in metadacite. (e) and (f) Progressive replacement of pyrrhotite by arsenopyrite in metadacite. *Abbreviation:* Ab: albite; Ank: ankerite; Kfs: K-feldspar; Apy: arsenopyrite; Au: gold; Chl: chlorite; Ms: muscovite; Po: pyrrhotite; Py: pyrite; Qz: quartz.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. 19.** Blocks diagrams showing the structural evolution of the Tabakoto deposit (not to scale). (a) Views of the three-stage structural evolution of the deposit phase by phase and relationships with the emplacement of gold-related hydrothermal features. The first stage started with folding of the metasedimentary units and the first dike generation ($D_{T1}$), which were affected by a second folding ($F_2$), a set of penetrative $S_2$ schistosity, shear zones, and mineralized $V_2$ veins ($D_{T2}$). These were followed by a network of conjugate faulting during $D_{T3}$. The faults were filled by mineralized $V_{3a}$ veins and cross-cut by fractures and barren $V_{3b}$ veins developed parallel to the shortening direction during $D_{T3}$. The second dike generation event (lamprophyre) cross-cuts previous structural features. (b) View of the entire finite deformation and related mineralization of the Tabakoto deposit. *Abbreviation: F: fold; S: schistosity; V: vein.*</figcaption></figure>

metarhyolite) and metasedimentary units. In this case, the spatial association of gold mineralization with dikes at Tabakoto highlights the role of magmatic dikes as competent heterogeneities within metasedimentary units. The dikes acted as favorable sites for fluid flow due to competence contrast with the host rock and their brittle rheology. Accordingly, the brittle rheology of the dikes compared to the metasedimentary units is a key parameter that controlled the spatial distribution of the mineralization at Tabakoto. However, the fluids of the first gold mineralization may be part of magmatic origin because of the presence of minerals such as Au-Bi-Sch-Po-Ag-As, which are common in intrusion-related gold systems (e.g., Thompson and Newberry, 2000; Hart, 2007). Here, we can point to a possible source of fluide from a hidden intrusion at depth or in the vicinity of the Tabakoto deposit, which could be the Yatea pluton located to the north of the deposit at 4 km.

In the light of lithological and mineralogical particularities at Tabakoto, two scenario can be highlighted: (i) an intrusion-related gold system (Thompson and Newberry, 2000; Hart, 2007) with the contribution of fluid mobilized during the intrusive dike event or (ii) an intrusion-hosted orogenic gold

system with an important auriferous quartz veins, which cut across the intrusive dikes and metasedimentary units where the mineralized veins are localized within shear zones and faults of the more competent units. At Tabakoto, we propose a combination of the two system. At first, the intrusions of dikes with low sulfides and low gold grade, which is portray by disseminated gold-bearing sulfides. This was followed by the bulk gold values associated with the new gold input marked by the late hydrothermal fluids and/or remobilization of gold in dikes due to the late brittle deformation. This process is also documented in other African deposits (e.g., Pampe in Ghana, Kalana in Mali, Edikan in Ghana, Bonikro in Ivory Coast, Moboma in Central African Republic, Brothers project in south amrica; Salvi et al., 2016a, b; Tourigny et al., 2018; Masurel et al., 2019; Kpeou et al., 2020; Combes et al., 2024).

# 8 Conclusion

Based on detailed mapping of the Tabakoto deposit, analysis of the litho-structural data as well as our interpretation, we can highlight the following conclusion:
(i) The bulk of the gold mineralization is associated to hydrothermal fluid circulation affecting both the dikes of different nature and the metasediments under ductile to brittle conditions.

(ii) The litho-structural architecture at Tabakoto is consistent with an intrusion-hosted orogenic gold system, where the dikes acted as favorable sites for fluid flow.

(iii) A possibly early magmatic fluid from hidden intrusion or in the vicinity of the deposit portray by the presence of Au-Bi-Sch-Po-Ag-As minerals.

(iv) The D$_{T2}$ deformation event at Tabakoto can be correlated with the D$_{2}$ deformation phase at the regional scale, as defined by Milési et al. (1989), Gueye et al. (2008), Masurel et al. (2017c), and Allibone et al. (2020).

(v) The D$_{T3}$ phase at Tabakoto can be correlated with the regional Eburnean D$_{3}$ and/or D$_{4}$ deformation widely observed in neighbouring Senegal (Dabo et al., 2016; Diatta et al., 2017) and in Mali (Diallo et al., 2024).

(vi) The Tabakoto deposit is an example of structural-magmatic-hydrothermal gold deposition during a continuous deformation with E-W shortening from vertical stretching during D$_{T2}$ to horizontal stretching during D$_{T3}$ from the climax of the Eburnean orogeny marked by ductile deformation, to subsequent lateral flow of the orogenic crust under ductile to brittle conditions.

## Acknowledgments

This study was achieved within the framework of the PhD thesis of the first author, funded by the French Ministry of Research through SCAC/Mali and by the West African Exploration Initiative (WAXI) project. AMIRA International, the industry sponsors, and sponsors in kind are gratefully acknowledged for their support of the WAXI-3 project (P934B). Endeavour Mining Mali is gratefully acknowledged through Julien Baumard (Exploration Manager) and John Barry (Technical Services Manager) who are no longer there. Special thanks to Moussa Kéita (currently at Syama), Hassan Said, and Oliver Benford for their help at Tabakoto.

The authors acknowledge the Raimond Castaing micro-characterization center and the scanning electron microscope (SEM) center in Toulouse (University Toulouse III-Paul Sabatier). Philippe and Fabienne de Parseval as well Thierry Aigouy and Sophie Gouy are thanked for their excellent scientific assistance during the different analyses.

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**Cite this article as:** Diallo M, Salvi S, Baratoux L, Béziat D, Vanderhaeghe O, Labou I, Baratoux D, Ly S. 2024. Geology of the Tabakoto gold deposit, Kédougou-Kéniéba Inlier, West African Craton, Mali, BSGF - *Earth Sciences Bulletin* 195: 24. https://doi.org/10.1051/bsgf/2024024.