Abstract

Metasedimentary rocks of the Banfora belt are examined for their petrography geochemistry and genesis. The main formations are pelite metapelites sandstone, and metagraywackes. The first ones show a fine interlocking layering with unclear graded bedding in the sandstone compared to the metapelites, while the metagraywackes show a clearly discordant graded bedding of light grey to light pink color. The low SiO₂/Al₂O₃ ratios (3.67 to 6.60) suggest their low sedimentary maturity. The metapelites and sandstone-pelites show moderate Na₂O/K₂O values (10.13 - 22.69), indicating moderate chemical maturity; in contrast, the metagraywackes show low Na₂O/K₂O values (1.94 - 5.80) suggesting low chemical maturity. The metapelites, sandstone-pelites and metagraywackes have Rb/Sr ratios of 0.866 - 0.004 and 0.173 - 0.607 respectively, indicating a moderate to low degree of chemical alteration. The chemical alteration index (CIA = 58 - 83) and the plagioclase alteration index (PIA = 53 - 81) suggest weak to moderate alteration of the source rocks. This alteration is different from simple alteration as the sole control of chemical composition, but is associated with metasomatism. The metapelites sandstone shows a mixture of mafic and intermediate igneous sources, which indicates that the protoliths could be basalts and andesites, but the metagraywackes would be derived from the erosion of a mixture of andesitic to granitic rocks. The studied rocks were generated from young undifferentiated to differentiated arcs. They are of low to moderate sedimentary and chemical maturity.

Keywords

Metasediments, Banfora Belt, Source, Alteration, Geotectonic Environment

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1. Introduction

The geochemical composition of sedimentary rocks records their sources and tectonic history [1]-[7]. During meteoric weathering of rocks, alkali and alkaline-earth elements with high solubility in meteoric waters have greater fractionation potential; thus, their concentration in fine-grained sedimentary rocks may not be representative of their origin. On the other hand, rare earth elements, yttrium, scandium and thorium, with low solubility in water, can provide information about their sources [3] [5] [6] [8]. In addition, sedimentary deposition can occur in a wide range of environments within evolving accretionary and collisional orogenic systems, from marginal basins in volcanic arcs to forearc and back-arc basins in collisional orogens [9] [10]. The types of supracrustal successions that develop within these systems can exhibit significant variability in terms of their geometry, stratigraphy, sediment provenance and structural event [10]. The Birimian Belts are characterized by a lithostratigraphy comprising tholeiitic to lower calc-alkaline basaltic suite with some intercalations of sedimentary rocks, overlain by a thick detrital series with carbonate intercalations in association with calc-alkaline plutonic and volcanic rocks [11]-[19]. However, sedimentation in the West African Craton (WAC) was diachronous during the Eburnean orogeny [10] [20]. This is also reflected in the lithostratigraphy, with shallow water deposits to the east of the WAC, while carbonates are only present in the younger successions to the west [10]. The deposits of the volcano sedimentary sequence occurred over more or less extended periods of time and can be grouped into deep water and shallow water series [10]. The shallow water series were deposited mainly on older volcanic series [10] [21] [22]. The deep-water sequences (2160 - 2065 Ma) were defined within a longer depositional interval than the shallow-water series (2160 - 2115 Ma), although both were deposited simultaneously during the later stages of regional sedimentation [10]. The present study deals with preliminary petrographic and geochemical data from the metasediments of the Banfora belt, which extends into Côte d’Ivoire as the Katiola Marabadiassa belt and the Bandama volcano-sedimentary basin [11] [15] [23] [24]. The Banfora belt appears to be poorly documented compared to the other western belts in Burkina (Houndé and Boromo) in terms of the nature of their sedimentary deposits [13] [25] [26]. Recent syntheses suggest that the sedimentary deposits of Banfora, Boromo and the western part of Houndé occurred in deep water over a long period that is supported by contemporary volcanic activity [10]. Unlike the Banfora and Boromo belts, the Houndé belt contains Tarkwaïan-type sediment units [13] [26] [27]. In the southern part of the Banfora belt, Pb-Pb zircon ages have been found in detrital sediments from the Bandama basin in Côte d’Ivoire, including detrital zircons dated between 2133 and 2107 ± 7 Ma, so the age of sediment deposition is estimated between 2108 ± 12 and 2097 ± 3 Ma [24]. Although some authors have shown the depositional environment of sediments in the Boromo and Houndé belts, that of the Banfora belt remains unknown. In this study, we will discuss the petrographic and geochemical characteristics of the sedimentary rocks in this part of the Birimian system.

