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Lithogenic Concentrations of Useful Elements and Residual Mineralization Indices in the Weathering Products Derived from Mineralized Rocks in Meїganga (Central Cameroon)

Tchaptchet. T.W.*, Tematio. P., Njiki. C.C., Guimapi. T.N., Hapi. F., Tiomo. I., Tchuenkam. D.B., Momo. N.N.M.

Department of Earth Science, University of Dschang, P.O. Box. 67, Dschang, Cameroon

Abstract: This paper focuses on the use of morphological, mineralogical and geochemical characterization of weathering profiles from mineralized rocks in Meїganga, a locality in the South-East of Cameroon. It is aimed at assessing weathering processes that induced the lithogenic concentrations of useful elements, and eventually residual mineralization indices in weathered products. Seven weathering profiles derived from four distinct mineralized rocks (micaschists, orthogneiss, granite, and quartzite veins) were studied. The profiles exhibit shallow weathered A/B/C or A/C soil profiles with a moderate thickness (less than 4m). Minerals identified in the weathered products in decreasing contents (%) were: quartz (60.3-93.9), kaolinite (0.8-22.3), phlogopite (0.2-15.3), goethite (1.9-13.0), hematite (0.5-8.5), halloysite (0.2-4.6) and smectite (0.5-4.3). The SiO₂ contents generally decrease upward in these weathering profiles, except for those from granitic parent rock. Inversely, Al₂O₃, Fe₂O₃ and TiO₂ contents increase upward, except for the weathering profiles from granite and orthogneiss. Alkaline and alkaline earths are more or less completely exported during weathering. Chemical weathering parameters have revealed intense rocks weathering in Meїganga, resulting to the important accumulation of quartz in association with 1:1 clay minerals. Trace elements that prevail in these weathered products arranged in decreasing order of abundance include: S, Ba, Sr, Zr, Cr, V, Zn, Rb, Ni, Y, Sb, Cu, Pb, Li, Co, Ga, Nb, Th, Sc, Cs, Hf, Sn, U, Mo, and W. The most significant useful elements identified in these weathered products are arranged in decreasing order of abundance include: Zr, Cr, V, Sb, U, Cu, Nb, Hf, Mo, and W. The weathered products present a CI-chondrite normalized pattern of REEs characterized by the fractionation of HREEs relative to LREEs. The accumulation of trace elements (Ga, W, Y, Sn, Hf, Nb, Cu, Sc, V, Zn, Cr, Sb, Pb, Ni, Co, Li, Mo, Th, Rb, Cs, U, Zr and Ba) and REEs in the weathered products of Meїganga has been attributed to the effects of weathering. The main host minerals are the residual primary minerals (epidote, apatite, pyrite, titanite, zircon, or opaque minerals) and the newly formed secondary minerals (phlogopite, hematite, goethite, kaolinite and smectite). Correlation matrices between useful elements (Cr, Mo, U, V, Nb, Zr, Sb, Hf and W) and major oxides (Fe₂O₃, TiO₂, P₂O₅ and Al₂O₃) indicates a strong affinity (>0.80), suggesting the trace elements are noble metals. Therefore, major oxides used as tracers for residual mineralization indices are: Al₂O₃ (Nb, Mo, U, W and Hf), Fe₂O₃ (Mo, Cr, V, Zr, Hf and Sb), P₂O₃ (U, Hf and Sb), and TiO₂ (Nb, Zr, V and Cr). In the Meїganga soil profiles, mineralization indices could be assigned to W, V, Hf, Nb, Mo, Cr, Sb and U.

Keywords: weathering products, useful elements, residual mineralization indices; African shear zone, Meїganga, central Cameroon

Received April 07, 2019; Revised May 17, 2019; Accepted June 10, 2019

Citation: Tchaptchet. T.W., Tematio. P., Njiki. C.C., Guimapi. T.N., Hapi. F., Tiomo. I., Tchuenkam. D.B., Momo. N.N.M., Lithogenic Concentrations of Useful Elements and Residual Mineralization Indices in the Weathering Products Derived from Mineralized Rocks in Meїganga (Central Cameroon). Journal of Geosciences and Geomatics, 7 (3). 112-133. January 2019. doi:10.12691/jgg-7-3-3

1. Introduction

Rock type plays a fundamental role in the degree and intensity of weathering, as well as the mobilization and concentration of chemical elements on the Earth surface [1,2]. The intensity of weathering also depends on a number of factors including climate and the tectonic activities. Meїganga is located in the Adamawa region of Cameroon and lies on the North-Eastern part of the Central African Fold Belt (CAFB) in Cameroon. This mobile belt is known to have been highly affected by tectonic movements during the Pan African orogeny (600-500 Ma) leading to a complex metamorphism and to the formation of different grades of metamorphic rocks [3,4,5,6,7,8,9]. The intensity of tectonic activities contributed to the rocks weathering and thus allowed the accumulation of trace metals in the weathering products at different elevations. The North-Eastern part of the CAFB in Cameroon as other orogenic belts is characterized by an important mineralization potential [9,10,11,12,13,14,15,16,17,18,19,20] which resulted from hydrothermalism. However, previous works on metallogenic products are mainly based on the use of basement rocks and alluvial deposits for prospection [10,11,13,14,15,16,17,18,19,21,22]. Meanwhile, the use of residual deposits derived from weathered products to prospect mineral and ore deposits is not well-documented [23,24]. Weathering processes and related chemical elements mobilization are premonitory precursors of some chemical element accumulation in weathered products and the occurrence of residual ores deposits on the Earth surface [11,12,25,26]. In this paper, we intend to propose a model for the future of mineralization identified in fresh rocks during weathering processes in Meїganga. This work will allow to clarify the implication of weathering intensity and trends into the processes inducing lithogenic concentrations of useful elements into the weathering products in tropical area which belong to anorogeny belt. Hence, we propose a model of residual ore deposits prospection based on major oxides used as indicators of useful elements mineralization indices in weathering products. Therefore, seven weathering profiles developed on four distinct mineralized rocks in Meїganga have been used to trace the pathway of mineral and chemical elements mobilization in one hand and their evolution in the related weathering products. Furthermore, we determine the weathering trends and intensity as well as the amount of useful elements mobilized.

