• The geological formations of the study area consist of an alternation of amphibolites, mica schists, quartzites and gondites which have undergone the vertical tectonics effects between a high-pressure amphibolite facies and greenschist facies conditions. The genesis of these rocks can be attributed to a volcano-sedimentary complex with a marly, arenaceous and pelitic sequences associated with submarine exhalations.

• Gondites of Tera show only spessartite-type garnet as primary manganiferous mineral, which by supergene alteration leads to the formation of lithiophorite, cryptomelane and pyrolusite as secondary oxide and hydroxide minerals of manganese during the manganese enrichment.

The Téra manganese deposit (western Niger) is part of the manganese deposits of the West African Craton, located in the Paleoproterozoic (Birimian) greenstone belts formations (Figure 1a). The first geological investigation on Tera manganese deposit was conducted by Reformatsky (1932). Subsequently, Machens (1964) further investigated the geological assessment of manganese deposit which involved mapping the manganese ores, a macroscopic description, and some ore geochemical analyses. Recently, Garba Saley et al. (2017) considered the protore of Téra manganese deposit as a gondite (garnetite), primarily composed of spessartite (a manganese silicate) that formed through metamorphic recrystallization during the Eburnean Orogeny (Soumaila, 2000). However, certain questions still remain unanswered: Do all the garnets belong to the spessartite type? Are there other manganese minerals present within these gondites?

Figure 1. Location of the study area. a-Simplified geological map of the Man’s Shield (Milesi et al., 1989), b- Location of the study area in the Niger Liptako province (Machens, 1967).

In terms of this deposit origin, Garba Saley et al. (2017) have proposed a model that involves two processes: primary mineralization of exhalative-sedimentary origin and secondary mineralization through supergene alteration. Focusing on the characterization of the alteration, Garba Saley et al. (2018) reported that supergene weathering led to the development of an alteration profile on the gondites, including manganese oxides and hydroxides development. These authors concluded that these manganese oxides and hydroxides were formed as a result of weathering processes, as indicated by the PIA (plagioclase index of alteration) evolution, which show two paths: from plagioclase-smectite to kaolinite and from illite to kaolinite.

This study aims to investigate the Mn ore deposit and its associated rocks including mica schists, amphibolites, and quartzites, determine the mineralogical composition, and highlight the ore genetic processes.

2.1. Location of the Study Area

The Téra manganese deposit is located in the Liptako region (West Niger), the northeastern part of the Man Shield, which is a component of the West African Craton.

The geological formations of a study area are part of the Diagourou-Darbani greenstone belt. This belt is sandwiched between the Téra-Ayorou granitoid pluton in the West and the Dargol-Gotheye in the East (Figure 1b). The studied outcrops are located precisely in the contact zone with the Téra-Ayorou pluton (Figure 1b). Gondites are the main manganese rocks which appear in the form of a folde chain hills over more than 1.50 km (Figure 2a). These gondite hills occur a compact crusting of manganese at the top (Figure 2b). The outcrops of a study area consist of an alternating of mica schists, amphibolites, quartzites, gondites (garnetites) and rare pegmatites that crosscut the mica schists (Figure 2c).

Figure 2. The manganese deposit of Téra. a-Google Earth images (2022) of Téra manganese deposit. b-Main hill of gondite (with manganese-rich blocks), c- sampling position and geological map of the studied area.

2.2. Geology of Man Shield of West Africain Craton

The West African Craton is made up of two ridges: the Réguibat shield in the North and the Man shield (or Léo-Man ridge) in the South. In each of these ridges, we distinguish formations of Archean age in the West and of Paleoproterozoic (Birimian) age in the East, generally limited by major tectonic contacts, such as the Sassandra fault in the Man shield (Lompo, 1991; Kouamelan, 1996; Naba, 2007).

The Paleoproterozoic (Birimian) formations are affected by the Eburnean orogeny (2.4 to 1.8 Ga according to Plumb, 1991). They occur in alternating belts of greenstone belts and granitoid massifs in elongated bands N-S to NE-SW. The granitoid massifs are generally made up of tonalites, trondhjemites and granodiorites (TTG) and the greenstone belts are generally made up of phyllades of sedimentary origin, metavolcano-sediments, metagrauwackes, metabasites and meta-ultrabasites.

