OJG  Vol.7 No.9 , September 2017
Structural and Lithological Study of Gold Mineralization in the Areas of Bena and Bakolobi, Mali, West Africa
ABSTRACT
The areas of Bena-Bakolobi are located in the eastern part of the Kedougou-Kenieba in layer. This sector contains several deposits of gold, including the mines of Sadiola, complex Gara-Yalea and Gounkoto, gold mines of Tabakoto and the gold deposit of Fekola, all located around the (SMS) Senegalo Malian shear zone. The work carried out is summarized in two parts: The first part is based on the detailed mapping of the Bena and Bakolobi target. It was necessary to make a detailed mapping in order to understand and assess the target. The second part is to determine the term and the litho-structural control of gold mineralization. This study is based on lithology, alteration, structures and mineralization of the two areas. The ultimate purpose is to draw a regolith map for a future research program. Two types of alteration affect the rock formations of the two zones: weathering (supergene) that consists in hematization, limonitisation and the kaolinitization. The hydrothermal alteration consists in albitization, chloritisation, the sericitization, the tourmalinisation, the sulphidation, silicification and carbonation. This paper improves structural and lithological study of gold mineralization in the areas of Bena and Bakolobi. The analysis of the results of the work carried out in the two zones allowed us to have an overview on the geology of these two zones, which are located in the Kédougou-Kéniéba window in the volcano-sedimentary rocks of Proterozoic Formation of Kofi. The lithological and structural studies carried out in the area have demonstrated the relationship between structures and mineralization and/or lithology.

1. Introduction

The area of Bena-Bakolobi is located in the eastern part of the Kedougou Kéniéba buttonhole. This area contains several gold deposits, all located in the vicinity of the Senegal-Malian shear zone (SMS) on the Kofi formation. These gold deposits, combined with gold panning and artisanal mining, underscore the highly promising character of the area. In Mali, a less developed country, gold export has substantially increased in the 1990s [1] . Mali suffers from artisanal and small-scale mining formalization problem as other mineral rich countries in the region [2] . It argues that the child labor, which is now widespread in many of the region’s small-scale mining communities, is a product of a combination of cultural issues, household-level poverty, and rural livelihood diversification [3] .

The study area is administratively attached to the Kéniéba circle, Kayes region in western Mali, near the Falémé river (natural border between Mali and Senegal). Located in 350 km west of Bamako, 250 km south of Kayes, Bakolobi is 12 km north of the Fekola and Bena, 15 km south of the Gounkoto gold mine. The ultimate purpose is to draw a regolith map for a future research program. The method which will be used is quick and cost-effective. According to the available literature [4] , [5] , [6] and [7] , there is limited research on gold resource in Mali.

2. Geological Setting

2.1. Regional Geology

The West African Craton: In 1663 - 1665, Rocci described the African continent as consisting of several blocks welded together by folded belts separated by moving zones. The West African craton, identified by Kennedy in 1964 is that part of West Africa

Consisting of Archean and lower Proterozoic ages, stabilized around 1600 - 1500 million years ago. It is partially covered with more recent Proterozoic formations higher than at present in its central part: the Taoudeni Basin. It is bordered to the west by the poly-orogenic belts of the Mauritanides and Rockelides (Panafricain and Hercynian); to the north by the Hercynian domain of the Anti-Atlas; In the East by the Pan-African, Pharisee and Dahomean belts.

The structuring of the West African Craton shows three units of outcrops: to the north the Reguibat ridge, to the south the ridge of Leo, have a great extension, the middle part windows of Kayes and Kéniéba is small but constitutes a landmark Important between The other outcrop areas (Figure 1).

2.2. Local Geology

From a geological point of view, the study area is located in the Birrimian formations of the Kédougou-Kéniéba window. The northwestern part of the Bakolobi zone is located in the volcano-sedimentary rocks of the Birrimian, while the southeast zone is interpreted as granitoid. Towards the east, the upper Proterozoic sandstones have been deposited unconformably on the Birrimian formations and the rate of outcrop of the zone is about 5%. The relief of this region is

Figure 1. Geological map and study area located in Kenieba (west African craton, source BRGM).

constituted by a peneplain dominated to the east by the plateau of the Tambaoura which is oriented NNW-SSE with 200 to 250 m of altitude. The peneplain corresponds to the region of the Bambouk or buttonhole of the base of Kédougou-Kéniéba, whose average altitude is of the order of a hundred meters.

3. Data and Research Methodology

Those maps of regolith have been established by a methodology, which consists of following lines along which are mapped on millimeter paper, the different facies, outcrops, armored plateaus, roads, drainage channels etc. The lines are spaced 100 m apart and total 24 lines. After field mapping, the carry-over is immediately carried out in parallel with the regolith mapping on a base map (scale 1:2000). To close the different polygons, a correlation is made between the boundaries of similar facies. Once the map is established, it will be crowned by a computer operation, which consists of scanning or to photograph this map. With the Geographic Information System (GIS), these maps have been digitized and added geological information. During the geological work, samples were collected for analysis at the laboratory. It was necessary to make a detailed mapping in order to understand, to assess the target, to determine the term and the litho-structural control of gold mineralization.