2. Geomorphology and Geological Setting of Meїganga

The study area is located in the Adamaoua plateau (Central Cameroon). It covers a surface area of about 12245 km². Geomorphologically, the Meїganga area is situated at the Southern border of the Adamaoua plateau which corresponds to a wide-range of lateritic paleo- surfaces with a mean altitude of 1,400masl (Figure 1A and Figure 1B). The Digital Elevation Model (DEM) shows three main morphological landscapes in Meїganga (Figure 1C): the Upper, the Intermediate and the Lower landscapes.

Figure 1. Geomorphology and Location map of the Meїganga study area. A. The main lateritic plateaus of Cameroon. B. Cross sections along the Adamaoua plateau. C. Digital elevation model (DEM) of the Meїganga study area. D. Topographic map of the Meїganga study site

Figure 2. Geological sketch of the Meїganga study area. A. Thrust belt between São Francisco, Congo and West African Cratons. B. Map of Cameroon showing the Cameroon section of the African shear zone (CSASZ). C. Geological map of the Meїganga study area. D. Synthesizing cross section of geology in the study area

The Upper landscape, with altitudes ranging between 1,200 and 1,500masl, is observed in the North-Eastern and the Northern borders of the study area. It covers a surface area of approximately 1,325 km², representing 10.8% of the study area.

The Intermediate landscape, with altitudes ranging between 900 and 1,200masl, covers a surface area of 10,044 km² and represents 82.0% of the study area. Thus, it represents the most wide-spread landscape and may be considered as the representative landscape of the Meїganga study area.

The Lower landscape, with altitudes ranging between 600 and 900masl, is located exclusively along the Lom River valley in the South, and the Vina River valley in the North-East. It covers a surface area of 876 km², representing 7.2% of the study area.

The Meїganga study area belongs to the north-eastern part of the CAFB in Cameroon. This belt is considered as resulting from the convergence of three main cratonic blocs: the São Francisco- Congo Craton (SFCC), the West African Craton (WAC) and the Saharan meta-craton Bloc (Figure 2A and Figure 2B). The rocks in this area were subjected to different degrees of metamorphism, leading to the formation of metamorphic rocks of different grades [3,4,5,6,8,9,27]. In Meїganga, different metamorphic rocks have been described (Figure 2C) including: orthogneiss, amphibolites, micaschists, mica bearing quartzites, granites, corneenes, quartzite veins and mylonitic zones [3,4,28,29]. A synthesis of the cross-section of the geology of Meїganga (Figure 2D) has helped to establish the geological history of the study area. First, the Meїganga region was affected by syn-, late- and post-tectonic granitic intrusions hosted in metamorphic rocks [19,30]. Metamorphic host rocks comprise amphibolites, meta-sediments made up of micaschists and quartzites (700-1000Ma) and orthogneiss dating back to 2.1 Ga. The entire host rocks were reworked during the Pan African orogeny [3,8,9,19,28,29,31,32]. Th-U-Pb monazite dating indicates an emplacement age of around 615±27 Ma for granitic intrusions in this region and 575±8 Ma for the mica-bearing granites.

Previous studies in this area have shown that mineralizations are hosted within micaschists, orthogneiss, granites, and the quartzite veins which correspond to the basement rocks [9,10,13,14,21,22]. Mineral associations in these rocks have been reported by [3,28,29]. They noted that orthogneiss are made up of quartz, K-feldspar, plagioclase, biotite, amphibole, and opaque minerals, (apatite, zircon, titanite and pyrites) occurring as accessory minerals. Micaschists are made up of quartz, biotite and feldspar, with calcite, epidote, pyrite and opaque minerals as accessory minerals. Granites also contain quartz, K-feldspar, plagioclase, biotite and muscovite, with apatite, titanite, zircon and pyrites as accessory minerals, whereas chlorite, calcite, epidote and opaque minerals behave like secondary minerals resulting from hydrothermal weathering. The quartzite veins contain mainly quartz and muscovite, but biotite, plagioclase and opaque minerals can be present in small amounts. Micaschists and the quartzite veins in this area have been reported to be mineralization bearing such as graphite (C), gold (Au), Zinc (Zn), Copper (Cu), lead (Pb), Barium (Ba), tungsten (W), Arsenic (As), uranium (U) and diamond (C) [10,13,22]. Granites were also admitted to be holders of gold (Au), sapphire (Al₂O₃), monazite [(Ce, La, Nd, Th)PO₄], silver (Ag), molybdenum (Mo), lead (Pb), tin (Sn), manganese (Mn), arsenic (As) and uranium (U) [13,14]. The presence of pyrite (FeS₂) in mica schists, granites and orthogneiss may be indicative of their substantial mineralizations [9,21,22].

