The studied area is located in zone 6˚22 and 6˚30 North Latitude and 13˚11 and 13˚18 East Longitude (Figure 1).

It is subjected to tropical Sudanese humid climate characterized by a long rainy season from March to October and short dry season from November to February [10] . The average annual rainfall is between 1500 and 1700 mm. The Sudano-Guinean anthropized savannah is the type of vegetation encountered in Ngaoundal [11] . The geomorphological features of the studied area are composed mainly of hills, with the Ngaoundal mountain which reaches an altitude of 1400 m. There are also some intermediate landforms made up of rolling land with some residual rises and plateaus that rarely exceed an altitude of 1100 m [12] . The slopes are steep and slightly inclined, separated by convexo concave interfluves coated by bauxitic duricrust formations, outcropping from the tops to the bottom of the hill.

From the geological point of view (Figure 2), the Ngaoundal sector belongs to the Central Cameroonian domain, also called the Adamawa-Yade domain of the Pan-African Chain of Central Africa [13] [14] . It includes Syn to late-tectonic granitoïds [15] , partially overlain by basaltic, trachytic and phonolitic flows [16] . In this region, the bauxite ores deposits are developed exclusively from the altered basalts [12] . Soils are dominated by ferralsols [17] associated with duricrusts which play an important role in the pedology of Adamawa region.

Based on the field relations and differences in duricrust facies, sixteen samples were collected (Figure 3). These were separated into two groups, the first referring to the samples from the trench’s profiles and the second consisting of the blocks of duricrust outcropping on the ground surface. The first group included

four samples, two from the first profile and one from each of the last two. The second group was constituted of twelve samples classified into four facies (scoriaceous, pisolitic, massive and nodular), each with three samples. After morphological characterization, all sixteen samples were subjected to mineralogical and geochemical analysis at Geoscience Laboratories (Geo Labs) in Canada, after their preparation in the Mechanical Ore Preparation Laboratory of School of Mining, Industry and Geology of Niamey (Niger).

The mineralogical composition was semiquantitatively determined by using X-ray diffraction (XRD-100) after pulverizing sample powders with an agate mortar and pestle. Samples were analyzed with Co radiation at 40 kV and 45 mA. The following parameters were used in the X’PertHighScore Plus software for the peak identification, and a Rietveld Refinement process [18] was applied for semi-quantifying the proportion of each mineral.

The compositional values of major and minor oxides along with the trace and rare earth elements were determined by using X-Ray Fluorescence Spectrometry (WD-XRF) coupled to ICP-AES and ICP-MS methods. The LOI (Loss on ignition) values were measured on the basis of weighting the samples before and after one hour of heating at 1000˚C. The performance of each method employed at the Geo Labs is monitored using control charts of the analysis of interlaboratory and in-house reference materials. The overall precision of the analytical methods can be assessed from the reproducibility of the analyses of the reference materials that were cycled through as quality assurance monitors during sample analysis.

The mobility of chemical elements during weathering was investigated using two indices of alteration, the Chemical Index of Alteration (CIA) and the Mineralogical Index of Alteration (MIA) [19] . The CIA evaluates cations (Ca²⁺, Na⁺ and K⁺) mobility in relation to Al³⁺ considered as immobile element while the MIA determines the degree of transformation of primary minerals into secondary minerals [20] . These indices are obtained by these formulae:

CIA = 100 × [ Al 2 O 3 Al 2 O 3 + CaO + Na 2 O + K 2 O ] . The MIA is deduced from the CIA

formula: MIA = 2 × ( CIA − 50 ) . The obtained data were processed by using Excel program. The various diagrams were performed with Sigma Plot 14.5 while the various maps were generated using GOOGLE MAPS and QGIS version 2.18.15.

The morphological characterization of the duricrusts outcropping in blocks on the ground surface (Figure 4) showed that the scoriaceous facies (Figure 4(a), Figure 4(b), Figure 4(c)) was the most distributed. It was observed from the top to the bottom of the hill. It was mainly characterized by a mottled porous

matrix; whose overall appearance gives them the character of a volcanic slag. The yellowish spots, which are very abundant in the summit formations (Figure 4(a)), tended to disappear in those of the mid-slop (Figure 4(b)) and the base of the hill (Figure 4(c)). The pisolitic facies (Figures 4(j)-(l)) was the least abundant and corresponded to the continuous bauxitic duricrusts which outcrop essentially at the foot of the hill. These formations were characterized by more or less friable spherical concretions contained in a red matrix. Massive and nodular facies outcropped at the top of the hill and at mid-slope. The former (Figures 4(g)-(i)) were very compact and very resistant. These were characterized by a nearly homogeneous red matrix and a total lack of nodules, but a few rare whitish spots within the matrix of the formations at the foot slope. The second ones (Figures 4(d)-(f)) presented red matrix containing millimeter to centimeter nodules in varying degrees of abundance. They also displayed white and gray punctuation on the formations of the top of the hill, while those of the mid-slope were stripped of the whitish spots.