2. Geological Setting

Various studies [22] [27]-[31] present the geological configuration of the Léo/Man shield in two domains (Figure 1), respectively the western Kénéma-Man domain > 2500 Ma and the eastern Baoulé-Mossi domain dated at 2000 Ma [28] [32]. The Baoulé-Mossi domain consists mainly of volcanic and sedimentary belts and granitoid intrusions [10] [12] [13] [22] [27] [33]-[37]. Burkina Faso, which extends over the Baoulé-Mossi domain, is composed of a Paleoproterozoic basement of greenstone and granitoid belts (80%) covered on its western, northern and eastern margins by Neoproterozoic to Cambro-Ordovician sedimentary formations (1000 - 435 Ma) (20%). The belts are essentially made up of metavolcanic rocks, ranging from meta basalts to meta rhyolites and metasedimentary rocks including quartzites and metapelites [13] [14] [38] [39]. More specifically, the Banfora belt, which is 160 km long and 35 km wide [14] in Burkina Faso, extends as far as Côte d’Ivoire and is intersected by the Greenville-Ferkéssédougou-Bobo-Dioulasso major shear zone (Figure 2). The eastern part of the Banfora belt is made up of interbedded units of basalts, andesites, volcano sediments and

Figure 1. Geological map of the Leo/Man shield and the most important gold deposits across the WAC (modified from Grenholm et al. 2019).

Figure 2. Simplified geological map of Banfora green belt (modified from Metelka et al., 2011) and location of the studied area.

rhyolites, 2 to 4 km thick, while the western part is composed solely of volcano sediments [13]. The overall stratigraphy of the belt, as proposed by [13] and revised by [14], suggests, from bottom to top: 1) deposition of ultramafic rocks, 2) deposition of mafic volcanics, 3) subsequent deposition of lava flows and projections (pyroclastites and lapilli tuff), 4) deposition of siliceous rocks (exhalite), 5) deposition of pelitic and turbiditic sediments, 6) deposition of the pre-collisional eastern granite and 7) deposition of the post-collisional western granite. The volcanic activity that prevailed in western Burkina Faso took place between 2190 Ma and 2160 Ma (Eburnean), according to [13] and has a juvenile character with respect to εHf (0 to + 8) [40]. Exhalites constitute the upper horizon of the litho-stratigraphy of volcanic formations in the eastern part of the belt, while subvolcanic rocks are mainly porphyritic micro diorite, sills and dolerite dykes [14].

Syn-tectonic intrusions (granite and granodiorite) emplaced by the Greenville-Ferkéssédougou-Bobo-Dioulasso regional shear zone known from Liberia and Côte d’Ivoire [13] [14] [41]. Based on the geochemical and geotectonic framework, [14] pointed out that the granitoids of the western and eastern margins of the Banfora belt are post- and pre-collisional, respectively. Geochemically, the granitoid intrusions are arc volcanic in nature, including negative Ta, Nb and Sr anomalies and mostly older than 2100 Ma [40]. In the neighboring area, the pegmatite associated with granitoids and migmatitic gneiss potentially contains rare metals [37] [42]. More specifically, the geology of the study area consists of a series of mainly metamorphosed volcanic formations (rhyolite, rhyolitic tuff, dacite, andesite, basalt, etc.). Rare outcrops of highly siliceous sulphidic felsic formations (rhyolite, cherts and quartzites) that have resisted alteration appear to be the main host rocks [39]. Late monzonite, granodiorite and diorite crosscut the volcanic assemblage.

3. Methodology

3.1. Sampling

The fieldwork consisted of outcrop description and diamond drill core logging, followed by sampling. Core samples were taken from two drill cores (K01 and K02), to ensure good spatial coverage of the study areas. In all, around ten rock samples from these boreholes were collected. Sampling was carried out on different facies. A dozen thin sections were taken at the Geosciences and Environment Laboratory (LaGE) at Joseph KIZERBO University. The detailed investigation was carried out under a Nikon Eclipse 50i POL polarizing microscope coupled to a camera. We use acronyms for rock-forming minerals in [43].

3.2. Analytical Method

Part of this study is based on whole-rock geochemical data from drill cores. Samples were analyzed by XRF diffraction for major elements, rare earth, and trace elements using inductively coupled plasma mass spectrometry (ICPMS) at the ACME laboratory in Vancouver. Twenty-one of the analyzed samples comprise andesite, dacite and rhyolite (Table 1). The samples were crushed and pulverized at 200 mesh, and sample fragments (0.2 g) were placed in graphite crucibles with the addition of lithium borate flux. The crucibles were placed in an oven and heated to 1025˚C for 25 min. Each molten sample was then dissolved in 5% HNO₃. A blank and an internal standard (STD SO15 reference material) were weighed, digested and analyzed for accuracy. Calibration standards, verification standards and reagent blanks were also added to the sample sequence for testing.