3. Methods of Study

Seven soil profiles representing the main weathering mantles developed on four mineralized rocks were hand-dug and described macroscopically [33]. A total of twenty-seven soil samples were collected and crushed into fine powder (<2mm) for mineral and geochemical analysis. Mineral identifications were performed by the X-ray diffraction (XRD) method using PW 1800 diffractometer upon the following conditions: cobalt anode (K-Alpha), wavelength, λ:1.79 Å, scanning speed: 0.033°/s, scan range: 5-78°, drive axis 2θ. The relative amounts of minerals were quantified from X-ray diffraction patterns using the Rietveld refinement method [20]. Major element contents were carried out by X-ray Fluorescence (XRF) using a Panalytical Axios advanced PW 4400 fluorescence spectrometer. Trace and Rare Earths (RE) Elements were determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) after three progressive acid [34]. The REEs fractionation was performed using fractionation index a; the Ce/Ce* and (La/Yb)N ratios, as well as the REEs normalization relative to CI-chondrite [35].

The weathering trends and intensity were assessed using the chemical index of alteration (CIA; [36]); the Weathering Index of Parker (WIP; [37]); the Total reserve in bases (TRB; [23]); and the index of lateritization (IOL; [38]). Correlations between major, trace and RE elements were performed using the Spearman correlation function [39] combined with the Principal Components Analysis (PCA) of the outcome correlation matrix. Indeed, the PCA is a powerful statistical method that helps to reduce the number of appropriate variables describing variation within a set of data. This is done by assessing linear combinations of the variables (chemical elements), which describes the distributions and relationships of the data. Ideally, each component might have been used to describe the geological process such as weathering and mineralization [40,41]. In this study, factor analysis with PCA performed using Statistica (v. 7.0) software, allowed reducing the size of the space of the variables [42]. Thus, the PCA main goal is to provide a small number of independent factors (principal components) which synthesize the associations between variables. The first factor (Fact 1) explains a major part of the total variance of the data set, and the other successive factors explain a smaller part of the remaining variance. The different factors are then related to common processes that affect the variables. The number of significant principal components for interpretation is selected on the basis of the Kaiser criterion with an eigenvalue higher than 1 [24] and a total of variance equal to or higher than 85%.

4. Results

4.1. Macroscopic Organization of the Meїganga Weathering Mantles

Soil profiles for weathering mantles developed on micaschists (P1M, P2M and P3M), orthogneiss (P1O and P2O), granite (PG), and quartzite vein (PQ) are presented in Figure 3.

4.1.1. Weathering Mantle Developed on Micaschists

The representative soil profiles are developed at different locations and altitudes; a profile on the top of an interfluve at 949 m (P1M), a profile on a lowland at 934 m (P2M) and a profile on the footslope (P3M) at 931 m (Figure 3A). Randomly disperse rock fragments are frequently noted in lowlands and footslopes. Almost every soils profiles in this group embody less than 4m thick shallow weathered A/B/C soils profiles. In these soil profiles, the surface horizon A corresponds to a moderate thick (20-30 cm thick) soils horizon made up of dark brown (10YR2/1) silty clayey to sandy-clayey soil matrix with crumbly structure. It grades downward to the underlying subsurface horizon B₁₁, made up of a fairly to strongly thick (70 to 130 cm thick) soil horizon with light reddish brown (2.5YR7/4) silty clayey soil matrix with fine polyhedral to coarse prismatic structure. In P1M and P3M, the subsurface horizon B₁₁ grades downwards to a weathered horizon BC characterized by dark yellowish brown (10YR4/6), sandy-clayey and massive saprolitic boulders embedded into reddish yellow (7.5YR7/8) and sandy loam soil matrix with coarse polyhedric structure. In this weathered horizon BC, the amount and size of the saprolitic boulders increase with depth and demarcate an almost continuous weathered horizon C. In all these soils profiles, the continuous weathered horizon C is made up of yellowish brown (10YR5/6) and more or less weathered rocks volumes embedded into a pinkish white (7.5YR8/2), clayey silt, massive and continuous saprolitic material.

4.1.2. Weathering Mantle Developed on Orthogneiss

The two profiles are located upslope (P1O) and downslope (P2O) at altitudes of 954 m and 951 m, respectively. Both profiles exhibit less than 4m thick shallow weathered A/B/C-type soil profiles. The surface horizon A corresponds to a slightly and moderately thick (10 to 30 cm thick) soil horizon with dark yellowish brown (10YR3/6) sandy loam and crumbly soil matrix, containing more or less weathered rock fragments. It grades downwards to the underlying brighter subsurface horizon B, made up of less than 100 cm thick soil horizon, with yellow (10YR8/8) and clayey sandy soil matrix, displaying a polyhedric to prismatic structure. In P1O, the subsurface horizon B grades downward to a weathered horizon BC, with yellowish brown (10YR5/6), sandy loam and massive saprolitic boulders embedded into reddish yellow (7.5YR7/8) and sandy loam soil matrix with coarse polyhedric structure. The amount and size of the saprolitic boulders increase with depth and demarcates an almost continuous weathered C horizon. In both soil profiles, the continuous weathered horizon C display a more or less weathered rock volumes embedded in a pinkish white (7.5YR8/2), clayey silty, massive and continuous saprolitic material.

Figure 3. Macroscopic organization of the Meїganga Soils profiles. Soils profiles developed on, A. Micaschist; B. Orthogneiss; C. Granite; D. Quartzite veins

4.1.3. Weathering Mantle Developed on Granite

The soil profile (PG) developed on granite is located on the foot of a rocky hill at an altitude of 974 m. It exhibits a 4 m thick shallow weathered A/B/C-type soil profile. The surface horizon A is made up of a moderate thick (20 cm) soil with dark grey (7.5YR3/1), sandy loam and crumbly soil matrix containing more or less weathered rock fragments. The surface A horizon grades downward to the underlying brighter subsurface horizon B with 190 cm thick soil horizon, displaying a yellow (10YR8/6), sandy-clayey and polyhedric soil matrix. The subsurface horizon B grades downwards to the weathered horizon BC with yellowish brown (10YR5/6), sandy loam and massive saprolitic boulders embedded into a soil matrix made up of a yellow (10YR8/6), sandy-clayey and polyhedric fine earth. The saprolitic boulders become larger with depth and exhibit a continuous weathered horizon C with more or less weathered rock fragments embedded in pinkish grey (5YR7/2) saprolitic materials.