Profiles description (Figure 5) focused on the variation in thickness of the horizons. It consists of: the structure or texture, color, traces of any biological activity, the limit between a layer and the one below, and the presence of vegetation cover.

The first profile (Figure 5(a)) with a thickness of 350 cm had three layers. The first (0 - 15 cm) was sandy-claybrown (7.5YR 4/3), rich in organic matter. It contained an abundance of fine roots and millimetric to centimetric nodules. Blocks of duricrust were found in outcropping. The limit with the underlying

level was clearly defined. The second level (15 - 195 cm) referred to a slab of reddish-brown duricrust (5YR 4/4), massive and compact. It showed relics of primary minerals and whitish spots. The transition was irregular throughout the trench and distinctive with the underlying level. The third and last horizon (195 - 350 cm) was marked by a yellowish-red clay layer (5YR 5/6), slightly compact and containing a high proportion of millimetric to centimetric nodules and abundant fine roots. It presented traces of biological activities and some relics of primary minerals.

The second profile (Figure 5(b)) was 380 cm in thickness and showed 3 layers. An upper level (0 - 70 cm) was brown nodular horizon (10YR 4/3). It enclosed fine roots and signs of biological activities. It contained millimetric to centimetric nodules. It presented clearly defined interface with the level below. An Intermediate level (70 - 290 cm) referred to slab of duricrust with a red background (2.5YR 4/6). It was very compact, very hard and contained very few fine roots, embedded in an abundance of rusty spots. Towards the transition to the bottom level, the matrix background changed to dark red. The limit was gradual with the inferior level. A lower layer (290 - 380 cm) was yellowish clayey red level (5YR 5/6), with a polyhedric structure and enclosing some rare fine roots, nodules and fragments of duricrust. It also included whitish patches disseminated within the complex.

The third and last profile (Figure 5(c)) was composed of 2 levels: the first one (0 - 150 cm) constituted of a dark brown duricrust (7.5YR 5/6), outcropped as a very compact and hard slab above loosen soils. A few rare vacuoles backfilled with dark brown soils were identified in a matrix that contained an abundance of rusty and whitish spots. The lower boundary was abrupt and regular. The second (150 - 350 cm) referred to red level (2.5YR 4/6), sandy-clay, containing many yellow spots and vacuoles filled with dark brown earth from biological activities. It contained fragments of duricrust, an abundance of fine roots and few yellowish spots.

According to mineralogical analyses (Table 1), gibbsite was the most abundant mineral particularly in scoriaceous facies. It represented more than 90% of minerals detected by X-ray. Hematite is also recorded, but in low quantities (3.88% - 6.1%) whereas kaolinite, quartz and goethite were totally lack in these materials. In the pisolitic facies, mineral association included gibbsite, kaolinite, hematite and goethite. Kaolinite was the most abundant mineral in these facies. It represented 52.21% to 76.01% of minerals detected by X-ray. Gibbsite grade was fairly low, but a significant content in the PIF2 sample (25.93%) was observed. Similarly, hematite was weak but it was a quite high proportion in PIF3 sample (16.84%). The nodular facies were mostly composed of hematite (41.85% to 42.01%) and kaolinite (30.79% to 45.97%). The quartz content was moderate. It varied from 12.22% to 25.19%. There was a total lack of goethite and gibbsite minerals in these formations. Three minerals were distinguished in the massive red-bottom facies: gibbsite (19.73% to 59.43%), hematite (24.87% to 68.13%) and quartz (9.17% to 15.7%). The first two minerals (gibbsite and hematite) were the most abundant and quartz the least important.

The two lower layers of the first profile were examined. The upper level showed a significant proportion of quartz mineral (66.06%), a moderate content of gibbsite (24.84%) and a slight content of kaolinite (9.1 %). It did not include hematite and goethite. The lower layer indicated the same pattern with a content of 55.91% quartz, 34.02% gibbsite and 10.07% kaolinite and lack of hematite and goethite. The second profile analyzed on its second layer, displayed 78.39% gibbsite, 11% kaolinite and 10.61% quartz. These results confirmed a significant content of gibbsite. The third profile analyzed on its first level indicated an interested content of quartz (48.82%) and gibbsite (36.25%), and weaker in kaolinite (14.93%). Figure 6 shows the X-ray patterns of the analyzed samples.