The methodology employed in this study involved field sampling followed by a microscopic examination. This examination involved the preparation and analysis of thin sections and polish sections in laboratory. Additionally, quantitative X-ray diffraction (XRD) analysis was conducted as part of the study.

3.1. Sampling and Field Relationships

All samples were taken from fresh parts of the outcrops (amphibolites, mica schists and quartzites) except the gondites which are the main Mn mineralization-bearing rocks. For the gondites, the weathered parts were also sampled by following the weathering profile from the bottom to the top of one of the gondite hills.

3.2. Microscopic Analysis

A total of thirty-two thin sections and fifteen polished sections were produced in the laboratory of the ‘‘Centre de Recherche Géologique et Minière’’ (CRGM) in Niamey. Petrographic analyses, in the laboratory, were conducted using an optical microscope (LEICA MICROSYSTEMS (SCHWEIZ) AG, Model/DM 750P/13613615) available in the Department of Geology at Abdou Moumouni University of Niamey, Niger. This optical microscope allows for observations under both transmitted and reflected light conditions.

3.3. X-ray Diffraction (XRD)

Seventeen pulverized samples (15 rocks and 2 ores) were sent to the Actlabs laboratory (Activation Laboratories) in Ontario, Canada, for quantitative X-ray diffraction (XRD) analysis. A portion of each pulverized sample was mixed with corundum and loaded into a standard holder, with corundum serving as an internal standard. The XRD analysis was conducted using a Bruker D8 Endeavour diffractometer, which utilized a Cu X-ray source and operated under the following conditions: 40 kV and 40 mA; range of 4 – 70 degrees 2θ; step size of 0.02 degrees 2θ; time per step set to 0.5 seconds; fixed divergence slit at an angle of 0.30; and sample rotation at 15 rpm. The mineral identification was performed using the PDF4/Minerals ICDD database. The quantities of the crystalline mineral phases were determined using the Rietveld method, which involves calculating the complete diffraction pattern based on crystal structure data. The amounts of crystalline minerals were recalculated, based on a known percentage of corundum, while the remaining portion up to 100% was considered X-ray amorphous material.

4.1. Rock Petrography

X-Ray Diffraction (XRD) analyses were conducted to provide detailed information about the mineralogical composition (Table 1). For each specific type of geological formation, the constituent mineral phases were identified and described.

Table 1 . Quantitative XRD mineralogical estimation for the samples from Téra area.

Mica schists:

The mica schists occur as hills or sporadic metric outcrops with a schistosity varying in NE-SW and NW-SE direction (respectively on the West and the East side of the study area) thus drawing a large kilometric fold with a submeridian axis. The mica schists are grayish in color and exhibit a granolepidoblastic texture. The XRD analyses (Table 1) presents the mineralogical composition as follow: quartz (29.3% to 52.4%), cordierites (2.9% to 16.4%), staurolites (1% to 13.1%), kyanites (≤6.8%), biotite (0,5% to 10.7%), chlorite (4,2% to 24.7%), the plagioclases are calcic of anorthite type (6. 4% to 36.9%), with rare muscovite, and garnet. The association of kyanite, staurolite and garnet indicates an amphibolite facies.

An example of diffraction pattern with the percentages of minerals identified from a mica schists sample (sample B3) is shown in Figure 3.

Figure 3. The percentages of minerals identified and their diffraction patterns of a mica schist (sample B3).

Under polarizing microscope (Figure 4a-f), the staurolites and plagioclases typically appear as poikiloblasts within the mica schists and exhibiting inclusions of quartz and opaque minerals that emphasize the internal schistosity (Is). Biotites and chlorites, on the other hand, mark the external schistosity (Es) (Figure 4a). Locally the inclusions in the poikiloblasts are either planar, helicitic or microfolded (Figure 4a-c). Sillimanite in fibrous form is found associated with cordierite (Figure 4d). Some replacement textures are observed (Figure 4d-f):

Figure 4. Photomicrographic analysis of: mica schists (a, b, c, d, e and f) for samples B9, B3 and G4 and, amphibolites (g, h and i) for samples C7 and C5. All images are observed under crossed nicols (polariser + analyser). a- poikiloblast of plagioclase (Pl) exhibiting an inclusion of an internal schistosity (IS) forming an angle of approximately 30° with the external schistosity (Es). b and c-microfolded internal schistosity (Is) on which the poikiloblastic staurotides (St) develop. d-cordierite (Crd) and fibrous sillimanite (Sil) paragenesis. e-replacement of staurolite (St) by muscovite (Mus), f-Destabilization of staurolite (St) into cordierite (Crd) and epidote (Ep). g-mineralogical composition of an amphibolite. h- destabilization of calcite (Cal) in favor of epidote (Ep). i-retromorphosis of garnet (Grt) in favor of calcite (Cal).