4.1.4. Weathering Mantle Developed on Quartzite Veins

The soil profile (PQ) developed on quartzite veins is located on a slightly undulating landscape with randomly dispersed rock fragments at an altitude of 933 m. It exhibits a 90 cm thick shallow weathered A/C-type soils profile.

The surface horizon A is a thin (10cm thick) soil horizon with dark-yellowish brown (10YR3/6), sandy loam and crumbly soil matrix with fragments of quartzite. It grades downwards to a continuous weathered horizon C with more or less weathered saprolitic materials with abundant quartzite fragments.

To summarize, the weathering mantles in Meїganga exhibit shallow weathered A/B/C or A/C-type soil profiles with a thickness not exceeding 4 m. Almost all these weathering mantles differentiate moderately to thin surface horizon A (10-30 cm), that lies on a thick subsurface horizon B (70 to 130 cm), with the exception of those developed on quartzite veins, where the surface horizon A is lying directly on the weathered horizon C. The weathering levels are made up of dominantly moderate thick (80 to 300 cm thick) weathered horizon C locally linked to the subsurface horizon B by a relatively thin (50-100 cm) weathered horizon BC. Moreover, all the weathering profiles in this area are located between altitudes 930 and 975 m, that represents the Intermediate landscape of the Meїganga study area.

4.2. Mineral Contents and Their Distribution in the Study Area

Mineral contents and their distribution in the seven soils profiles are summarized in Figure 4.

4.2.1. Weathering Profiles Developed on Micaschists

The mineral association of weathered profiles developed on mica schist is made up of the following minerals in decreasing abundance (%); quartz (67.0-89.2), kaolinite (8.2-12.7), goethite (1.9-13.0), hematite (0.5-8.5), phlogopite (0.7-5.0), and smectite (0.7-4.3). the abundances of quartz and kaolinite increases upward in all the soil profiles with increasing ratios of 0.07% and 0.29%, respectively. The content of hematite increases upward only in P1M (maximum increasing ratio: 1.38), while P2M and P3M profiles are characterized by alternating increasing and decreasing values. On the other hand, phlogopite and smectite abundances decrease upward the soil profiles with maximum decreasing values of 0.85% and 0.77%, respectively. Goethite is observed only in P2M soil profile.

4.2.2. Weathering Profiles Developed on Orthogneiss

The mineral association of weathered profiles developed on orthogneiss includes the following minerals in decreasing abundance (%); quartz (71.5-95.5), kaolinite (3.8-22.3), halloysite (0.2-4.6), goethite (2.0-2.3), and smectite (0.7). Quartz contents generally increases upward (maximum increasing ratio 0.13), while kaolinite and halloysite contents decrease upward, with the maximum decreasing ratios of 0.49 and 0.89, respectively. Smectite appears in a very low amount (0.7%) in the surface A horizon in profile P1O. On other hand, goethite presents a complex characteristic in profile P2O with alternation of increasing and decreasing contents.

Figure 4. Minerals detected by XRD and their relative contents in the Meїganga weathered products

4.2.3. Weathering Profile Developed on Granite

The mineral association of weathered profiles developed on granite includes the following minerals in decreasing abundance (%); quartz (60.3-93.9), kaolinite (0.8-21.1), phlogopite (0.2-15.3), hematite (1.7-3.2), and smectite (0.5-3.2). Quartz and kaolinite contents increase upward, with the maximum increasing ratios of 0.13 and 25.38, respectively. In contrary, hematite, phlogopite and smectite contents decrease upward, with the maximum decreasing ratios of 0.47, 0.99, and 0.47, respectively.

4.2.4. Weathering Profile Developed on Quartzite Veins

The mineral association of weathered profiles developed on quartzite veins include the following minerals in decreasing abundance (%); quartz (87.1 - 93.2), kaolinite (5.9-12.4), halloysite (0.2-1.2), smectite (0.7) and hematite (0.5). Smectite and hematite are observed in a low amount (0.7% and 0.5%, respectively) in the surface A horizon and the weathered horizon C respectively. Quartz contents increases upwards with maximum increasing ratio of 0.07.

Table 1. Major elements contents and some weathering parameters in the Meїganga weathering profiles; TRB, total reserve in bases; CIA, chemical index of alteration; IOL, index of lateritization; Ki, silica exportation ratio; WIP, Weathering index of Parker

4.3. Geochemistry

4.3.1. Major Elements Contents and Related Weathering Parameters

The major element data for the weathered mantles are presented in a Table 1.

In the profiles developed on mica schist, the contents of SiO₂, alkaline and alkaline earths in decrease upward. In contrary, the abundances of Al₂O₃, Fe₂O₃ and TiO2 increase upward. About the weathering parameters, TRB and WIP contents are decreasing upward (TRB: from 206 to 11mg.kg⁻¹; WIP: from 44 to 4%) and CIA values are above 70% (CIA: 74 - 92%) and increase upward. The silica exportation ratio above 1 (Ki: 4 - 9) and the index of lateritization (IOL) is below 30% in almost all the weathering products (IOL: 18 - 29%).