Chemical elements may have certain relationships during the bauxitization process. Binary plots among the major elements indicate strongly positive correlations between Fe₂O₃ and P₂O₅; Al₂O₃ and TiO₂; Nb and TiO₂; Ta and TiO₂ (Figure 7). A moderate positive correlation is observed between Fe₂O₃ and MgO whereas negative correlations exist between Al₂O₃ and SiO₂, and Al₂O₃ and Fe₂O₃. These patterns suggest two groups of components: a first group of positively correlated components including Al₂O₃ vs. TiO₂ diagram (Figure 7(f)), Fe₂O₃ vs. P₂O₅ (Figure 7(e)), Fe₂O₃ vs. MgO (Figure 7(d)), Nb vs. TiO₂ (Figure 7(h)) and Ta vs. TiO₂ (Figure 7(i)) and a second group of negatively associated components composed of Fe₂O₃ vs. Al₂O₃ diagram (Figure 7(a)), Fe₂O₃ vs. SiO₂ (Figure 7(b)), Al₂O₃ vs. SiO₂ (Figure 7(c)) and Al₂O₃ vs. Cr (Figure 7(g)).

Weathering of basalts developed during the upper Cretaceous was the origin of concentrated bauxite deposits on the platform of Ngaoundal and Minim-Martap in the Adamawa Region of Cameroon [7] [8] [9] [12] . Morphological facies variation (scoriaceous, nodular, pisolitic and massive) corroborate with the fact that these materials are derived from the erosion and re-sedimentation of older formations and possibly also from eroded bauxitic formations, formed on the exposed platform [6] . This degradation process was favored by humid equatorial climate [21] and tectonic instability [22] of the region which belongs to the Cameroon volcanic line. According to [4] and [23] , the African morphologies depend on the progressive erosion and peneplanation which are favored by the hydrographic network which generally get along with the faults and are responsible for the reduction and variations of the thickness of the duricrust formations above the plateaus and on the flanks of hills. The white and gray punctuation displayed by certain bauxitic facies would result from the weathering of plagioclases, substituted in volume and form by gibbsite and ferruginous minerals [6] [12] . The transformation of the bauxitic formations into the pisolitic bauxitic facies by individualisation of pisoliths derived from ferruginous spots and nodules, led to the development of a thick bauxitic mantle with pisolitic facies [12] [24] . The landscape and the pedological coverage of the study area reveal that the hills are very narrow with very steep slopes, unlike the plateaus where the slopes are slightly inclined [6] [12] . The presence of the gibbsitic-dominated duricrust has contributed to the development of a thick soil-cover at the level of the hills, a compact pedologic horizon at the top of the plateaus and in the whole of a spotted horizon rich in quartz reflect the climatic contrast of the locality [5] [9] .

Bauxitic mantles that are regularly located at the top of hills and interfluves in the Ngaoundal area constitute a residual surface dominated by duricrust formations. These were probably formed during the upper Cretaceous under contrasted climate in an oxidizing-leaching environment that favored the crystallization of hematite more than goethite [25] [26] . The sustained presence of hematite mainly in the nodular and massive duricrust facies implies neutral to sligthly alkaline conditions in a later stage of bauxite formation. Moreover, its presence implies epigenetic processes during bauxitization, including Fe-mobilization, Fe-reprecipitation as amorphous Fe-hydroxides and subsequent recrystallization to hematite during diagenesis. In this context, hematite was shown to form contemporaneously to Al-oxyhydroxides [27] . The residual phase of the altered basalt cover included iron hydroxides (hematite, goethite), clay minerals, kaolinite and a considerable volume of aluminium-free hydroxides [12] . Thus, lateritization in Ngaoundal favored bauxitization through aluminium mobilization with individualization of the iron bearing lateritic bauxites which develop thick bauxitic mantles [8] . This suggests that bauxitization in this environment may correspond to intense hydrolysis leading to allitization with losses of SiO₂ and various cations, and monosialitization leading to the formation of kaolinite [28] . This intense weathering leads to the accumulation of significant immobile elements as Al, Fe and Si.

Climate conditions during bauxitic formations control the distribution of chemical elements between textural components [29] . For the major elements, we found the usual pattern of lateritic weathering [21] : progressive silica leaching, almost total alkali and alkaline earths leaching, gradual accumulation of iron and aluminum. Some elements are mostly concentrated in the matrix while other elements are associated with the ooids. This is the case of SiO₂ which is generally confined in the matrix, whereas Al₂O₃, TiO₂, and Fe₂O₃ are mostly associated with the ooids [30] [31] . However, the SiO₂ enrichment of the Sardinia bauxites is considered to be due to the alteration process caused by the circulation of Si-rich solutions, which caused the re-silicification of hematite as a consequence of the emplacement of Tertiary volcanites on top of the bauxites [28] . Relatively high iron contents (average value of 23%) indicate that these belong to iron-rich bauxites. Thus, the Ngaoundal bauxite deposits can be classified as ferruginous lateritic bauxites [32] . The most abundant trace elements (Zr, Cr and V), represent the elements that are not very mobile during alteration, and are most often enriched in bauxites [29] .

