Gabbros

Gabbros are the most dominant lithology in the study area. They are found in the localities of Gouméré, Dakoua, Siago and Koboko. They are melanocratic, massive in appearance (Figure 3(a)). Microscopic study of these rocks shows gritty to microgranular porphyritic textures (Figures 3(b)-(f)).

· Plagioclase: it represents 25% to 30% of the matrix with rarely automorphic forms. It often occurs in phenocrysts (1 to 2 mm long and 1 mm wide). It is almost unrecognizable on some sections because of alteration. It alters mainly to sericite (Figure 3(e)) and damourite and sometimes to epidote (Figure 3(c)).

Plagioclase is found in places in inclusion in the green hornblendes.

· Pyroxene: abundant 35% to 40% in some thin sections and less in others where we have observed an almost total ouralitization in amphibole. Several types of pyroxenes have been described, they are augite showing a macle of albite (Figure 3(f)), and hypersthene.

· Green Hornblende: it represents 20%, in general automorphic. This hornblende is millimetric to centimetric, very pleochroic (dark green to light green). It is omnipresent in some parts of the thin section and comes essentially from the ouralitization of pyroxenes. It is presented in phenoblasts of more or less elongated shape (up to 2 mm long and 1 mm wide) with two sections: a basal section with two cleavage planes making 120˚ and a longitudinal section.

1) Basalt

The basalt is of massive or deformed aspect (Figure 4(a)). It is generally melanocratic (blackish) and sometimes traversed by veins and veinlets of quartz and calcite. This rock of basaltic composition has been observed in the Koboko and Siago localities.

Under the microscope, the basalt has a microlitic porphyry texture (Figure 4(b) & Figure 4(c)). The phenocrysts observed are generally pyroxene and amphibole.

· Green Hornblende: abundant, occurs in microliths and often alters to epidote.

· Pyroxenes: not very abundant, automorphic most often in phenocrysts with two cleavage planes at 90˚, most often augite.

· Mesostasis: composed of plagioclase and amphibole rods, is devitrified and partially recrystallized into epidote and sericite minerals.

Carbonate are also observed, most often associated with sericite, epidote, oxides and sulfides (Figure 4(d) & Figure 4(e)).

2) Volcanoclastite

These outcrops are located in the locality of Koboko. These rocks are melanocratic. Facies with dark minerals (amphiboles and pyroxenes) of millimeter to centimeter size are the most numerous. These occur locally as breccias and lapillis tuffs (Figure 5(a)). Microscopically, we observe a porphyritic microlithic texture.

· Mesostasis: contains glass and minerals of carbonates, amphibole, sericite and quartz. We also note the presence of quartz-carbonate veinlets. The clasts generally destabilized in carbonates and also phenocrysts of quartz, amphibole and carbonate are mainly composed of chalcedony and carbonate (Figures 5(b)-(e)).

· Chalcedony: subautomorphic to automorphic, consists of a quartz crown, there are two generations of quartz depending on their size and carbonates (Figure 5(c) & Figure 5(d)).

· Carbonates: mainly subautomorphic calcite, recognizable by its Carlsbad macle.

3) Rhyodacite

These outcrops are located in the Siago and Koboko localities. These rocks are mesocratic (brownish) with dark minerals (amphiboles) of millimeter to centimeter size being the most numerous. Quartz phenocrysts are also present (Figure 6(a)). The microscopic mineralogy of the rhyodacites shows a porphyritic microlithic texture, composed of plagioclase, amphibole and quartz in a plagioclase matrix.

· Plagioclase: abundant and automorphic (rectangular), sometimes in phenocrysts up to 1.5 mm long and 1 mm wide. It may exist as a micrograin associated with the matrix. It is frequently altered to sericite and sometimes shows zonation (Figures 6(d)-(f)).

