Received 27 June 2015; accepted 23 August 2015; published 26 August 2015

ABSTRACT

To achieve the current study, geoelectrical surveys along six (06) profiles of 4 km long in a 100 m × 200 m grid defined according to the triangulation principle in the Tindikala-Boutou (Eastern- Cameroon) area along the Kadey River have been made through electrical sounding and profiling following Schlumberger array. The instrument is the resistive meter Syscal Junior 48 (IRIS Instrument). The data have been processed and modelled with Res2Dinv and Winsev softwares, and then interpolated with Surfer software. Investigation method used is the Direct Current (DC) method. Interpretations and analyses of results from the investigation method highlight weak zones or conductive discontinuities. The latter has been identified as shear zones within granitic structures of the Precambrian basement, according to the geologic and tectonic background of the area. The structural trend of these shear zones is E-W approximately. The mineralization characterized by conductive zones proves the presence of clay minerals disseminated in weathered quartz vein, which cross the shear zones. The intense activities of gold washers encountered in the studied area are able to attest the presence of clay minerals concentrations.

Keywords:

Geoelectrical Surveys, Direct Current Method, Resistivity, Shear Zones, Clay Minerals

1. Introduction

The reliability of any geophysical exploration result is supported by the geological background assessment of a given area, no matter its purpose. In accordance with this, a geoelectrical investigation has been made in the Tindikala-Boutou village of the Ngoura subdivision (Eastern-Cameroon). The approach consists in collecting apparent resistivity data using the Direct Current Schlumberger’s geoelectrical investigation (sounding and profiling). Data will be processed with Res2Dinv, Winsev and Surfer softwares. Results, given as pseudo sections of resistivity, resistivity maps will be able to plot tectonic unevenness of the area under study and also to characterize the associated mineralization. The geological sections realized through the interpretation of electrical soundings will bring out the correlation between the resistivity data and the geological background of the studied area.

2. Geological and Tectonic Setting

The study area (Tindikala-Boutou village) is located in the Ngoura subdivision in the heart of the East Cameroon’s region. Its easting stretches from 426,000 m to 437,800 m and the northing from 548,750 m to 557,750 m following UTM33 WGS84 system (Figure 1 and Figure 2).

The Ngoura area is located in the transition zone between the Pan African domain and the Congo Craton [1] [2] , in the northern edge of the Cameroon faults’ zone. This region of Cameroon is essentially made up of a Precambrian basement comprising metamorphic and magmatic rocks [3] - [5] . These are Precambrian rocks which sometimes outcrop. These are mainly granites and migmatites rejuvenated during the panafrican event. The area is mainly made up of (Figure 1) [6] [7] :

- Biotite and muscovite quartzites, sericitic muscovite and conglomeratic quartzites, chloritic and sericitic schists, paraamphibolites, orthogneisses, biotite gneisses and, pegmatites and quartz veins constituting the precambrian basal complex aged between 2.5 and 1.8 billion years.

- Plutonic and metamorphic formations made up of calc-alkaline granites with a porphyroic and alkali facies; granodiorites and syenites containing heterogeneous and undifferentiated biotite facies; quartz-diorites; micaschists, migmatites and embrechite gneisses.

- The main sedimentary rocks encountered are sandstones, sand, marl, limestones and, Paleozoic and Mesozoic conglomerates.

The tectonic facts revealed that, the study area is characterized by four deformations phases D1-D4 [8] . The observed tectonic lines are directed SW-NE below, and turned to be SE-NW above the 4 N parallel (Figure 1). Previous geophysical studies [9] [10] have shown evidence of some buried faults directed W-E and have confirmed tectonic nappes with a southern vergency. According to Olinga et al. (2010), the Pan-African deformation affecting the study area which appertains to the southern segment of the Neoproterozoic fold belt of Central Africa in Cameroon, is controlled by thrust tectonics and late strike-slip shear zones: the thrusting of the Pan-Af- rican Nappe over the Congo Craton (D2 deformation phase) is followed by a strike-slip shearing trending ENE- WSW (D3 deformation phase). During these stages deforming conditions were ductile to brittle-ductile. The dominant structural features of the D3 phase are penetrative foliation steeply dipping N or S, an associate ENE- WSW stretching lineation, and an N-S to NE-SW folding. Deformation criteria in the distinguished rock units indicate dextral sense of shear. A dextral trans-pressional model is assumed by Olinga et al. (2010) to explain the observed thrust and shear movements.

