The prefecture of Boké is located in northwestern Guinea ( Figure 1(b) ) between latitudes 10˚30' and 11˚45' north, and longitudes 13˚45' and 15˚00 west. It covers 11,453 km² ( Mamedov et al., 2010 ) and belongs to the Lower Guinea natural zone. The Boké region is made up of sedimentary formations of the Plateformian cover, crossed by numerous Mesozoic intrusions. This sedimentary cover is identified by the Ordovician formations, which are marked by fine quartz sandstones, sometimes with aleurolite intercalation, observed to the southwest ( Seliverstov, 1970 ). The Silurian formations are represented by finely bedded black argillites, compact argillites, aleurolites and lenses of micro oolitic iron with intercalation of quartz sandstones ( Seliverstov, 1970 ). The Devonian is underlain by Silurian argillites and aleurolites represented by quartz sandstones ( Samozvantsev et al., 1976 ). At the end of the Paleozoic and Mesozoic, the study region experienced intense tectonic activity linked to the break-up of the Gondwanan continent and the opening of the Atlantic. This activation is marked by the formation of a system of NE-SW transform faults and several secondary faults, through which intrusions have accessed. These magmatic intrusions are represented by dolerites ( Boufeev et al., 1970 ; Samozvantsev et al., 1976 ). There are Cenozoic marine formations (quartz sands, bedded clays) and Paleogene weathering crusts resting on Paleozoic formations. The Neogene is characterized by conglomerates that lie concordantly on the Paleogene. Quaternary deposits are represented by sands, sandy silts, marine kaolinitic clays, fluvio-lacustrine clays and sands. These deposits are found on the coastal plain and sometimes in the river valleys ( Figure 1(c) ).

The Boundou-Waadé bauxite deposit is represented by Devonian aleuro-argillites, argillites and aleurolites into which Mesozoic dolerite sills and dykes are injected ( Samozvantsev et al., 1976 ). On the deposit under study ( Figure 2 ), the oldest faults strike northwest and north-south, while northeast-trending brittle dislocations are associated with recent transform faults. Northwest- and northeast-trending faults are the best developed. Tectonic activation occurred during the neotectonic phase when inherited landforms were established ( Samozvantsev et al., 1976 ). The Mesozoic tectonomagmatic reactivation of the source rocks the Boundou-Waadé favored the emplacement of dolerite sills. These sills, of modest dimensions (4 - 10 m), are sub-concordant and gently sloping in relation to the surrounding Devonian sedimentary rocks ( Mamedov et al., 2010 ). They alternate with intercalation of the sedimentary rocks of the Faro suite, whose thickness is comparable to that of the lateritic cover. The deposit under study is associated with the core of the Bowé syncline ( Figure 2 ).

The main material studied is the Boundou-Waadé bauxite deposit. To carry out this study, the following tools were used:

During our internship from January 24 to March 30, 2020 at Compagnie of Bauxites in Guinea, geological observations and studies (core analysis, stratigraphy, etc.) were carried out.

The Boundou-Waadé deposit is underlain by ferruginous bauxites 1.5 to 3 m thick at the surface, covered in places by vegetation. Analysis of stratigraphic logs C, D, E, H and J ( Figure 3 ) shows two (2) types of formation: source rocks at the base, more than 1 m thick, topped by the alteration crust ( Bhukte et al., 2020 ). This weathering crust is subdivided into four (4) distinct horizons ( Sidibe and Yalcin, 2019 ): from bottom to top, there is the initial decomposition horizon (4 m thick), the clay horizon (1.5 m to 3 m), the transition zone (3 m to 7.5 m), and the free oxide or bauxite zone (9.5 m) ( Figure 3 ). This bauxite zone is marked by gelified and lateritic bauxites. Isolated intercalation of nodular ferriplantites is observed.

Macroscopic and microscopic studies were carried out on bauxites and ferruginous laterites.

The Boundou-Waadé deposit contains two (2) groups of bauxites:

These bauxites are most often referred to as structural bauxites;

Both types of bauxite are characterized by their low quality and low aluminum monohydrate content. This is due to the predominance of gibbsite. Alumogoethite has a low alumina content. Titanomagnetite is present in apodoleritic bauxites. Rutile and anatase are typical for bauxites developed at the expense of Devonian rocks ( Figure 5(a) and Figure 5(b) ). Lateritic bauxites can be distinguished by their textural features and platelet-like divisions for structural bauxites ( Figure 5(d) ) and shell-like divisions for apodoleritic bauxites.