· Hornblende: automorphic (rhombic) phenocrysts of about 1.5 mm long and 1 mm wide. It has two cleavage planes at 120˚ in basal section. It also exists in

automorphic micrograin (Figures 6(b)-6(f)).

· Quartz: less abundant, corroded and showing rolling extinction (Figure 6(b)).

· Mesostasis is essentially composed of plagioclase microliths, showing a sericitization.

These rocks were collected in the localities of Lomo and Iguéla, of brown color, they are characterized by grains of quartz taken in a clayey cement (Figure 7(a), Figure 7(b)). Microscopy reveals quartz and muscovite minerals organized in a granular texture (Figures 7(c)-(f)).

· Quartz: very abundant, 70% to 80% of minerals. It is subrounded and angular with a diameter of up to 1 mm. It is often in the form of cracked porphyroclasts

showing a direction of deformation and also in micro grains (Figure 7(f)). These grains have a preferential orientation.

· Muscovite: rare, showing a wavy extinction and a bright hue. It is found only in the matrix (Figure 7(f)).

· Matrix: less than 20%, it is made of ferro-titanium clay with an important sericitization.

1) Plutonic rocks

These rocks (GOU, DAK, KOB 2, SIA 2) are characterized by SiO₂ contents of

48.72% to 54.2% and alkalis (Na₂O + K₂O) of 2.85% to 3.87%, which give them a gabbro and gabbroic diorite composition on the classification diagram of [30] (Figure 9). Al₂O₃ contents vary between 13.11% and 16.66%; MgO between 5.29% and 9.07%; Fe₂O₃ between 9.72% and 15.15%. CaO contents vary from 8.08% to 11.3%, Na₂O from 1.95% to 3.29% and K₂O from 0.18% to 0.8%. MnO contents are between 0.14% and 0.23%. The TiO₂ values are lower than 2% (Table 1).

2) Volcanic rocks

The basalt (KOB 1), (Figure 10) has SiO₂ contents of 52.57% (Table 1); MgO of 6.47%; Fe₂O₃ is 11.69%; Al₂O₃ of 14.57% and CaO of 9.07%. Na₂O and K₂O contents are 4.09% and 0.21% respectively. The low TiO₂ content of 1.37% of the analyzed basalt resembles those of plutonic rocks of magmatic arcs [31] , but are different from intraplate basalts, which often possess high TiO₂ contents (>2%). This mafic volcanoclastite (KOB 3) is characterized by SiO₂ contents of 50.14% (Table 1); MgO of 3.61%; Fe₂O₃ of 8.84%; Al₂O₃ of 12.85% and CaO of 10.01%. Na₂O and K₂O contents vary respectively by 2.32% and 0.38%; MnO by 0.16%. Low levels of TiO₂ (0.81%) are also observed. This rock corresponds to basaltic andesites as shown in the diagram of [30] , (Figure 10).

Rhyodacite (KOB 4) shows SiO₂ contents of 67.02%; MgO of 1.51%; Fe₂O₃ of 4.63%. Al₂O₃ varies by 15.02%; CaO by 3.3%; Na₂O by 5.28%; K₂O by 1.06% and MnO by 0.07%. TiO₂ values are less than 2% (0.42%).

Reference [29] , allows us to discriminate magmatic to sedimentary rocks. We

have highlighted two samples of sedimentary origin: IGU 1 and LOM 1 (Figure 8). This diagram is based on P₂O₅/TiO₂ ratios versus MgO/CaO. These ratios give values for sandstones (metasediments) between 0.09 - 0.14 for P₂O₅/TiO₂ and 4.14 - 4.33 for MgO/CaO.

Reference [32] diagram based on the log (SiO₂/Al₂O₃) and log (Fe₂O₃/K₂O) ratios, shows a litharenite and sublitharenite composition respectively for the samples of IGU 1 and LOM 1 (Figure 11).

Gold mineralization in the Gouméré area occurs in mafic volcanics and in microconglomerates. The processes that affected these formations result in the formation of quartz veinlets and are accompanied by hydrothermal alterations.