3. Method

The resistivity is a suitable parameter in characterizing the nature and the weathering of materials [11] . Hence electrical prospecting methods have been used for a long time in geological and geotechnical engineering. These both qualitative and quantitative methods are based on the Ohm law [11] . They consist in the injection of a direct current in the ground and the measurement of the electrical potential which enables to obtain the true resistivity of encountered formations. In our study area, while considering the geological setting, Schlumberger sounding and profiling methods have been used to determine: the thickness, the lateral extension and the nature of formations encountered along a profile; and to highlight the geometry of geological bodies related to contrasted electrical characteristics [12] .

Field electrical methods (D.C.) consist in injecting an electrical current in the ground between two electrodes A and B (Figure 3), and then, measuring the induced potential drop between two so-called potential electrodes M and N [12] . For the current intensity is known and the potential drop measured, it is therefore possible to determine the ground apparent resistivity. This apparent resistivity depends on the current and potential electrodes array. The apparent resistivity ρ_a (Rho) can be expressed function of the potential drop and the current intensity [13] :

(1)

where r_a is in Ohm∙m, and K (in m) is the geometric factor depending on the electrodes’ array which is given by the Formulae (2) below.

(2)

- V_MN: potential drop between electrodes M and N, in mV;

-I_AB: electric current injected between electrodes A and B, in mA.

For the Schlumberger symmetrical configuration (Figure 3), the apparent resistivity ρ_a is given by Equation (3) as follow [14] :

(3)

This resistivity value enables to characterize a formation in the point (o) or station (Figure 3). The resistivity of an earth’s material depends essentially on the humidity and the clay proportion in a given volume of that material [15] . While clay and water fill in any vacuum in a rock, one assumes that the resistivity is function of parameters such as fracturing, fractures and fissures clay filling in, porosity, the clayey clogging of alluvium [11] [15] .

4. Material and Data Acquisition

For a suitable coverage of the study area, data have been collected through 236 VES (vertical electrical sounding) along six (06) profiles of 4 km long in a 100 m × 200 m grid, defined according to the triangulation principle (Figure 2). This grid has been designed according to the topographic data of the area whose coordinates have been expressed in UTM33 WGS84. The acquisition data is made by combining electrical sounding and profiling following the Schlumberger configuration along a profile (Figure 3). The AB maximum length was held at 600 m to detect desired lithological formations at an approximate depth between 114 and 130 meters [16] . To avoid miscellaneous due to formations’ anisotropies, sounding and profiling surveys were E-W oriented (Figure 2).

We used the direct current resistivimeter, Syscal Junior 48 (IRIS Instrument) system. This unit runs under the Rho mode which enables to measure the resistivity (Rho) of ground structures. Acquisition of electrical data (sounding and profiling) is made using the Schlumberger direct method where the current is injected through A and B electrodes (Figure 3) and the potential difference is measured through receiving electrodes M and N [17] .

The apparent resistivity data from Schlumberger electrical profiles were processed and modelled using the Geotomo Res2Dinv software [14] [18] to obtain pseudosections or inverse pseudosections that reflect the true resistivity values of local subsurface structures. The electrical soundings were interpreted using WinSev from Geosoft [19] which permits to obtain the depth distribution of layers at each station, hence enabling to plot geological sections. Resistivity maps were plotted using Surfer software [20] which determines the spatial distribution of resistivity by interpolating their values in the area under study.