In the profiles developed on orthogneiss, SiO₂ and Fe₂O₃ show alternate increasing and decreasing contents up the profile. Like those developed on mica schist, alkaline and alkaline earths, with TiO₂ contents moderately decreases upward. On the contrary, Al₂O₃ contents increase steadily upward. The TRB and WIP values also decrease highly upward (TRB: from 271 to 37mg.kg⁻¹; WIP: from 46 to 11%) and CIA values are still above 70% (CIA: 74 - 87%). Once again, the silica exportation ratios are above 1 (Ki: 3 - 6) and the index of lateritization (IOL) is below 40% in all the weathering products (IOL: 18 - 39%).

In the profiles developed on granite, SiO₂, Fe₂O₃ and TiO₂ contents increase upward, while the abundances of alkaline and alkaline earths decrease conspicuously up the profile. The TRB and WIP values also decrease strongly upward (TRB: from 416 to 43mg.kg⁻¹; WIP: from 56 to 9%) when CIA values are above 60% (CIA: 65 - 79%). The silica exportation ratios remain above 1 (Ki: 2 - 5) and the IOL values is still below 40% (IOL: 24 - 37%) in these weathering products.

In the profiles developed on quartzite veins, SiO₂ contents decrease upward while the abundances of Al₂O₃, Fe₂O₃, TiO₂, MgO, Na₂O and K₂O increase upward. The TRB and WIP values suddenly increase upward (TRB: from 2 to 100mg.kg⁻¹; WIP: from 0 to 29%) when the CIA values are highly increase (from 84 to 90%) than the values obtained in the other weathering product. The silica exportation ratios remain above 1 (Ki: 4) and the IOL are values below 25% (IOL: 24%) in the weathering products.

In the soil profiles studied in Meїganga, major oxides behave differently. SiO₂ contents globally decrease upward in the weathering profiles from micaschists, quartzite veins, and orthogneiss, but slightly increase in granite. Al₂O₃, Fe₂O₃ and TiO₂ contents increase upward, except in the weathering profiles from granite and orthogneiss. Alkaline and alkaline earths contents decrease upward, except in the weathering profile from the quartzite veins with increasing contents. Chemical weathering parameters indicate that TRB and WIP values decrease upward in all the weathering in the exception of the weathering products developed on quartzite vein where they suddenly increase upward. Generally, the CIA values are above 65% (CIA: 65 - 92%) when the IOL do not exceed 40% and the silica exportation ratio remain above 1 (Ki: 2 - 9) in all the weathering products.

4.3.2. Trace Elements Contents

Trace element abundances (ppm) for samples collected from different soil profiles in Meїganga are presented in Table 2.

In the weathering products from micaschists, trace elements in the studied profiles show characteristics that can be generally grouped into increasing or decreasing trends. The trace element behaviors are not the same in all the profiles. In soil profiles developed on mica schist, trace elements with increasing abundance up the profile include; V, Cr, Ni, Sb, and Pb, while those with decreasing abundance up the profile include; S, Zr , Y, Nb, Ga, Hf, Sn, Mo, and U. The most abundant useful elements is Cr (63 - 289ppm), followed by Zr (85 - 268ppm), V (20 - 225ppm), Sb (1 - 62ppm), Cu (4 - 61ppm), W (9 - 44ppm), Nb (4 - 20ppm), Mo (0 - 9ppm) and Hf (2 - 5ppm).

In the weathering products from orthogneiss, trace elements with increasing contents up the soil profiles include; Y, Pb, Th, Ga, Sc, Nb, Cs, U and Mo, while trace elements showing a decrease in abundance up the profile include; S, Ba, Zr, Sr, Rb, Zn, Cu, Ni, Li, Co, Hf, W and Sn. The V and Cr display alternately decreasing and increasing contents. Useful elements contents in decreasing abundance are: Zr (215 - 383ppm), V (22 - 175ppm), Cr (62 - 170ppm), Cu (15 - 40ppm), Nb (7 - 21ppm), Hf (6 - 8ppm), Mo (2 - 5ppm) and W (1 - 3ppm).

In the weathering products from granite, most of the trace element abundances Zr, V, Cr, Y, Cu, Ni, Co, Ga, Nb, Th, Sc, Sn, Hf, U, Mo and W increase upward, while some decrease upward (Ba S, Sr, Rb, Li, Pb and Cs). Useful elements contents in decreasing abundance are: Zr (232 - 536ppm), V (116 - 175ppm), Cr (94 - 151ppm), Cu (15 - 40ppm), Nb (17 - 31ppm), Hf (5 - 11ppm), Mo (1 - 3ppm) and W (0 - 3ppm).

In the weathering products from the quartzite vein, almost all the trace elements display increasing contents. They are, in decreasing abundance: Ba, Zr, Rb, S, Cr, V, Pb, Zn, Sr, Nb , Ga, Ni, Sb, Y, Th, Cs, Cu, Li, Sc, Co, Sn, Hf, U, Mo and W. No trace of Li, Sc, Ga, Y, Nb, Cs, Hf, W, Th and U were noted in fresh rock. Useful elements contents in decreasing abundance are: Zr (135 - 182ppm), Cr (64 - 90ppm), V (29 - 66ppm), Nb (20 - 23ppm), Sb (4 - 19ppm), Cu (4 -11ppm), Hf (4ppm), Mo (1 - 2ppm) and W (2ppm).

Thus, trace elements that prevail in the Meїganga weathering products are, in decreasing order: S, Ba, Sr, Zr, Cr, V, Zn, Rb, Ni, Y, Sb, Cu, Pb, Li, Co, Ga, Nb, Th, Sc, Cs, Hf, Sn, U, Mo, and W. Traces elements enriched are: Sc, V, Cr, Sb, Cs and Pb. Trace elements depleted are: S, Rb and Sr. Useful elements that were identified in the Meїganga weathering products are, in decreasing contents: Zr (2 - 547ppm), Cr (44 - 289ppm), V (7 - 225ppm), Sb (1 - 62ppm), Cu (4 - 61ppm), Nb (4 - 31ppm), Hf (2 - 12ppm), Mo (0 - 9ppm) and W (0 - 7ppm). Thus, the most significant useful elements in the Meїganga weathering products are Zr, Cr and V.