4. Results

4.1. Lithology

4.1.1. Meta Pelitic Sandstone-Facies

The metapelites sandstone-pelitic facies are made up of metapelites, metasandstone and meta arkoses. The metapelites form an ill-defined contact with black cherts, but are easily identified by their alternation of dark and light beds (Figure 3(a)). They form the top of the lithostratigraphic sequence. These rocks are alternating black and whitish, giving a ribbon-like structure. They are locally characterized by millimeter to centimeter-high beds alternating with fine beds of black

Table 1. Litho geochemical data of the metasedimentary rocks of the Banfora belt.

cherts. Their primary bedding is preserved and marked by alternating dark beds of glauconite-graphite and light beds of carbonate-biotite (Figure 3(b)). The carbonate-biotite beds are in close contact with the glauconite beds and vary in thickness from 5 mm to 1 cm. Quartz can represent up to 90% to 95% of the quartz-biotite beds, which are also in close contact with the glauconite beds. In places, this unit consists of folded graphitic beds one centimeter thick, which appear as a turbidite sequence.

Figure 3. Samples from drill core and microphotograph of the main lithologies. (a): Banded metapelite; (b): Metapelite with alternating quartz—biotite, glauconite and carbonate—biotite; (c): Sandstone; (d): Heterogranular quartz in sandstone; (e): Alternating beds of quartz-glauconite and sericite in sandstone; (f): Tourmaline porphyroblast in sandstone. Qtz: Quartz; Bt: Biotite; Glt: Glauconite; Cb: Carbonate; Tm: Tourmaline; St: Staurolite.

The metasandstone has a fine, crisscrossed stratification (Figure 3(c)) with unclear graded bedding (Figure 3(d)). They consist of ±foliated, mylonitic texture formed by staurolite porphyroblasts (Figure 3(e)). The metasandstone in contact with the monzogranite shows tourmaline porphyroblasts (Figure 3(f)). Their mineralogy essentially consists of staurolite porphyroblasts, quartz beds (50%), white micas (25%), and glauconites (5%) parallel to subparallel in a quartz matrix, with tourmaline micro-clusters. The staurolite porphyroblasts are moulded into the white mica beds and dwell unaltered. Tourmaline porphyroblasts are in micro-clusters in a white quartz-mica matrix background and often show fine recrystallisation of phyllite of white micas and quartz inclusions.

4.1.2. Metagraywackes

The metagrauwackes are bedded and show clearly discordant light grey to light pink graded bedding (Figure 4(a)). The bedding is fine and coarse, composed of intergrown quartz in which some phyllite minerals of white micas and biotite, and cherts and felsic clasts are encrusted (Figure 4(b)). The cherty clasts are composed of quartz, while the felsic clasts, about 1 to 2 cm long, are sub-angular and contain small crystals of white micas and quartz. The metagraywackes are composed of breccias supported by angular clasts in a carbonatitic matrix. They appear to be variable, both in terms of grain shape and mineralogical composition. Mature minerals are composed of quartz grains (75%) with blunt contours and, secondarily, plagioclase (5%), while immature one is made up of angular to sub-angular quartz-feldspar grains (70%). The mineralogy is presented by fragments of quartz (0.5 mm) and albite (0.5 mm) of around 1% to 2% in quartz cement (50% to 60%) white mica (30% to 40%) in the mature deposits (Figure 4(c)), while the immature ones are made up of 30% to 35% quartz, 1% amphibole, 5% orthoclase, 10% biotite and 40% to 50% white micas (Figure 4(d)).

Figure 4. Samples from drill core and microphotograph of the main lithologies. (a): Grauwackes showing granoclassing; (b): metagraywackes with felsic clasts and chert clasts; (c): Feldspar-rich metagraywackes; (d): metagraywackes’ rich in white mica.

4.2. Geochemical Data

4.2.1. Composition of Major Elements

In the metapelite sandstone, SiO₂ ranges between 41.04 and 62.28 wt%, and Al₂O₃ values range from 7.42 to 16.94 wt%. Majors such as MgO and CaO show high contents of 8.56 - 12.99 wt% and 0.92 - 29.94 wt% respectively, while K₂O and Na₂O show low values of 0.008 - 1.466 and 0.001 - 0.064 wt% respectively. TiO₂ and Fe₂O₃ concentrations vary between 0.37 - 0.75 wt% and 0.6 - 1.55 wt% respectively.