5. Results

5.1. Pseudo-Sections

Electrical sections of profiles L1 to L6 (Figures 4(a)-(c) and Figures 5(a)-(c)) were plotted with Res2Dinv [21] . These figures reveal an approximate investigation depth of 128 m for each profile. This depth corresponds to the maximum cable length (AB = 600 m) according to x_l position along a profile [16] [22] .

For the L1 profile (Figure 4(a)), the resistivity is slightly low (Rho < 1500 Ω∙m) from near subsurface to about 40 m depth in the eastern part and less than 20 m in the western part. Downward along E-W, one notices a bedding of layers and an increase of resistivity values above 10,000 Ω∙m. These resistive and bedded layers suggest unweathered granitic structures [23] . Along the profile (1100 < x_l < 1900 m), the resistivity is fairly low (Rho < 2500 Ω∙m) and iso-resistivity lines are nearly subvertical. Low resistivity values and the shape of iso-re-

sistivity curves are significant to a weakness zone or conductive discontinuity. It may exhibit a fracture, a fault or a shear zone along the profile [16] [22] [23] .

N-oriented, profiles L2 and L3 (Figure 2 and Figures 4(b)-(c)) clearly highlight a strong resistivity contrast. The western part of the area under study (0 < x_l < 2100 m) is generally low resistive (Rho < 2500 Ω∙m). The x_l width increases progressively from profile L2 to profile L3. The electrical responses of these profiles (L2 and L3) look like resistive bedded layers (Rho > 3000 Ω∙m) in the eastern side (2500 < x_l < 3800 m and x_l > 3000 m). Effects of these layers go downward to deep structures and they are evidences of fresh granitic structures which sometimes outcrop [22] [23] . The bedding and the resistance of layers disappear at the west; and a weakness, an accumulation or infiltrations’ zone sets itself progressively. Thus, in some parts of the area, there are resistive shallow deposits (Rho > 3000 Ω∙m) which may be laterite layer [24] .

Along profiles L4 and L5 (Figures 5(a)-(b)) deep structures are low resistive. They are bounded top and down by more resistive structures. This structural geoelectrical morphology characterizes an accumulation zone of weathered structures or a fluids’ infiltration zone [22] [23] . Meanwhile around 630 m and 450 m along profiles L4 and L5 respectively, an involvement of deep resistive materials on surface characterized by conic domes (Figures 5(a)-(b)) is observed.

On profile L6 (Figure 5(c)), granitic structures characterized by high resistivity values (Rho > 3000 Ω∙m) cover the centre (1250 < x_l < 2250 m). Weakness zones or discontinuities zones characterized by low resistivity values (Rho < 2500 Ω∙m) cover the two ends of the profile (0 < x_l < 1250 m) and (1250 < x_l < 3500 m).

5.2. Resistivity Map

The Figure 6 represents the space distributions of the resistivity computed by the surfer software for three depth

levels corresponding to the three arrays used: AB = 50 m and MN = 5 m, AB = 300 m and MN = 30 m, AB = 500 m and MN = 50 m. These arrays represent depths 9.5 m, 57 m and 95 m respectively [16] .

In the centre of the study area and along the E-W direction, resistivity maps highlight (Figure 6) a conductive discontinuity characterized by low resistivity values (Rho < 1500 Ω∙m). The extension of this area decreases to deep structures (Figure 6). It is located at boundaries of resistive zones characterized by high resistivity values (Rho > 2500 Ω∙m) and narrowing of iso-resistivity curves. This curves’ narrowing prove an inner to outer variation of geological structures along the N-S trend. Central structures materialize decayed rocks, accumulation or infiltration structures [11] [22] . They show up the presence of weakness zones in the central part of the study area [11] [22] . Otherwise resistive structures bounding these zones characterize fresh granitic formations [25] .