Carbonation, silicification, sericitization and sulfidation are the most important alterations in the study area. Carbonation leads to the impregnation of the surrounding rocks by carbonates. The commonly encountered carbonates are calcites and to a lesser degree, dolomites and/or ankerites. They are found pervasively in rocks (Figure 12(d), Figure 12(f)). Carbonation is greatest in mafic lava where all calcic plagioclase and other minerals are destroyed and replaced by calcite, dolomite or ankerite.

Silicification indeed appears as the process of impregnation of the surrounding rocks by silica (quartz, chalcedony). It includes at least two phases: one being a quartz-feldspar vein (Figure 12(a), Figure 12(b), Figure 12(e)) and the other being related to sulphides and carbonates (Figure 12(a), Figure 12(d), Figure 12(f)). The first phase would correspond to vein alteration; these are phenomena of clogging of fractures by hydrothermal fluids highly enriched in metals. The second phase is manifested by the abundance of fine quartz and chalcedony

minerals in mafic volcanic rocks, associated with calcite (Figure 12(a)). Sericite minerals are generally common in feldspar-rich volcanic and sedimentary rocks. Indeed, the strongly sericite zone developed around gold-bearing quartz veins or in mineralized zones is of hydrothermal origin. Sericitization accompanies the establishment of gold mineralization (Figure 12(e)). The process of forming sulfides is called sulfidation. In Gouméré, sulphides are mainly formed of pyrite and chalcopyrite (Figure 12(f)). The oxidation of these sulfides gives titanium iron oxides in occurrences magnetite and hematite (Figure 12(f)). In the area studied, sulfidation is more significant in highly deformed zones. Sulfides are generally concentrated in mineralized zones invaded by numerous quartzcarbonate veins and/or veinlets.

The mineralized zones are intensely deformed and have undergone pervasive alteration. There is a gradual reduction in the intensity of the alteration as one moves away from the mineralized zones. The formulas of [33] and [34] made it possible to make a quantitative evaluation of the alteration minerals. These two parameters were plotted on the same diagram to evaluate the enrichment of the surrounding rocks in alteration minerals (Figure 13). All rock samples were plotted on the IA-ICCP diagram. It appears that most of the samples are affected by hydrothermal alteration. They are all generally grouped in the ankerite-sericite-chlorite triangle and converge more towards the ankerite-dolomite poles. The microconglomerates, on the other hand, are richer in sericite (Muscovite).

Metalliferous paragenesis is mainly formed of pyrite, followed by chalcopyrite. The sulphides occur either in the form of veinlets parallel to the schistosity or in the interstices of the minerals, or in the form of automorphic phenocrysts. The

former are anteschistose while the latter demonstrate post to late tectonic recrystallization. Gold mineralization appears to have a close relationship with sulphides. The grains of pyrite are either disseminated in the host rocks or either in filling of the microfractures in the host (Figure 14(a), Figure 14(f)). Pyrite occurs in the form of fine xenomorphic crystals, elongated rods, aggregates or subautomorphic porphyroblasts (Figures 14(b)-(d)). There are two generations of pyrite: early pyrite in fine grains aligned along the S1 schistosity and late pyrite in the form of randomly oriented aggregates. The preferential orientation of the pyrite crystals suggests that they formed before or during deformation (Figure 14(b), Figure 14(c)). Some pyrites are interpreted as replacement minerals (Figure 14(d)). They often show flowing or distorted movements

(Figure 14(b), Figure 14(d)).

Chalcopyrite appears in the form of fine xenomorph to automorph grains or phenocrysts. They are isolated or often associated with pyrite, hematite and magnetite (Figure 14(f)). They are often inclusions in pyrite (Figure 14(e)). Chalcopyrite also occurs in the form of fine, disseminated xenomorphic crystals and in aggregates (Figure 14(f)).