From a macroscopic view point, the Alahina metasediments have a schistose facies, ranging from greenish-gray to yellowish-gray or brown (Figure 4(a)), sometimes blackish with more remarkable schistosity (Figure 4(b)). The schistosity is marked by an alternation of greenish-gray beds, sometimes dark, 8 - 10 cm thick, affected by mineral stretch lineations, with thin dark-brown beds (1 - 3 mm thick). The greenish-gray bands consist mainly of quartz, feldspars, sericite and chlorite while the dark beds are mainly composed of quartz, feldspars, limonite, hematite and sericite.

Multiple fractures and intense hydrothermal alteration affected the meta-sediments. The corridors created by these fractures are materialized by quartz veins, of thickness varying between 2 mm and 30 cm. Hydrothermal alteration is characterized by carbonation (ankerite and calcite), chloritization, seritization, and pyritization of the hanging wall of the veins (Figure 4(a)). Gold is often not visible to the naked eye because it is extremely thin and associated with sulphides. In the areas of intense weathering, there is a discoloration of the rocks that changes from a greenish color due to the abundance of chlorite to a beige or gray color due on the one hand, to carbonates, and secondly the leaching of pervasive chlorite (Figure 4(a)).

From a microscopic point of view, the analyzed meta-sediment samples generally have a lepidoblastic texture with the following mineralogical association:

· quartz: not very abundant (20%) with remarkable rolling extinction. It is an aggregate of almost isometric crystals, dispersed and stretched parallel to the direction of the major schistosity, about 0.005 mm in size (Figure 5(a));

· sericite: abundant (50%), in small flakes, evenly distributed. Its size varies between 0.005 and 0.01 mm (Figure 5(a));

· chlorite: less abundant than sericite (20%), in subautomorphous and evenly distributed small lamellae varying in size from 0.005 to 0.01 mm with a mean positive relief in polarized light;

· epidote: less common (6%), crystallized in the form of small interlocking grains (0.01-0.005 mm), very strong relief and oblique extinction 30˚;

· magnetite: fairly rare (3%), opaque, isometric or slightly elongated, with a size of about 0.01 to 0.02 mm (Figure 5(a));

· apatite: accessory mineral (1%), hexagonal, gray with a strong relief in polarized light, it is in the form of crystals of small dimensions, of the order of 0.005 to 0.0025 mm.

In the vicinity of the hydrothermal quartz veins, the meta-sediments have particular textures: porphyroblastic (Figures 5(b)-(d)), granoblastic, lepidogranoblastic (Figure 5(e)). The porphyroblastic texture appears with a remarkable proportion of rectangular or square phenocrysts (15%) (Figure 5(b)). Porpyroblasts are tremo

lite and actinolite lenses, 11˚ - 17˚ oblique extinction, with fine crystals (0.005 mm) of neoformed quartz. The size of the phenocrysts varies from 0.5 to 1 mm. Coarse, almost round isometric and round grains (0.15 mm) with wavy extinction and a gray tint are also observed (Figure 5(b) and Figure 5(c)). The epidote is observed on the photomicrograph (Figure 5(b)) in the vicinity of a hydrothermal quartz vein. The same is true for opaque minerals (4%, Figure 5(c)). These opaque minerals are in almost isometric clusters, ranging in size from 0.005 to 0.0025 mm (Figure 5(c)). Rounded or rectangular cavities, filled with carbonates, are also observed on the framework formed by quartz, sericite and chlorite (Figure 5(c)), with a combination of sulphide minerals (1%), especially cubic pyrite (0.75 mm of side (Figure 5(d)). The granoblastic texture, the pidogranoblastic, locally serrated (suture) (Figure 5(e)) emanates from the development of microscopically coarse quartz veins (0.5 mm) and wavy extinction, on the finer grain sericite frame of quartz (0.005 mm). The quartz veins thickness is about 2 mm. The lepidogranoblastic texture band does not exceed 0.3 mm. It contains neoformed metallic crystals, magnetite, and probably hematite and leucoxene (Figure 5(e)).

The hydrothermal quartz veins contained in the meta-sediments, generally of a multi-centimeter thickness, have a porphyritic texture. They consist of oxides, hematite and iron hydroxides (limonite), deposited within the quartz fragments (Figure 5(f)). These quartz fragments are uniaxial positive and show a gray hue, sometimes with a dull yellowish hue in polarized light. They are characterized by wavy extinction and their size often exceeds 0.5 mm (Figure 5(f)). Some hydrothermal quartz veins display a cataclastic, porphyritic, sometimes collomorphic texture (Figure 5(g) and Figure 5(h)). The fragments are angular, disaggregated (breched) and consist of quartz and potassium feldspars. These fragments ranging in size from 2 mm to 1 cm are clogged with collomorphic hematite deposits (Figure 5(g) and Figure 5(h)).

The study of major and trace elements provides information about rocks and minerals formation conditions. Authors [17] [34] [35] [36] have studied rocks similar to those of this study and none of them reveal anomalies likely to modify the geochemical characters. Thus, we use the major and trace elements to highlight the petro-geochemical characteristics of the Alahina meta-sediments. Table 1 and Table 2 below mention respectively the composition in major and trace elements of the studied meta-sediments. Table 2 also presents PAAS compositions: Post-Archean Average Australian Shale and EP GRAY: Average Chemical Composition of Early Proterozoic Grauwackes.