5.3. Geological Section

The interpretations of electrical sounding curves from the study area and geological surveys have permitted to realize the geological sections [26] of the subsurface crossed by profiles L2 and L4 (Figure 7 and Figure 8). These geological sections quantitatively illustrate the geology of the area. East of profile L2 geological section, the geological profiling shows a four layered model structure (Figure 7). The superficial layer or topsoil lies on a lateritic cover with a variable thickness which remains less than 10 m (Figure 7). The third layer corresponds to a weathered or conductive layer. It lies on the basement. To the western part of profile L2 geological section, the geological profiling shows three layers (Figure 7). The superficial layer or topsoil lies on the slightly thick weathered granite. The third layer represents the rocky or granitic basement. The geological section of profile L4 (Figure 8) is similar to the eastern part of profile L2 geological section. The weathered layers or conductive layers evidenced by geological sections permits to highlight tectonic unevenness of the previous results [8] [9] [22] [25] .

6. Discussion

The analyses of resistivity variations through pseudo-sections provide quantitative and qualitative information upon the conductivity of the studied area’s subsurface. During these analyses, we have delineated two different geological zones [27] [28] :

- The first zone is characterized by high resistive bedded structures. It fits the geological background of fresh granitic structures which sometimes outcrop [11] .

- The second zone is characterized by low resistive structures. It fits the weakness zones or conductive zones [23] . These conductive zones or conductive discontinuities characterize clay minerals intrusions inside barren structures of the granitic basement which underwent shearing and weathering process of shallow structures [11] .

The tectonic setting of the study area enables to match that tectonic unevenness with shear zones. There, fluids infiltration’s ability is considerable, thus increasing the weathering ability of in situ metamorphic or magmatic structures [25] .

In addition, analyses of the resistivity variations through resistivity map show that the space distributions of resistivity, for each subsurface level, are not uniform. The resistivity contrasts highlight weakness zones or fluids’ infiltration zones within granitic structures [29] [30] . These weakness zones considered as tectonic unevenness also characterize shear zones with a high accumulation capability of in situ weathered or dissolved structures [22] . They are located at the middle of the study area and have an E-W strike. The mineralization characterized by conductive zones is located in shear zones [25] [28] . It reflects the presence of disseminated clay minerals [22] [27] [29] .

In the mining and hydrogeological researches, weathered layers or conductive layers (Figure 7 and Figure 8) characterize the target zones [31] . They are identified as mineralogical deposits and groundwater zones for the mining and hydrogeological researches respectively [22] [24] [26] . We suggest that conductive layers may represent target zones of clay minerals concentrations [28] - [30] . The gold washing activity exerted along stream banks in the area [32] [33] suggest these are gold bearing structures located in weathered quartz veins [34] .

7. Conclusion

The electrical Schlumberger data acquisition (sounding and profiling) in the Tindikala-Boutou area along the Kadey River, using Direct Current (DC) method, has permitted to identify the tectonic unevenness and to characterize its mineralogical nature. Results which enabled this identification come from pseudo-sections of resistivity and the resistivity maps particularly. Interpretations made from different models are able to highlight weakness zones or conductive zones which characterize shear zones following an E-W strike. In these zones, the geological sections bring out weathered layers or conductive layers which constitute the target zones for mining and hydrogeological researches. According to the geological and tectonic background of the study area, clay minerals have been identified as the associated mineralization along shear zones within the Precambrian granitic basement. The presence of gold washers attests the existence of gold bearing clay mineralized structures disseminated in weathered quartz veins along the Kadey River.

Acknowledgements

The authors are grateful to the reviewers for their kind remarks making the manuscript clearer and more pertinent. The authors are also grateful to the Artisanal and Small Scale Unit of the Ministry of Mines, Industry & Technological Development for providing the Syscal Current Iris instrument to collect the data sets on site.

Cite this paper

Daniel HervéGouet,ArsèneMeying,Stéphane PatrickAssembe,ThéophileNdougsa-Mbarga, (2015) Clay Minerals Channels Identification in the Tindikala-Boutou Area (Eastern-Cameroon) along the Kadey River Using Direct Current (DC) Method. Journal of Geoscience and Environment Protection,03,123-133. doi: 10.4236/gep.2015.36018

References

NOTES

^*Corresponding author.