The geophysical data used in this work come from gravimetric data downloaded from the Bureau Gravimétrique International (BGI; http://bgi.omp.obs-mip.fr/) in dat file format.

As part of this project, rock samples were taken in the field to validate gravity anomalies. The equipment used consisted of: a GPS (global positioning system), a geologist’s hammer, a clinometer compass, a sledgehammer, a chisel, a digital camera, a road map of the region, and the geological map of the area.

Laboratory work was carried out at the Laboratoire de Géologie, Ressources Minérales and Energétiques of the Université Félix Houphouët Boigny. Thin sections were made to describe the mineralogical composition of the formations in each zone. A polarizing microscope and camera were used to study and photograph the various thin sections.

Data processing was made possible using the following software:

- QGIS 3.18 is used for digitizing and producing location and geological maps;

- Geosoft 8.1. This enabled us to combine road and geological maps, to facilitate fieldwork.

The gravimetric prospecting geophysics method applied to sub-surface studies aims to detect variations in terrain density. The physical quantity involved is gravity ( g → ). The data used in this approach are those of the Bouguer anomaly. The value of the Bouguer anomaly measured at a given point on the earth’s surface reflects the density of the underlying rock [14] . There is therefore a correlation between spatial variations in Bouguer anomaly and lithologies through their density.

Data processing led to the production of the Bouguer anomaly map using Geosoft software by the spatial kriging interpolation method of Matheron (1962, 1963a, b) in [15] . A vertical derivative (dz) was applied to the Bouguer anomaly to highlight lithological contacts. Anomaly interpretation is based on the local geological map and geological field observations.

Macroscopic petrography consists in describing the lithological units of the samples taken from the outcrops encountered. The aim is to find or propose a name for the rock, based on the various classification criteria of texture, structure, mineralogical composition and color.

This involved making observations on the rocks and their deformation, identifying structural elements (stratification, schistosity, foliation, lineation, fracture, fold, etc.) and taking measurements to study deformation.

Rock samples were taken to make thin sections for microscopic study. The mineralogical assemblages observed were used to refine the petrographic and structural studies carried out in the field. They also enable us to characterize the different metamorphic facies.

The Bouguer gravity anomaly map shown in Figure 3(a) and Figure 3(b) reveals the geological heterogeneity of the study area through color variation. Depending on the color scale, Bouguer anomaly amplitudes range from 110 to 139 mGal. Regions of high density (anomalies with amplitudes above 121 mGal and colors ranging from orange-yellow to magenta) are the most dominant. The relatively rounded shape suggests the existence of a major massive intrusive dominating the region. Approaching the sub-surface (Figure 3(b)), this important structure is well marked and extends as far as western Ghana. In the field, these high-density anomalies correspond to a series of formations composed of granodiorite (CIB10, GOUM1 and 2) and tonalite (CIB4), locally overlain by cinerites.

Macroscopic and microscopic descriptions indicate that this granodiorite is porphyroid and oriented. It has even been observed in the Soko locality to the northeast of the study area (Figure 4(a) and Figure 4(b)). Granodiorite is grainy in texture, with occasional enclaves of porphyritic metabasite and oriented amphibole minerals. The granodiorite is often altered in places, and sulfides have been identified in the metabasite.

The tonalite observed in the Gouméré locality also has a grainy texture with a mesocratic color. It contains abundant feldspar minerals, amphibole and quartz (Figure 4(c)).

On the gravity maps (see Figure 3(a) and Figure 3(b)), the cold color anomalies (blue) highlight all low-density formations. On the map of the vertical derivative (dz) of the Bouguer anomaly (Figure 3(b)), the anomalies corresponding to this low density are observed to the west, south-west and north-east. Field and laboratory descriptions reveal a complex of Tarkwaïan formations (CIB3, CIB18, SO2, SO4, SO5) and associated formations (DUA1, SO6).