- Chlorite forms as a result of the alteration of biotite or cordierite

- Staurolite is destabilized and replaced by muscovite, cordierite, or epidote.

Amphibolites:

The amphibolites have the same type of outcrops as the mica schists described above. The amphibolites are greenish in color and show an alternation of amphibole bed and garnets or saussuritized plagioclase, which highlights the foliation.

Amphibolites exhibit a grano-nematoblastic texture, and primarily consist of (table 1) : quartz (4.3% to 37.4%), amphiboles are calcic of the magnesio-hornblende type (22.4% to 50.5%), plagioclases are also calcic of the anorthite type (7.1% to 45.2%), garnets are of almandine (8.3% to 15.2%) and grossular (≤16.7) type with little spessartite (≤ 1.7%), pyroxenes are calcic of the augite type (3.3% to 12.7%), epidotes are of the clinozoisite type (4.2% to 11.2%) and the chlorite is magnesian of the clinochlor type (≤3.3%). The appearance of hornblende and garnet indicates an amphibolite facies.

An example of diffraction pattern with the percentages of identified minerals results from a sample of amphibolite (sample C5) is in Figure 5.

Figure 5. The percentages of minerals identified and their diffraction patterns of a amphibolite (sample C5).

Under polarizing microscope (Figure 4g-i), the quartz grains display a heterogranular polygonal to anhedral shape with undulatory (undulose) extinction due to the effects of stress. The amphiboles are rod-shaped, sometimes poikilo-blastic that mark the foliation of the rock. Garnets (ranging from millimeter to centimeter scale in size) are poikiloblastic and contain quartz inclusions. Epidote and calcite are secondary minerals phases. Epidote are formed as a replacement of calcite (Figure 4h), while calcite comes from the destabilization of plagioclase or garnet (Figure 4i).

Quartzite:

Quartzites are with or without manganese oxide and occur in the form of a sporadic metric outcrops. The quartzites are generally Brunish color but blackish for the garnet- quartzite. The Quartzites have a granoblastic texture composed, according to the XRD analysis (Table 1), of quartz (53% to 76.6%), plagioclases are calcic of the anorthite type (≤31.8%), amphiboles are calcic of the magnesio-hornblende type (from 1% to 14.2%), garnets are almandine type (0% to 1%) and spessartite type (0% to 21.4%) with rare lithiophorite (1%) (sample A8, manganese quartzite) (Figure 6). we noticed, in microscopic analyses, a rare kyanite, staurolite and sillimanite which indicate an amphibolite facies for the metamorphism conditions.

Figure 6. The percentages of minerals identified and their diffraction patterns of a spessartite-bearing quartzite (sample A8).

Under polarising microscope (Figure 7), garnet occurs as a poikiloblast with either hecilitic or planar quartz inclusions highlighting the internal schistosity (Is). The quartz constitutes the mesostasis with intergrowth contacts. The kyanites present a sub-parallel orientation with the internal schistosity (Is). Locally the sillimanites are folded with a fold axis sub-parallel to the orientation of the staurolites (Figure 7).

Figure 7. Photomicrographs (under crossed nicols +) of a quartzite (sample A9) showing garnet (Grt), kyanite (Ky), staurolite (St) and folded sillimanite (Sil) paragenesis. Internal schistosity (Is) is observed in the poikiloblastic garnet (Grt).

4.2. Ore Petrography

Manganese mineralization-bearing rocks in Téra area are mainly gondites, which constitute the highest hills in the area. A supergene alteration developed on gondites a saprolitic profile (Figure 8) with relatively fresh rocks at the bottom (Figure 8a), gradually moving towards a compact crust of manganese oxides at the top of the hills (Figure 8c).