In these formations, some correlations between major and trace elements were noted. Thus, a significantly negative correlation was observed in Al₂O₃ vs. SiO₂ diagrams (r = 0.815) and slightly negative between Fe₂O₃ and Al₂O₃ (r = 0.418). On the other hand, the correlation was significantly positive for Fe₂O₃ and P₂O₅ (r = 0.754); Nb and TiO₂ (r = 0.893) and Ta and TiO₂ (r = 0.830). Similarly, it was also positive for Fe₂O₃ and MgO (r = 0.504) and Al₂O₃ and TiO₂ (r = 0.830), but not significant. During the bauxitization process, a fractionation of major, minor and trace elements takes place [31] . It is widely accepted that the study of immobile elements is a powerful tool to unravel the parental material and the genetic history of bauxites [33] . Therefore, an identification of mobile and immobile elements during the bauxitization process is of vital importance. Furthermore, it has been shown that titanium oxides in bauxite can accumulate substantial amounts of Nb, Ta, Th [34] . In this study, the observed positive correlation between TiO₂ and Al₂O₃ might be attributed to the fact that Al-oxyhydroxides have been found to contain Ti-nanominerals [27] . Trace elements, such as Nb, Ta, Zr, Th or U show a good correlation with TiO₂ and Al₂O₃, meaning that they also behaved rather immobile during bauxitization. The good correlation between Nb and Ta with TiO₂ indicate that these elements are hosted in Ti-oxide phases within bauxitic samples. These correlations are typical of strongly weathered deposits where immobile elements accumulate and mobile elements are leached out [35] . This is also confirmed by the low contents of alkali and alkaline earth metals, which are highly mobile during chemical weathering [36] [37] .

Allitization, which refers to lateritization process, may be related to abundant rainfall (around 1500 to 1700 mm mean annual rainfall), dense and dendritic hydrographic network and steep slopes which favored a highly leaching milieu [38] . Bauxitization process of Ngaoundal duricrust formations was implemented by factors such as deferrugenization-ferrugenization, desilisification, and fluctuation of underground water table level. Chemical Index of Alteration (CIA) and Mineralogical Index of Alteration (MIA) derived from major element composition of the studied bauxites, showed extremely high values (>99). This is caused by the intense weathering resulted from high temperature, rainfall and regional groundwater system in the region [39] . The consequence is almost total exportation of alkali and alkaline earths elements (Na, Ca, Mg, K) and the accumulation of an important aluminum enrichment as well as abundant accumulation of gibbsite.

The Cameroonian territory is full of more than fifty mineral substances and about 50% of the national territory has been explored. Between 1960 and 1990, the government with their international partners like UNDP and BRGM carried out studies in order to investigate the mineral potential of the country [40] . Nowadays, the government through their development strategy (SND30), oriented projects on mining. This is emphasized by the creation of SONAMINES which is in charge of mining exploitation. But at first, the need of understanding processes which control establishment of minerals and relationship with environment are important for sustainable management.

The geology of Cameroon is favorable to the mineralization of substances as precious metals, base metals and rare metals [12] . Indeed, in Cameroon, lateritic weathering and erosion concentrated both metallic and non-metallic ores, as well as valuable substances, of which main indices and deposits have been identified. This is the case of bauxite deposits from Ngaoundal and Minim Martap in Adamawa [8] [12] and from FongoTongo in the Western region [6] . For example, 554 million tons of bauxite reserves have been found in the Adamawa region as of 2009 and 50 million tons in FongoTongo and 4 million tons in Foumban [8] . According to experts, Cameroon is the second largest bauxite reserves deposit in Africa.

Duricrusts described indicated the successive steps in the establishment of the bauxitic formations in Cameroon in general and in the Adamawa region in particular. The alteration of basaltic rocks of the Adamawa Plateau resulted in the deposit of alumina-rich bauxites in the high landscapes notably those of Ngaoundal [7] [12] . The study revealed interesting bauxite deposits in Ngaoundal area, thereby allowing the mining properties of the studied formations to be established. Indeed, significant concentrations of Al₂O₃ (over 45% in grade) on the one hand, and gibbsite (more than 90% in grade) on the other hand, notably in the most abundant scoriaceous duricrust facies in this area, sufficiently indicate favorable mining perspectives. The mineralogical and geochemical investigations conducted in this study allowed for a better assessment of industrial and technological properties of the bauxitic formations developed [33] [41] . However, further investigations including drillings and prospecting wells are required for establishing the geologic properties of deposit.