Chemogenic bauxites represent only 2%, and are simply the product of the chemical redistribution of elements at the base of lateritic bauxites. The latter is subdivided into two (2): gelomorphic bauxites, which are less widespread on the deposit, and gelified bauxites, which are more widespread ( Figure 5(c) ). They have a massive texture and a microcrystalline structure. In comparison with lateritic bauxites, they are similar in textural features, with a slight difference in pink-white coloration; lighter pink.

Ferruginous laterites have a massive, cavernous texture and an aphanitic colomorphic structure (bauxite derived from dolerites). As a rule, they are composed of ferrigels and hematogels that form rhythmically zoned patches encrusting the walls of cracks and caverns. Large grains of newly-formed secondary gibbsite, 2 - 3 mm in diameter, are also found.

They have a pseudobrecciated texture with yellow, yellowish-red coloration and pores. Fragments range in size from 1 - 2 to 6 - 9 mm, with a sharp-edged shape and a very bright dark-red color. The cement is composed of hematogel and ferrigel with a yellowish-red, variable-yellow color. The fragments have a massive texture and an aphanitic collomorphic structure, occupying a large part (80% -90% and more) composed of ferruginous minerals (goethite, hematite). The cement is developed in the form of veinlets, and presents an aphanitic texture and a collomorphic structure. It is semi-transparent under piercing light. The cement is composed of ferrialumogel ( Figure 6 ).

XRD analysis shows that mineralogical composition is similar in all log bauxites (C, D, E, H, J). The results of quantitative phase analysis are given in tables (1 and 2); these values represent the relative quantities of crystalline and amorphous phases normalized to 100%. Boehmite and diaspore are absent in some samples. The bauxitic horizon of log C, D and E—most samples contain gibbsite, alumogoethite, goethite, hematite, anatase and rutile, accompanied by a mineralization of hydromica.

Several mineral combinations: kaolinite (C06 and C07); kaolinite, quartz (C07); kaolinite, quartz and hydromica (E01) ( Table 1 ). In the clay horizon of sections C, D and E, most samples contain kaolinite, quartz and hydromica. In the Log H and J bauxitic horizons, most of the samples contain kaolinite, quartz and hydromica. contain gibbsite, alumogoethite, goethite and hematite, accompanied by one or more minerals such as rutile and kaolinite (J01) ( Table 2 ). In the clay horizon. From the results of mineralogical analysis ( Table 1 and Table 2 ), given the absence of boehmite and diaspore, we can deduce that gibbsite is the economic mineral for aluminum at Boundou-Waadé.

Geochemical analysis clearly shows that Boundou-Waadée bauxite is essentially composed of: Al₂O₃ (23% - 68.4%); Fe₂O₃ (3.00% - 51.25%); SiO₂ (0.5% - 2.9%) and TiO₂ (1.32% - 8.1%).

Alkaline and alkaline-earth: FeO (0.04% - 0.43%), CaO (0.05% - 0.16%), MgO (0.05% - 0.42%). MnO (0.0064% - 0.072%). K₂O (0.002% - 0.5%). Na₂O (0.01% - 0.7%). P₂O₅ (0% - 0.1%) et PAF (011% - 29.5%) have low values ( Table 3 ).

Clays (C07, J04 and J03) have component values ranging from 0.05% to 41.25%.

Al₂O₃ (24.07% - 29.9%). SiO₂ (32.5% - 41.25%) Fe₂O₃ (18.6% - 21%) TiO₂ (1.68% - 2.52%) FeO (0.13% - 0.16%) CaO (0.09% - 0.15%) MgO (0.05% - 0.42%) MnO (0.08% - 0.23%) K₂O (0%) Na₂O (0%) P₂O₅ et (0%) and (13.22% - 15.5%) PAF. While the aleuro-argillite samples: (CO5; CO6; H08; JO1; JO2) have respective values in: Al₂O₃ 28.58% and 41.47%; SiO₂ 1.72% and 7.00%; Fe₂O₃ 30.8% and 48.50%; TiO₂ 1.50% and 2.68%; FeO 0.04% and 0.43%; CaO 0.05% and 0.08%; MgO 0.05% and 0.05%; MnO 0.02% and 0.04%; K₂O 0.00% and 0.03%; Na₂O 0.00% and 0.03%; P₂O₅ ≤ 0.05% with a PAF ranging from 0.21% to 23.88% ( Table 3 ).