Figure 8. Weathering profile of one of the gondite hills showing the petrographic variation from the bottom (a) to the top (c) passing through an intermediate level (b). (a): fresh rock, (b): altered rock, (c): manganiferous duricrust, (d): Gondite hill

-The fresh rock

The gondite sample is greenish to brownish in color (Figure 8a). It has a medium to fine granoblastic texture with a folded foliation. The foliated structure is represented by an alternating bed of garnets and quartz-rich beds. According to Garba Saley et al. (2018), the MnO, Al2O3, SiO2 and CaO content of gondites are 19.15%, 14. 09%, 45.58% and 6.13% respectively. XRD analysis specifies the mineralogical composition such as: quartz (35.8% to 53.1%), garnets are of the spessartite type (from 30.4% to 53.8%) and almandine (≤ 12.2%), amphibole is of the magnesio-hornblende type (2.7% to 5.5%) slightly ferrous and rare titanite, tourmaline and microcline. Some gondites samples have only spessartite as garnet in their mineralogical composition (Sample D1) (Figure 9).

Figure 9. The percentages of minerals identified and their diffraction patterns of a fresh gondite (sample D1).

Under polarising microscope, the quartz are granular or anhedral crystals with undulatory (undulose) extinction and are arranged according to the foliation (Figure 10a). Amphiboles appear euhedral and subhedral, elongated and locally molded around garnet crystals. Garnets appear as polygonal grains in atoll, porphyroblastic and poikiloblastic with inclusions of quartz and amphibole and are highly fractured. This underlines a phase of deformation subsequent to the inception of the foliation. The titanite (sphene) is subhedral with cracks. It has parallel orientation to the foliation and intersects the amphibole. The microcline is associated with quartz in the foliation.

Figure 10. Image of gondites. All pictures are under crossed nicols +, excepted (c) and (d) which are under parallel nicols // and (b) which is a macroscopic image. a- photomicrograph of a fresh gondite (sample G2) showing garnet (Grt) grain foliation alternating with quartz (Qtz). b-outcrop image of the manganese-bearing carapace showing part of the altered gondite on which the botryoidal structure develops. c-photomicrograph of a gondite showing the development of manganese oxide at the periphery of the garnets (Grt) and in the fractures and the tourmaline (Tur) molding around the garnets (Grt) or in inclusion, d-photomicrograph of the part with botryoidal structure showing the replacement of lithiophorite (Li) and pyrolusite (Py) and the epigenization of garnet (Grt) by pyrolusite with cryptomelane (Cry) and pyrolusite (Py) which form a concentric banded texture.

-An intermediate level

In this level (Figure 8b), gondite weathering is at an advanced stage (Figure 10b). The rocks are brownish to reddish with whitish kaolinic spots. In this level, garnet and quartz crystals are rare.

Under microscope (Figure 10c & d), this alteration is characterized by the beginning of silica leaching, a transformation of feldspars into kaolinite and an epigenization of garnets (spessartite and almandine) into oxide. The alteration of garnets (spessartite) into manganese oxides takes the form of opaque granules that develop at its periphery and in its fractures, where they gradually replace the garnet (Figure 10c). In this level tourmaline occurs as small nematoblastic crystals oriented in the same direction as the foliation and sometimes it occurs as inclusion in garnet or either molded around garnet crystals (Figure 10d). According to Garba Saley et al. (2018), the MnO, Al2O3, SiO2 and CaO content of this level are 27.87%, 18. 2%, 19.98% and 1.35% respectively.

-Duricrust cap

At the top of the gondite hills (Figure 8c), the supergene weathering led to the development of a secondary manganese-bearing deposit in the form of carapace layer about 2m thick with a botryoidal structure. This carapace covers a very altered level with a massive structure and which locally presents white spots of kaolinite (Figure 10b).

The observation in reflected light under microscope shows two features depending on the levels with a massive or botryoidal structure:

-In a massive structure level: manganese oxides generally crystallize in the form of lithiophorite, replacing spessartites. Some cryptocrystalline pyrolusites (amorphous) replacing the lithiophorite are also observable (Figure 10e & f).

-The level with a botryoidal structure presents a fibrous cryptomelane and colloform cryptocrystalline (amorphous) pyrolusite (Figure 10g & h). Pyrolusite and cryptomelane, entirely formed as supergene oxidation products of manganese silicates (spessartite).

According to the XRD analysis, this manganese carapace is constituted of kaolinite (17.6% to 38.4%), of lithiophorite (≈7.9%), cryptomelane (≈23.8%), and pyrolusite (amorphous) which varies from 51.7% to 56.5% (Figure 11). This level constitutes the ore (Figure 8c) with the highest MnO (47.92%) and Al₂O₃ (22.5%) content and the lowest SiO₂ (12.86%) and CaO (1,14%) content (Garba Saley et al., 2018).