For the major elements, the meta-sediment samples studied have a SiO₂ composition range of between 48.7% and 71.25% and Al₂O₃ between 13% and 28.30%. The other major elements are in the following proportions: Fe₂O₃ (3.05% - 61%), CaO (0.20% - 2.97%); MgO (1.24% - 4.71%); Na₂O (1.37% - 6.06%); K₂O

(1.09% - 6.81%); TiO₂ (0.35% - 1.14%); P₂O₅ (0.03% - 0.33%) and V₂O₅ (0.02% - 0.5%). These rocks have been subjected to a metamorphism causing a redistribution of some elements such as K₂O and Na₂O while others are immobile (SiO₂ and TiO₂). These immobile chemical elements (SiO₂ and TiO₂) can provide information on the nature of the protholite of our samples as well as their provenance.

Thus, in the TiO₂ versus SiO₂ classification scheme [27], all the meta-sediments analyzed are rich in SiO₂ with varying concentrations of TiO₂ and are positioned in the field of sedimentary rocks (Figure 6(a)). Identical results are obtained using classification scheme based on Log (Fe₂O₃/K₂O) versus Log SiO₂/Al₂O₃) [28]. The meta-sediments analyzed showed low SiO₂/Al₂O₃ and Fe₂O₃/K₂O ratios and would come from shales (sometimes rich in iron) and grauwakes (Figure 6(b)).

The alteration degree of meta-sediment precursor is evaluated from the chemical alteration index (CIA) [37]: CIA = 100 * [Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O)]

(CaO * corresponds to the concentration of CaO incorporated in the silicate fraction of the rock). For example, rocks with CIA > 92 values are strongly altered due to the transformation of feldspars into clay minerals [39]. Rocks with CIA values between 60 and 80 with atmospheric alteration and CIA < 60 show no or little alteration [29]. The meta-sediments analyzed have CIA values between 62.5 and 77.51 and indicate an atmospheric alteration of the protholite of these rocks.

The intensity of the alteration as well as the nature of the original sediments was indicated by the ternary diagram A-CN-K and CIA [29] (Figure 6(c)). Thus, our samples plotted in this diagram (Figure 6(c)) are positioned in the field of an intermediate alteration and prove that sediments in the Alahina sector derive from andesite, basalt and granodiorite. It should be noted that some of the meta-sediments analyzed are smectite rich and others in illite and muscovite, indicating the diversity of sources of provenance.

Rare earth elements (REE) patterns of Alahina meta-sediments (Figure 7) were compared to PAA: Post-Archean Average Australian Shale [40], as well as to EP GRAY: Early Proterozoic Chemical Composition. Greywackes [41]. These samples show a moderate LREE/HREE fractionation of (La/Yb)_N = 6.31 - 13.24, a flat pattern for heavy rare earth elements (HREE), (Gd/Yb)_N = 0.98 - 2.05 and negative anomalies in europium at low amplitude. The characteristics of these meta-sediments are similar to those of PAAS and EP GRAY, providing additional evidence concerning the pelitic and semi-pelitic composition of Alahina metasediments. In addition, the values of Eu/Eu* and Gd_N/Yb_N ratios are in the range of values (Eu/Eu* > 0.7; Gd_N/Yb_N > 1.0) found in Archean meta-sediments by [30] and [31] (Figure 8).

The geodynamic deposition environment of the detrital sediments that led to the formation of the metasediments object of the present study is appreciated from the

discriminating diagrams of [32] and [33]. Thus, in the Roser and Korsch diagram, the analyzed meta-sediments are positioned in two domains: the oceanic island arcs margins domain and the active continental margins domain (Figure 9(a)). However, the discriminant diagram from Bhatia shows a membership of metasediments in oceanic island arcs domain (Figure 9(b)).

The petrographic study showed mineralization within the meta-sediments already affected by tectonic deformations.

· Veins and mineralization

Fluid associated with metamorphic and tectono-sedimentary event affecting meta-sediments in the Alahina sector has spread as veins of three types V₁, V₂ and V₃.

Veins V₁, of a millimeter thickness, are placed in the schistosity planes S₀ - S₁ (Figure 10(a)). They are affected by a ductile-brittle strain D1 (Figure 10(a) &

Figure 10(b)) generating open folds whose axes have a direction varying between N80˚ and N100˚ (measured in situ). Veins V₂ are parallel to S₂ schistosity or NE shear zones and intersect veins V₁ (Figure 10(a)). They are arranged “in step” and affected by ductile-brittle D2 deformation characterized by driving folds and fractures of direction NE. Their thickness often varies from 2 cm to 30 cm. Veins V₃ intersect V₂ veins associated with S₂ schistosity following NE dextral shear (Figure 10(a)).

These different veins are generally recrystallized in silica, carbonates, micas, sulphides, oxides and precious metals thanks to the circulation of hydrothermal fluids responsible for neoformations.