4.3.3. REEs Contents and Fractionation

In the Meїganga weathering products, the REEs contents and related fractionation parameters are reported in Table 3. Besides, the REEs normalized patterns of the related weathering products are shown in Figure 5.

In the weathering products from micaschists, the total contents of the REEs vary from ∑REEs: 76 to 272ppm, in which LRREs display the highest amounts (∑LREEs: 67 to 256ppm) (Table 3). The most abundant REE is Ce (29 - 110ppm), followed by La (26 - 71ppm), Nd (14 - 63ppm) and Pr (4 - 17ppm). The contents of these REEs decrease upward. Fractionation index a, vary from 5 to 16, suggesting important accumulation of LREEs in these weathering products compared to HREEs. The (La/Yb)N ratios from 5 to 44 show evidence of intense fractionation of the REEs in these weathering products. The Ce/Ce* ratios which refer to Ce anomaly vary from 0.3 to 1.2. This is consistent with both positive and negative Ce anomaly prevailing in these weathering products as confirmed by the normalization patterns in Figure 5A. The Eu/Eu* ratios related to Eu anomaly are below 1 (Eu/Eu*: 0.4 to 0.9), giving evidence of negative Eu anomaly in these weathering products as confirmed by the normalization patterns (Figure 5A). These weathering products are depleted in REEs compared to CI-Chondrite [35]. In addition, REEs are more or less highly fractionated; with fractionation intensity increasing from LREEs to HREEs as shown by the progressive scattering of the normalization patterns rightward (Figure 5A).

In the weathering products from orthogneiss, the total contents of the REEs vary from ∑REEs: 210 to 345ppm, in which LRREs display the highest amounts (∑LREEs: 189 to 316ppm) (Table 3). The most abundant REE is still Ce (72 - 130ppm), followed by La (38 - 84ppm), Nd (34 - 70ppm) and Pr (9 - 20ppm). The contents of the REEs in these weathering products increase upward. Fractionation index a, 7 to 12, suggests important accumulation of the LREEs in these weathering products compared to the HREEs.

Table 2. Trace elements contents in the Meїganga weathering profiles

Table 3. REE contents, fractionation and normalization parameters in the Meїganga weathering profiles

The (La/Yb)N ratios from 6 to 21 give evidence of intense fractionation of the REEs in these weathering products. The Ce/Ce* ratios vary between 1.5 and 2.7. This is consistent with positive Ce anomaly prevailing in these weathering products, as confirmed by their normalization patterns (Figure 5B). The Eu/Eu* ratios are above 1 (Eu/Eu*: 1.4 to 1.9), giving evidence of positive Eu anomaly in almost all these weathering products (Figure 5B). All these weathering products are much more enriched in REEs with respect to CI-Chondrite. Thus, the REEs in these weathering products are highly fractionated compared to CI-Chondrite; but the fractionation intensity decreases from the LREEs to the HREEs as indicated by the strengthening tendency of the normalization patterns rightward (Figure 5B).

In the weathering products from granite, the total contents of the REEs vary from ∑REEs: 182 to 474ppm, in which LRREs display the highest amounts (∑LREEs: 166 to 444ppm) (Table 3). The most abundant REE remains Ce (76 - 273ppm), then La (33 - 91ppm), Nd (38 - 83ppm) and Pr (10 - 22ppm). The contents of the REEs in these weathering products increase upward. Fractionation index a, varies from 11 to 15, suggesting important accumulation of the LREEs in these weathering products. The (La/Yb)N ratios also vary from 18 to 31, giving evidence of strong fractionation of the REEs in these weathering products. The Ce/Ce* ratios vary between 1.4 and 2.7. This is consistent with strong positive Ce anomaly in these weathering products, as shown in their normalization patterns (Figure 5C). The Eu/Eu* ratios vary from Eu/Eu*: 0.9 to 1.4, indicating alternately positive and negative Eu anomaly in these weathering products (Figure 5C). These weathering products are more or less enriched in REEs with respect to CI-Chondrite (Figure 5C). Thus, REEs in these weathering products are slightly fractionated compared to CI-Chondrite (Figure 5C).

In the weathering products from the quartzite veins, the total contents of the REEs vary from ∑REEs: 191 to 228ppm, in which LRREs display the highest amounts (∑LREEs: 178 to 213ppm) (Table 3). The most abundant REE remains Ce (85 - 99ppm), followed by La (39 - 48ppm), Nd (36 - 45ppm) and Pr (10 - 12ppm). Contents of the REEs in these weathering products strongly increase upward. Fractionation index a, varies from 15to 16, highlighting important accumulation of the LREEs in these weathering products. The (La/Yb)N ratios also vary from 34 to 44, giving evidence of strong fractionation of these REEs. The Ce/Ce* ratios vary between 69.7 and 80.9. This is consistent with strong positive Ce anomaly. The Eu/Eu* ratios vary from Eu/Eu*: 32.6 to 40.4, suggesting strong positive Eu anomaly. These weathering products are highly enriched in REEs with respect to CI-Chondrite (Figure 5D). Thus, the REEs in these weathering products are also strongly fractionated compared to the CI-Chondrite (Figure 5D).