Figure 11. The percentages of minerals identified and their diffraction patterns of a manganese carapace with a botryoidal structure (sample TCR).

4.3. Alteration Features

In the study area, according to the alteration, the manganese ores display four distinct types of textures which were observed either at the hand specimen scale or on the polished sections under the microscope. The four types are as follows:

Granular Texture: The granular texture is observed at the bottom of the gondite hills where the gondite is less altered. This texture is marked by the aboundance of grain-shaped minerals such as garnets (spessartite and almandine) and quartz in the fresh gondites. The garnets occur in the form of subhedral to euhedral grains with cracks in which manganese oxidation develops.

Epigenization texture: This texture has been observed in both levels with altered gondite and in the top with manganese carapace. The textural relationships show that the hosted euhedral spessartite grains are corroded by pyrolusite and grading inward until the spessartite to be transformed into pyrolusite; in other cases, the remnants of the original spessartite (as relics) are found as small enclaves in the subhedral to euhedral grains of pyrolusite.

Replacement Texture: The replacement texture has been observed in altered gondite under microscope with transmitted light, where garnet is replaced by a secondary manganese oxide. This replacement texture is formed either at the edges or within fractures of the garnets. Additionally, this texture has been observed in manganese ores, where lithiophorite is replaced by pyrolusite.

Colloform Texture: This type of texture is mostly observed in the manganese ore at the top of gondite hills. Colloform texture is typically exhibited by cryptomelane and pyrolusite. It is also observed in hand specimen as well as polished sections. This texture shows a small nucleus (garnet or quartz) surrounded by concentric bands of cryptomelane and pyrolusite exhibiting a banded texture with a successive alternated rhythmic concentric band. Sometimes, it is observed as spheroid fibrous, mammillary or botryoidal forms. According to Salem et al. (2010), the presence of colloform texture and amorphous materials support colloidal transport of thermal water.

5.1. Mineralogical Composition and Metamorphism

In the amphibolites, the presence of minerals such as calcic amphiboles (magnesio-hornblende), calcic plagioclases (anorthite), calcic pyroxenes (augite), allumino-ferrous or calcic garnets (almandine or grossular, respectively) are characteristic of marly sequence having undergone meta-morphism under amphibolite facies conditions. Soumaila and Garba (2006) assign the sequence of these amphibolites as ancient greywackes.

In quartzites, the presence of quartz (up to 76.6%) coupled with that of aluminous minerals (almandine and spessartite) and calcic plagioclase (anorthite) strongly support the interpretation of these quartzites as belonging to an arenaceous sequence. The mineral assemblage including garnet, kyanite, staurolite, sillimanite indicate an amphibolite facies.

Gondites are manganese-bearing metamorphic rocks made up mainly of quartz and manganiferous silicates (spessartite, braunite, rhodonite…). The presence of tourmaline in gondite paragenesis shows that the primary manganese mineralization is of submarine exhalative origin. Garba Saley et al. (2017 and 2018) reported a SEDEX origin (Sedimentary exhalative deposit) based on geochemical and the mineralogical analysis of the gondites. Indeed, tourmaline is associated with several mineralizations of submarine exhalative origin known from the Archean to the Mesozoic (Cleland et al., 1996). According to Banerji (2020), tourmaline indicates a boron metasomatism in manganese deposits from hydrothermal origin.

Mica schists are known to be of pelitic sequences. Petrographic analysis of mica schist and quartzite samples reveals the presence of an internal schistosity (Is) marked by quartz or opaque inclusions (linear, folded or helicitic) in poikiloblasts of garnet, plagioclase, kyanite and staurolite. This is characteristic of a prograde metamorphism in amphibolite facies conditions. This prograde metamorphism is at the origin of the transformation of volcano-sedimentary sediments rich in manganese into gondites. Other authors (Soumaila, 2000; Soumaila and Garba 2006; Ganne et al., 2011; Garba Saley et al., 2017) interpret this metamorphism as high-pressure amphibolite facies, from 600 to 700 ᴼC and from 7 to 9 kbar conditions. Pons et al. (1995) concluded that the assemblage of kyanite, garnet and staurolite is the witness of a medium to deep structural level, ascent by the granitoids during their implementation.