According to the mineralogical classification ( Aleva, 1994 ), all bauxitic samples (C02, C04, D04, H01, H03, H04, H05, H07, D01 and E04) from the Boundou-Waadée plateau fall into the bauxite and ferric bauxite fields ( Figure 7 ). The argillite samples (C07, J04 and J03) fall in the ferrite and laterite fields. Ferriplantites (C03, D05, H09) and ferruginous laterites (C07 and H08) fall into the ferrite field. We note that some clay samples (C07, J03, J04) fall into the kaolinite field, while some clays (D01, D03, E02 and E04) and aleuro-argillite (C07) fall into the bauxitic kaolinite field ( Figure 7 ).

The triangular diagram of the degree of lateritization of bauxites by ( Schellmann, 1986 ) shows that most samples (C05, C06, J01, J02, E03, H02, H06, H08, C01, D02, E01, C02, C04, D04, H01, H03 and H04) from Boundou-Waadé are strongly lateritized ( Figure 8 ), while samples (C01, D02, E01, D01, D03, E03, E04) are weakly lateritized. A bauxitic sample (E02) lies at the boundary between strong and moderate lateritization. Finally, a clay sample (J04) lies in the kaolinization band ( Figure 8 ).

In the classification diagram of ( Boulange et al., 1996 ), all the bauxite samples (C02, C04, D04, H01, H03, H04, H05, H07, C05, C06, J02, E03) from Boundou-waadé fall within the bauxite and ferruginous bauxite fields. Only one bauxitic sample (E04) lies between the two bauxite fields ( Figure 9 ). Ferruginous laterites (J02 and J04) lie between ferruginous and kaolinitic bauxites. The ferriplantites (C03, D05, H09) lie between the kaolinitic interval. In short, the clays (C07, J03, J04) and aleuro-argilites (CO7) are in the kaolinitic field ( Figure 9 ).

In the classification diagram of ( Tardy, 1997 ), the geochemical dismantling trajectories demonstrate numerous trends followed during the bauxitization process: kaolinite preservation, deferruginization, kaolinite destruction and dehydration ( Figure 10 ). Thus, the clay and aleuro-argillite samples (C07; J03; J04) follow the kaolinite preservation trend; in addition, the ferruginous laterite samples (C05; C06; H08; J01; J02) lie in the ferruginous shadow zone. Finally, the bauxite samples (C01; D02; E01; C02; C04; D01; D03; D04; E02; E03; E04; H01; H02; H03; H04; H05; H06; H07) follow the trend of kaolinite destruction, which explains why bauxites from the Boundou-Waadée deposit originate from the destruction of kaolinite ( Figure 10 ).

The correlation diagrams for SiO₂, Fe₂O₃ and TiO₂ were plotted against Al₂O₃, the main chemical element, to study the behavior of the elements. The correlation diagrams for SiO₂, Fe₂O₃ and TiO₂ as a function of logs C, D, E, H and J are virtually identical ( Figure 11 ). In logs C, D and E, SiO₂ content remains low despite high Al₂O₃ content, while Fe₂O₃ and Al₂O₃ content increases in these logs. Sample C07 has a high quartz content. Overall, they show low values in logs C, D and E. This can be explained by the fact that the content is constant in these logs, indicating a negative correlation on the other hand, in these same logs, Fe₂O₃ content shows a positive correlation with Al₂O₃. Whereas TiO₂ content shows a negative correlation with Al₂O₃ in these logs. In log H, the SiO₂ and TiO₂ versus Al₂O₃ plots show that their content decreases with Al₂O₃, indicating a negative correlation; while the Fe₂O₃ vs. Al₂O₃ diagrams show that Fe₂O₃ content increases with Al₂O₃, indicating a positive correlation. On the other hand, in log J, SiO₂ and Fe₂O₃ diagrams as a function of Al₂O₃ show that contents decrease with Al₂O₃ while TiO₂ increases with Al₂O₃, indicating a positive correlation. This positive correlation can be explained by the fact that rutile, anatase and gibbsite contents are increasing ( Figure 11 ). Thus, in bauxitic horizons, the major element correlation diagrams show silica leaching and residual enrichment in aluminum and titanium.