The metagraywackes are more siliceous with (SiO₂ = 70.58 - 79.22 wt%. These samples show MgO = 0.28 - 0.91 wt% and CaO = 0.33 - 2.62 wt%) and moderate K₂O = 1.74 - 3.26 wt% and Na₂O = 0.562 - 0.896 wt%. TiO₂ and Fe₂O₃ contents ranged from 2.71 - 5.40 wt% and 0.13 - 0.55 wt% respectively.

The sedimentary rocks were classified using the log (Fe₂O₃/K₂O) versus log (SiO₂/Al₂O₃) binary diagram [44]. In this diagram, the metapelite sandstone belongs to the iron-rich sandstone group, while the metagraywackes belong to the wakes group, evolving towards the litharenites group (Figure 5).

Figure 5. Classification of the studied metasedimentary rocks using log (Fe₂O₃/K₂O) versus log (SiO₂/Al₂O₃) (after Herron, 1988).

4.2.2. Trace Element Composition

The composition of high field strength trace elements (Nb, Ta, Zr, Hf, Sc, Y); large Ion lithophile elements (Rb, Sr, Ba, Cs, Cu, Pb, U, Th) and transition metals (Mn, Sc, V, Cr, Co, Ni, Cu, Zn) vary greatly in metasedimentary rocks (Table 1). The levels of Nb (1.32 - 5.36 ppm), Ta (0.18 - 0.44 ppm), Zr (71.9 - 128.19 ppm), Sc (4.19- 3.36 ppm), Rb (52.79 - 89.33 ppm), Sr (127.26 - 305.07 ppm) confirm this variability in the metagraywackes. Similarly, the metapelites sandstone show variable contents of Nb (6.35 - 10.32 ppm), Ta (0.46 - 0.75 ppm), Zr (113.20 - 128.19 ppm), Sc (19.44 - 30.19 ppm), Rb (22.05 - 46.88 ppm), Sr (54.10 - 74.92 ppm).

5. Discussion

5.1. Lithological Characteristics

The sedimentary units of the Banfora belt are essentially composed of metasandstone—pelitic and metagraywackes rocks. Metapelites sandstone and metagraywackes are the main component of the Birimian basins [7] [10] [45]-[49]. The metapelites rocks studied are mainly composed of glauconite and graphite, assuming that they were formed in a marine environment. In the metagraywackes facies, the sedimentological characteristics are more or less preserved through the fine- and coarse-grained bedding. The felsic volcanoclastite clasts observed in the metagraywacke indicate the initial emplacement of felsic volcanoclastite prior to the filling of the basin. They were deposited over an extended period of time that is supported by contemporary volcanic activity [10] [13] [17] [22] [24] [50]-[52]. The geochemistry of sedimentary rocks provides information on several factors: 1) sedimentary maturity indicated by the SiO₂/Al₂O₃ ratio which is a function of the proportion of quartz and clay minerals/feldspars and chemical maturity marked by the Na₂O/K₂O ratio which highlights the presence of feldspars [44]; and 2) the mineral stability index marked by the Fe₂O₃/K₂O ratio relating to the proportion of ferromagnesian minerals to plagioclases [53]. The low SiO₂/Al₂O₃ ratios (3.67 to 6.60) in the rocks suggest their low sedimentary maturity. The metapelites sandstone record moderate Na₂O/K₂O values (10.13 - 22.69), indicating moderate chemical maturity; on the other hand, the metagraywackes show low Na₂O/K₂O contents (1.94 - 5.80), suggesting low chemical maturity. In fact, the geochemical and isotopic data for Birimian sedimentary rocks suggest an immature nature with very little recycled material [7]. Moreover, compared with the upper continental crust (Figure 6), these Birimian rocks are depleted in alkaline and alkaline-earth elements and enriched in transition metals [7] [47].