· Phases of mineralization and their location

Three hydrothermal phases have been distinguished: 1) a phase with silico-sericite-chloritized and sulphurized parageneses, often diffuse or stratiform, consisting of Qz_I-Ser-Chl ± Ep ± Tr ± Act ± Cb ± Ap-Py_I ± Apy_I ± Mg_I ± Leu_I ± Hem_I for veins V₁ which is localized in meta-sediments (Figure 10(b) & Figure 10(c)); 2) two phases with sulphurised, locally serititized and/or chloritized sulphide feldspathic parageneses, consisting of Qz_II-Cb_II-Mc_II ± Ser ± Chl ± Ep_II-Py_II ± Mg_II ± Leu_II ± Hem_II-Cpy_II-Au_II et Qz_III-Cb_III-Mc_III ± Ser ± Chl ± Ep_III-Py_III ± Mg_III ± Leu_III ± Hem_III-Cp_III-Au_III, forming respectively the stock of veins V₂ and V₃ (Figures 10(d)-(f)).

The metasediments studied are sericite and chlorite schists traversed by quartz veins containing auriferous sulphides and oxides. The minerals that characterize these schists are: quartz, potassic feldspars (rather rare), sericite, chlorite, epidote, apatite, actinolite, tremolite, carbonate, biotite, sulphides, magnetite, hematite and ilmenite. The presence of certain metamorphic minerals such as chlorite, epidote and actinol testifies the sedimentary origin of our samples and prove that they were subjected to low grade metamorphism (Temperature = 250˚C - 500˚C, Pressure = 2 - 10 kilobars, depth = 5 - 30 km), with shale facies [36] [43] [44]. These shales consist of shales and greywackes, set up in an oceanic island arc and active continental margin environment, as well as the metasediments located in Yanfolila belt, in western Mali [38], in Agbahou Fettêkro furrow in Ivory Coast [45] and the paragneisses of Granja in northeastern Brazil [44]. The basaltic, andesitic and granodioritic source of our samples is consistent with the juvenile character of the Birimian formations.

This study has shown the archaic character that the Birimian schist could have. The rare earth elements pattern of the meta-sediments are similar to the Early Gray Protein Gray Chemicals of [39], and have in addition values of the Eu/Eu* and Gd_N/Yb_N ratios in the range of values (Eu/Eu* > 0.7; [Gd/Yb]_N > 1.0) found in Archean meta-sediments by [30] and [31]. The existence of Archean relics (3200 Ma) in the paragneissic area of Tabou and Grand-Bereby in southwestern Côte d’Ivoire [17] [35] [36] is in agreement with this result. It is therefore obvious that Alahina’s green shale originates from the disintegration of Paleoarchean migmatitic gneisses and metagabbros (~3500 Ma [9]. These Archean metamorphic rocks are adjacent to the Siguiri volcano-sedimentary basin and outcrop in southeastern Guinean territory at the base of the Nimba Mountains [2] [3] Meta-sediments with iron-rich shales as protoliths are probably Archean.

Although present in meta-sediments, trace elements such as Au, Pt, Pd, Ba, Be, Co, Cs, Ga, Sr, As, Ag, Zn, Cu, Pb, In, Ge, Nb, Rb, Sn, Mo, Sr, Th, U, V, W, Sb, Zr, Tl and Ni have not been specifically analyzed. However, their presence should be indicated because of the mineralogical associations that accompany gold areas, including arsenic, zinc, lead, copper, and silver in gold sulphides.

The Alahina metasediments are affected by deformations and traversed by hydrothermal fluids with deposition of silica and sulphide [46] and/or oxidized minerals such as pyrite, chalcopyrite, gold, magnetite, hematite, ilmenite and to a lesser extent, arsenopyrite. Gold mineralization occurs in a context of polyphase (ductile-brittle and brittle) tectonic deformation marked by NE shear. The work from [3] on the Siguiri gold mine in Guinea (eastern zone of this study area) indicates four tectonic deformation phases: NS compression (D1S), EW compression (D2S) progressively evolving into a transpression then in a transtension (D3S) and finally compression NW-SE (D4S). [47] and [48] reported that the Ashanti gold deposits (Ashanti, Konongo, Bogosu, Prestea and Abawso mines) are found in NE-SW shear-zones at NNE-SSW and intersecting Birimian sediments near the contact with the volcanites. Like these deposits, gold mineralization in Alahina sector is associated with shear zone-type deformations. The genetic mineralization model commonly accepted in such a geological context is that of a continuum of fluid flows from ductile stages to brittle stages in relation to the triggering of earthquakes, according to the “fault-valve” model proposed by [49]. Indeed, each time the fluids pressure exceeds the lithostatic pressure, it causes rocks fracturing in localized zones which will thus constitute drains facilitating hydrothermal fluids circulation, creating the mineralized zones [50]. With the decrease of pressure, these fluids precipitate their mineral charge which will be found thus in filling of veins. Two types of mineralization can be mentioned in the Alahina sector: disseminated and veined. The existence of disseminated mineralization that would be syngeneic is based on the presence of nanoparticles of pyrite, magnetite, hematite, ilmenite within the metasediments independently of the veins or veins.