1) Tarkwaïan formations

- Polygenic conglomerates

These rocks have been observed on the Bondoukou-Appimadoum axis and outcrop in the form of a hill. The surface is altered, with boulders and healthy pebbles partly individualized by the passage of the road. This rock has also been observed on the Soko-Dua Kouamé axis. These conglomerates are also made up of quartz pebbles, rhyolite, quartzite, basalt (containing sulfides) and the presence of cinerite and deformed quartz veins (Figure 5(a)). In slide CIB 03, microscopic observation shows that the rock is composed of a granular-textured sandstone cement, made up of quartz, feldspar and other dark sedimentary grains (Figures 5(b)-(f)).

- Arkosic sandstone

These Tarkwaïan formations are vast in extent. In the field, they occupy almost the entire study area. They have been mapped along the Gouméré-Dadiassé axis, then around 300 m from Koun-fao, towards Tanda (CIB 18, Figure 6(a)). These rocks are composed of abundant quartz, sometimes in pebbles or large grains of variable size, less than 5 cm. As a result, they can be assigned microconglomeratic facies levels (Figure 6(b)). Next, there is a notable abundance of finer grains of orthoclase, giving these formations a pink color. This is metaarkose. The latter forms the cement in the micro-conglomeratic levels (Figure 6(b)). In places, quartz-like formations are encountered, with fine, dark interlocking beds of biotite and ferro-titanium oxides (magnetite, hematite, ilmenite) and/or sulfides. This is the effect of gneissification due to metamorphism. These formations can be described as grauwackes (Figure 6(c) and Figure 6(d)).

2) Associated formations

Like the Tarkwaïan formations, the low-density anomalies are also described by the presence of volcanics composed of tuffs and cinerites (Figure 6(e) and Figure 6(f)).

Tuffs (DUA1) are also observed at Dua-kouamé (north-east of the study area). They present stratified levels with vertical granoclassing containing quartz pebbles and fine sediments. They show no trace of deformation (Figure 6(e)).

The kinerites were described in the field at Siago, a village located on the main Gouméré-Bondoukou road (see Figure 1). They form a contact with the polygenic conglomerate on the Bondoukou-Appimadoum axis. The rock has a slightly silicified appearance with a conchoidal fracture and a greenish-gray color (Figure 6(f)). Its texture is microlithic, with no apparent grain and a whitish-yellow ash appearance in the altered phase. There are two facies marked by variations in color intensity. Microscopically, cinerites have an oriented microgranular matrix and small colorless and whitish crystals.

- Fractures

Several fracture networks have been observed in the arkose along the Gouméré-Dadiassé axis, running NNE-SSE, NE-SW, E-W, NW-SE (Figure 7(a)). These directions are clearly visible on the various interpreted gravity maps. Well-cleared fracture planes (fracture schistosities) have been observed in the Siago kinerite along the Gouméré-Bondoukou axis, trending from NW-SE to NE-SW with subvertical dips (Figure 7(b)).

- Faulting and displacement

A fault has been observed in the Soko granodiorite. This fault shows a fault mirror with a plane of 64˚ towards WSW and striations plunging 70˚ towards NNW (Figure 7(c)). Several ENE to SSE conjugate fractures were encountered in the area, with NW-SE aplite veins. These fractures are probably evidence of a stall.

- Schistosities

These structures were mainly observed in the arkosic microconglomerate with quartz pebbles and phyllite minerals, around 300 m from Koun-Fao on the Koun-fao-Tanda axis. The outcrop is a metaconglomerate with quartz and phyllite clasts oriented N10˚ and N166˚ (Figure 6(b)).

- Folds

Although not clearly identifiable on the various gravity maps, folds have been described in several formations in certain localities. In the Siago kinerite, for example, the folds observed are isoclinal and chevron-shaped (Figure 7(d)). The direction of the axial plane is N124˚, with a clear hinge axis dipping 40˚ towards SSW, observed in the vertical plane (Figure 7(e)). The direction of the axial plane is N124˚ with an axis dipping 40˚ towards SSW (Figure 7(e)). On the Gouméré-Dadiassé axis, the arkose is subvertically inclined in a NNW direction, folded by crenulation and dipping 64˚ to the NNE. The direction of the axial plane is N20˚. The grauwacke is also creased by crenulation. The direction of the axial plane is N20˚ (Figure 7(f)).