5.2. Alteration and Diagenesis

Although petrographic data on the alteration of sedimentary rocks can be useful, no quantification studies have yet been carried out. Furthermore, diagenesis also commonly involves the alteration of feldspars and other unstable minerals to clay, so it can be difficult to determine these processes through the use of geochemical data [4] [54]. However, the Rb/Sr ratio and the chemical index of alteration (CIA) have been established as a general guide to the degree of alteration [4]. The metasandstone-pelites and metagraywackes have Rb/Sr ratios between 0.866 - 0.004 and 0.173 - 0.607 respectively, indicating a moderate to low degree of chemical alteration. In addition, the studied samples have ACNK values ranging from 58 - 83 and plagioclase alteration index (PIA) values from 53 - 81 respectively, suggesting weak to moderate alteration of the source of the rocks. In the ACNK diagram, the samples show more or less linear trends, but this is not comparable with simple alteration as the only control on chemical composition (Figure 7). This pattern indicates the contribution of K to the samples as a result of metasomatism.

5.3. Geotectonic Setting and Sources of Sedimentary Rocks

Several authors have used the chemical composition of sedimentary rocks to distinguish their geotectonic context [4] [55] and defined four types of sedimentary terrain: 1) ancient upper continental crust (UGC), 2) recycled sedimentary rocks (RSR), 3) undifferentiated young arc (YUA) and 4) differentiated young arc (YDA). The Banfora sedimentary rocks compared to the upper continental crust show relatively low to moderate but variable values of SiO₂/Al₂O₃, K₂O/Na₂O and lower ratios of IAC and incompatible to compatible elements (Figure 8) such as Th/Sc (0.16 - 1.53) and Zr/Sc (5.24 - 17.11). These geochemical characteristics suggest that the metasandstone and metagraywackes originate from young undifferentiated to differentiated arcs.

The SiO₂ vs K₂O/Na₂O diagram of [55] shows that the meta sandstone-pelite samples formed in active continental margin and passive continental margin environments, whereas the metagraywackes formed in an active continental margin environment (Figure 9(a)). Furthermore, the Th vs Th/U diagram of [4] indicates

Figure 6. Upper continental crust normalized major element plots for the studied metasedimentary rocks. Upper continental crust values are from Rudnick and Gao (2014).

Figure 7. Al₂O₃-(CaO +Na₂O)-K₂O diagram for the metasedimentary rocks of the Banfora belt (after Nesbitt and Young, 1984).

Figure 8. Plot of Th/Sc versus Zr/Sc for the metasedimentary rocks of the Banfora belt. The compositional variation trend lines are from McLennan et al. (1993).

Figure 9. (a) Na₂O/K₂O versus SiO₂ diagram from Roser and Korsch (1986) applied to metasedimentary rocks of the Banfora belt. (b) Plot of Th/U versus Th for the metasedimentary rocks of the Banfora belt (after McLennan et al., 1993).

that the meta-pelite sandstone and metagraywackes samples formed in an active margin context (Figure 9(b)). In the diagram of [55], the metapelite sandstone have a mafic to intermediate igneous origin, while the metagraywackes come from a felsic source with a mixture of a quartzite sedimentary rock source (Figure 10). Observation of the representative points in the ACNK diagram shows that the metapelite sandstone and metagraywackes derived from the erosion of andesite basalt and andesite-granite, respectively. Similar results have been found in the Ivory Coast in the Comoé basin [48], where the protoliths evolve between gabbros and granites, and in the SASCA domain, where the source evolves from basalts to andesites [56].

6. Conclusion

Preliminary petrographic and geochemical data show that the Banfora metasediments

Figure 10. Discriminant function diagram Roser and Korsch (1988) applied metasedimentary rocks of the Banfora belt. Discriminant function 1: (−1.773 * TiO₂) + (0.607 * Al₂O₃) + (0.760 * Fe₂O₃) + (−1.500 * MgO) + (0.616 * CaO) + (0.509 * Na₂O) + (−1.224 * K₂O) + (−9.090). Discriminant function 2: (0.445 * TiO₂) + (0.070 * Al₂O₃) + (−0.250 * Fe₂O₃) + (−1.142 * MgO) + (0.438 * CaO) + (1.475 * Na₂O) + (−1.426 * K₂O) + (−6.861).

are essentially meta sandstone-pelites and metagraywackes. The meta sandstone-pelites contain a mixture of mafic and intermediate igneous sources, indicating that the protoliths could be basalts and andesites, but the metagraywackes are thought to have been derived from the erosion of a mixture of andesitic and granitic rocks. The rocks studied come from young undifferentiated to differentiated arcs. They have low sedimentary maturity and low to moderate chemical maturity. The Chemical Alteration Index (CIA) and Plagioclase Alteration Index (PIA) suggest weak to moderate alteration of the source rocks, but this is not comparable to simple alteration as the sole control on chemical composition. The samples underwent K input as a result of metasomatism.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

References