Finally, in the Meїganga weathering products, proportions of the REEs vary from ∑REE: 76 to 474ppm, with the highest contents assigned to the LREEs (∑LREE: 67-444ppm). The most abundant REEs are in decreasing contents: Ce (29 - 273ppm), La (26 - 91ppm), Nd (14 - 83ppm) and Pr (4 - 22ppm). These REEs contents in the Meїganga weathering profiles increase upward, except in micaschist with decreasing contents upward. Thus, in the Meїganga weathering products, there is an important accumulation of the LREEs compared to the HREEs, and positive Ce and Eu anomalies, except in micaschists with negative anomalies. In Meїganga, almost all the REEs are strictly enriched in the weathering products with respect to CI-Chondrite, in the exception of those derived from micaschists in which the REEs are more or less depleted. the REEs are strongly fractionated in the weathering products, with the fractionation intensity increasing from the LREEs to the HREEs.

Figure 5. REEs/CI-Chondrite [35] normalized patterns in the Meїganga weathering profiles. Weathering profiles developed on: A. Micaschist; B. Orthogneiss; C. Granite; D. Quartzite vein

Figure 6. The PCA aggregation circles showing the relationship between major, trace and RE elements in the Meїganga weathering products, Weathering mantles developed on, A. Micaschist; B. Orthogneiss; C. Granite; D. Quartzite veins

4.3.4. Affinity between Chemical Elements

In the weathering products, the affinity between chemical elements was defined by using the principal components analysis (PCA). Consequently, the PCA aggregation circles showing the relationship between major, trace and RE elements in the samples are presented in Figure 6.

In the weathering products derived from micaschists, two major parameters (factors) accounting for 63.17% of the total variance have been defined (Figure 6A). Factor 1 expressing 37.37% of the total variance (Figure 6A1), indicates that Ga, W, Y, Sn, Hf, Zr, Nb and all the REEs are positively correlated with TiO₂, suggesting their trapping into epidote or phlogopite since TiO₂ in these minerals behave like substituting element. Factor 2 expressing 25.80% of the total variance (Figure 6A2) points out groups of variables. The first group refers to Cu, Sc, V, Zn and Cr positively correlated with Fe₂O3 and P₂O₅, giving evidence of their likely incorporation into iron oxides (hematite or goethite), and apatite respectively. The second group is represented by Ba, Cs and Rb positively correlated with Al₂O₃ and K₂O. This may suggest that they are either trapped into clay minerals such as kaolinite or phlogopite (Al₂O₃) or simply leached during weathering (K₂O).

In the weathering products derived from orthogneiss, two principal components (factors) accounting for 72.94% of the total variance were considered (Figure 6B). Factor 1 expressing 50.05% of the total variance, reveals two groups of variables (Figure 6B1). The first group is assigned to Y and all the REEs that are strictly enriched in the weathering products, suggesting their incorporation as substituting elements or impurities into residual minerals such as zircon or titanite. The second group refers to Sr, S, Zn, Ba, W, Hf, Li and Zr positively correlated with CaO, Na₂O, MnO and MgO, giving evidence of their depletion during weathering. Factor 2 expressing 22.44% of the total variance also defines two groups of variables (Figure 6B2). The first group is represented by Cr, Nb, Ga, Sc, Th and V positively correlated with Fe₂O₃, Al₂O₃ and TiO₂, suggesting their expected incorporation into hematite or goethite; kaolinite, smectite or halloysite; and titanite or zircon respectively. The second group refers to SiO₂ that is more or less depleted, highlighting relative development of clay minerals such as smectite, halloysite or kaolinite.

In the weathering products derived from granite, two principal parameters (factors) accounting for 81.90% of the total variance were considered (Figure 6C). Factor 1 expressing 53.62% of the total variance defines two groups of variables (Figure 6C1). The first group corresponds to Cr, Sc, Ni, Cu, W, Nb, Sb, Th, U, Y, Er, Tm, Yb and Lu which are correlated positively with TiO₂ and give evidence of their likely incorporation into titanite, epidote or phlogopite.

The second group refers to Sr that is correlated positively with P₂O₅, Na₂O and CaO. Therefore, Sr in these weathering products may be leached during weathering. Factor 2 expressing 28.28% of the total variance defines two groups of variables (Figure 6C2). The first group corresponds to Zr, Mo and Hf. These elements are correlated positively with SiO₂, suggesting their trapping as substituting elements or impurities into newly formed clay minerals such as smectite or kaolinite. The second group refers to V, Cs, Sn, Ga, Zn, Rb, Li, Ba, La, Pr, Eu, Nd and Sm which are correlated positively with Al₂O3 and MnO. This may be consistent with their incorporation into newly formed clay minerals such as smectite, kaolinite or phlogopite.

In the weathering products derived from the quartzite veins, two principal components (factors) accounting for 97.29% of the total variance were defined (Figure 6D). Factor 1 expressing 83.68% of the total variance, reveals two groups of variables (Figure 6D1). The first group refers to V, Ni, Cr, Co, S, Sr, Li, Pb, Sc, Zr, W, Y, Th, Hf, Nb, Sn, Ga, Zn, Cs and all the REEs. They are positively correlated with P₂O₅ and Na₂O, suggesting their leaching out during weathering. The second group refers to SiO₂ which is more or less depleted, favoring the development of newly formed clay minerals such as kaolinite, halloysite or smectite. Factor 2 expressing 13.61% of the total variance also defines two groups of variables (Figure 6D1). The first group corresponds to Rb and Ba positively correlated to MgO and K₂O. This may be indicative of their leaching out during weathering. The second group is represented by Sb, Mo and Cu which are positively correlated to Fe₂O₃, suggesting that they may have been trapped into residual opaque minerals.