4. Results and Discussion

The result obtained is the present regolith maps (Figure 2). The dominant regime is the residual cuirass regime and some greywackes outcropping salaries and breach. In the mapped area (Figure 3), there are three (3) regimes, namely:

1) The cuirass regime represents about 85%;

2) The erosion regime represents about 5%;

3) The depositional regime represents about 10%.

Figure 2. Regolith of the northern part of Bakolobi.

Figure 3. Sinsinko target regolith (Bena area).

Two types of alteration affect the rock formations of the two zones: weathering (supergene) that consists in hematization, limonitisation and the kaolinitization. The hydrothermal alteration consists in albitization, chloritisation, the sericitization, the tourmalinisation, the sulphidation, silicification and carbonation.

Measured structures: Many structures were measured in the trench. These structures are represented on the following stereogram in (Figure 4 and Figure 5).

Figure 4. Wulf pattern of the structures measured in the trench No. BKTR009. According to the stereogram, these structures have a general direction N191/75 with a strong dip.

Figure 5. Wulf pattern of shear measured in trench No. BKTR009, the shear in trench No. BKTR009 has a general direction N191/72 with a strong dip.

4.1. Relationship between Mineralization and Structures

1) In the Dioula target, mineralization was observed in three sediments. This allows us to conclude that the mineralization in this target therefore has no lithological control. It appears mainly linked to the structures and therefore has structural control. Several mineralized zones have been intercepted; Controlled by steering structures with shear NNE-SSW steering and N-S steering structures.

2) In the Gamaye target, mineralization has structural control. It appears mainly linked to structures. Three mineralized zones were intercepted, controlled by NNW-SSE shear structures.

3) In the Koliguinda target, some lithologies are affected by mineralization. It appears mainly linked to the structures and therefore has structural control. Three mineralized zones were intercepted; Controlled by steering structures with NNW-SSE steering shears and NW-SE steering structures. However, gold deposits in the Syama and Tabakoroni goldfields in southern Mali occur along a north-northeast trending mineralised litho-structural corridor that trends for approximately 40 km [8] .

We notice that the primary gold mineralisation at Yatela is low-grade and sub-economic [9] . However, gold mineralization at Morila is hosted in flat-lying meta-graywacke associated with a Birimian-aged Massigui schist belt in Mali [10] .

Even though the three gold occurrences are aligned along the same major structure, each one has distinct geological features including the nature of the dominant host rock in the mineralized zone (for example a basaltic volcanic sequence at Syama, a diorite pluton at Tellem and a felsic volcanic sequence at Tabakoroni) [11] .

4.2. Relationship between Mineralization and Lithology

The interpretation of the various works carried out in the zone showed that the mineralization is dispersed in the locally brecciated quartzite.

Some lithologies are affected by mineralization. It appears mainly linked to the structures and therefore has structural control. Controlled by structures with intense shear N-S. However, in the other zones:

We notice that the Alamoutala gold deposit is part of the Yatela gold district, which is located in the Kédougou-Kénieba inlier (KKI), a window of deformed Birimian rocks (Paleoproterozoic, ca. 2200 - 2050 Ma) that outcrop in eastern Senegal and western Mali [12] .

While the Loulo-Gounkoto complex in the Kédougou-Kéniéba Inlier hosts three multi-million ounce orogenic gold deposits, situated along the Senegal-Mali Shear Zone [13] .

Therefore, the interaction of hydrothermal fluids with metamorphosed carbonaceous matter (CM) could be one of the causes of the reduction of hydrothermal fluids and formation of the respective mineralization [14] .

The further comprehensive interpretation concern a process separating freshly-panned gold concentrate from residual mineral inclusions.

5. Conclusions

This paper improves structural and lithological study of gold mineralization in the areas of Bena and Bakolobi. The analysis of the results of the work carried out in the two Bena-Bakolobi zones allowed us to have an overview on the geology of these two zones, which are located in the Kédougou-Kéniéba window in the volcano-sedimentary rocks of Proterozoic Formation of Kofi Formation. The lithological and structural studies carried out in the area have demonstrated the relationship between structures and mineralization and/or lithology. The main characteristics of the gold mineralization in the Bakolobi zone appear to indicate hydrothermal mineralization in connection with a shear zone.

The process of structural and lithological study in this work indicates two aspects.

Firstly, the interpretation of the various works carried out in the zone showed that the mineralization is dispersed in the locally brecciated quartzite.

Secondly, some lithologies are affected by mineralization. It appears to be mainly linked to the structures and therefore has structural control.

Acknowledgements

The Minister of Mining of Mali under (Bena-Bakolobi project 2015), that we would like to thank sincerely, has financially supported this study research. Special thanks to my supervisor in state key laboratory of geological processes and mineral resources, China University of Geosciences, Wuhan. Thanks also all colleagues from National School of Engineers Abderhamane Baba TOURE of MALI for advice.

Cite this paper
Tounkara, F. , Chen, J. , Konaté, A. and Wang, C. (2017) Structural and Lithological Study of Gold Mineralization in the Areas of Bena and Bakolobi, Mali, West Africa. Open Journal of Geology, 7, 1318-1326. doi: 10.4236/ojg.2017.79087.
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