The destabilization of staurolite to cordierite, epidote, or muscovite marks a retrograde metamorphism from high-pressure amphibolite facies conditions (characterized by staurolite, kyanite, and garnet paragenesis) to low-grade assemblage marked by calcite, epidote, muscovite and chlorite paragenesis in greenschist facies conditions. This retrograde metamorphism can be linked to a quick rise of the rock which did not allow the transformation of sillimanite or kyanite into andalusite. This indicates a vertical movement which emphasizes the vertical tectonics (Pons et al., 1995 ; Vidal et al., 1996, 2009; Lompo, 2009 et 2010; Baratoux et al., 2011).

5.2. Manganese Ore

Spessartite is the only primary or metamorphic manganiferous mineral present in Téra gondites, unlike the vast majority of manganese deposits in West Africa. Only the Mokta manganese deposit (Cote d'Ivoire) is similar to the Téra deposit, as it has very little or no rhodochrosite in the primary manganese-bearing minerals, which consist solely of silicates (spessartite and braunite) (Grandin and Perseil, 1977). Whereas rhodochrosite is dominant in the other manganese deposits of west African Craton such as Nsuta deposit (Ghana), Tambão-Beliata deposit (Burkina Faso) and Ansongo deposit (Mali) (Mücke et al., 1999; Beauvais et al., 2008; Nyame, 2008; Ilboudo, 2010; Hein and Funyufunyu, 2014; Hein and Tshibubudze, 2015).

The absence of rhodochrosite may be due to the intensity of metamorphism, as reported by Mohatapra and Nayak 2005 for Mn carbonate–silicate rocks from the Gangpur Group in India. According to these authors, the carbonate minerals can be transformed into silicate and oxide minerals through decarbonation-oxidation process during prograde metamorphism.

Following the alteration profile, the decrease in SiO₂ and CaO contents associated with an increase in K₂O, Al₂O₃ and MnO contents from the bottom to the top of gondite hills, characterize the supergene weathering process. Which involves hydrolysis and leaching of SiO₂ and CaO followed by a concentration of K₂O, Al₂O₃ and MnO in the form of manganese oxide and hydroxide. Supergene weathering is known in west Africa to be the cause of the development of bauxitic, lateritic and manganiferous formations during the periods from Cretaceous to Eocene (102-45 Ma) (Brown et al., 1994; Burke and Gunnell, 2008; Beauvais et al., 2008 Hein and Tshibubudze, 2015).

Pyrolusite, lithiophorite and cryptomelane are the secondary manganese minerals resulting from the weathering of spessartite in the Téra manganese deposit. The aluminum released by the alteration of tourmaline and garnets kaolinites allows the formation of lithiophorite while the alteration of microclines introduces potassium into the solution which allows the development of cryptomelanes. A more oxidized manganiferous phase leads to the destabilization of lithiophorite and cryptomelane in favor of pyrolusite. According to Beauvais et al. (1987), Cryptomelane and lithiophorite constitute the more stable forms of manganese in the superficial layers of the lateritic profiles.

Other rare secondary manganese minerals such as nsitute and psilomelane have been detected under an optical microscope and reported by Garba Saley et al. (2018). Because these minerals are in the minority, their presence did not reach the detection limits of the XRD analysis method.

Based on the main mineralogical results of the present study, the following genetic considerations can be proposed for the Téra manganese deposit.

The geological formations of the study area consist of an alternation of amphibolites, mica schists, quartzites and gondites. This alternation resulted from metamorphism under the high-pressure amphibolite facies conditions of a volcano-sedimentary complex with a marly, arenaceous and pelitic sequences associated with submarine exhalations.

The primary manganiferous mineral contained in the Téra gondites is of a silicate type composed only of spessartite-type garnet.

Supergene alteration of spessartites, by replacement and epigenization, led to the formation of secondary manganese minerals such as lithiophorite, cryptomelane and pyrolusite.

This manganese deposit is originated from the supergene alteration of spessartite protores which allowed the enrichment of manganese contents.

This study is part of the Ph.D. thesis of the first author and was supported financially by the African Union through Pan African University of Life and Earth Sciences Institute (PAULESI) of Ibadan, Nigeria.

Many thanks are due to Mr Aliyu Ohiani Umaru for the scientific exchanges.