Therefore, we have noticed that many trace elements (Ga, W, Y, Sn, Hf, Nb, Cu, Sc, V, Zn, Cr, Sb, Pb, Ni, Co, Li, Mo, Th, Rb, Cs, U, Zr and Ba), as well as almost every REEs are positively correlated with some majors elements. All the above major oxides are known to be premonitory for the presence of residual primary minerals (titanite, apatite, and zircon) or newly formed secondary minerals (phlogopite, hematite, goethite, kaolinite and smectite) in the weathering products.

4.3.5. Correlation between Major Oxides and Useful Elements

The correlation matrices between major oxides and identified noble metals are shown in Table 4.

Cr, Mo and U are strongly correlated with Fe₂O₃ and P₂O₅ oxides, with the correlation ratios varying from 0.87 to 0.88 for Fe₂O₃ and from 0.80 to 0.87 for P₂O₅. V, Cr, Nb and Mo are strongly correlated to Al₂O₃, Fe₂O₃ and TiO₂ oxides, with the correlation ratios from 0.86 to 0.91 for Al₂O₃, 0.81 to 0.90 for Fe₂O₃, and 0.83 to 0.91 for TiO₂ for weathering products derived from orthogneiss. V and Nb are strongly correlated with Fe₂O₃ (0.95) and TiO₂ (0.83) oxides respectively for soil developed on granite. In the weathering products derived from the quartzite veins, V, Cr, Zr, Nb, Mo, Hf, W, and U are strongly correlated to Fe₂O₃, TiO₂, P₂O₅, MnO, and Al₂O₃ oxides, with the correlation ratios varying from 0.80 to 1.00 for Fe₂O₃; 0.85 to 1.00 for TiO₂; 0.83 to 0.98 for P₂O₅; 0.92 for MnO; and 0.95 to 1.00 for Al₂O₃ respectively. Thus, according to the above correlation matrices, Cr, Mo, U, V, Nb, Zr, Sb, Hf and W, can be considered as useful elements having strong affinity (> 0.80) with major oxides like Fe₂O₃, TiO₂, P₂O₅, Al₂O₃ and MnO.

Based on the above assumption, each giving useful elements was later on used to draw up regression functions with the corresponding major elements in order to calculate the useful elements contents in each weathering product based on the major elements contents. The error factor between measured and calculated useful elements contents was used to draw up the distribution matrices of the theoretical errors between both contents (Figure 7).

Table 4. Correlation matrix between less mobile major elements and noble metals in the Meїganga weathering products

Figure 7A-C

Figure 7. Distribution matrix of theoretical errors between measured and calculated useful metals contents using major elements contents in the Meїganga weathering products. Weathering products from, A. Micaschist; B. Orthogneiss; C. Granite; D. Quartzite vein

Hence, the must appropriated major elements that may be used to estimate, Mo and Cr contents is Fe₂O₃, and U contents is P₂O₅ for soil samples associate to micaschists. In the weathering products derived from orthogneiss, the must appropriated major elements to be used to estimate, V and Cr contents is Fe₂O₃, Nb and Mo contents is Al₂O₃. For weathering products derived from granite, Fe₂O₃ and TiO2 are the most appropriated major elements that can be used to calculate V and Nb contents respectively. In the products derived from the quartzite veins, the most appropriated major element that can be used to calculate U, W and Nb contents is Al₂O₃; Zr and V contents are Fe₂O₃ and TiO₂; Cr contents is TiO₂; Hf contents are Al₂O₃, Fe₂O₃ and P₂O₅; Sb contents are Fe₂O₃ and P₂O₅, and Mo contents is Fe₂O₃. Definitely, in the Meїganga weathering products, major elements that can be used as tracers for the useful elements prospections are: Al₂O₃ for Nb, Mo, U, W and Hf; Fe₂O₃ for Mo, Cr, V, Zr, Hf and Sb; P₂O₃ for U, Hf and Sb; and TiO₂ for Nb, Zr, V and Cr.

5. Discussion

5.1. Weathering Pattern, Weathering Trends and Weathering Intensity in Meїganga

Rock type plays a fundamental role in weathering intensity, as well as the mobilization and concentration of chemical elements on the Earth surface. That can be used as suitable tools for analyzing and understanding trace metals concentrations in weathering products through its main evaluation parameters such as weathering pattern, weathering trend and weathering intensity [43,44,45]. This may be particularly interesting in Meїganga, a part of the mobile belt belonging to the Central African Fold Belt (CAFB) in Cameroon with varieties of mineralized rocks that has been submitted to a long-period of weathering under a hot and humid tropical climate.

Based on the weathering patterns in Meїganga, minerals association reveals high contents (up to 22.3%) and recurrence of kaolinite as the main newly formed mineral.

Thus, in Meїganga, the prevailing rocks weathering process seem to be strong hydrolysis during which part of silica, but almost all the alkali and alkaline earth elements are leached, causing in situ crystallization of 1:1 clay minerals preferably [46,47,48]. The global increasing contents of kaolinite upward in these weathering profiles may express continuous and progressive weathering.

Also, the recurrence of smectite in the Meїganga weathering products, even in very low amounts (0.7 to 4.3%), may suggest much localized low leaching microsites that allowed crystallization of 2:1 clay minerals preferably [7,49]. In addition, the persistence of iron oxides like goethite and hematite in these weathering products (goethite: 1.9 to 13.0%; hematite: 0.5 to 8.5%), supports the fact that lateritization process occurs in this area [50,51,52]. Thus, in Meїganga, rocks have been subjected to chemical weathering (hydrolysis) under a relatively high leaching milieu that favored crystallization of 1:1 clay minerals preferably in a hot and humid tropical climate conditions, inducing low lateritization process and the development of weathered A/C or A/B/C soil profiles globally not exceeding 4 m thick.

In order to assess weathering intensity and trend in Meїganga, valuable weathering parameters (TRB, WIP, CIA, and Ki), the SiO₂-Al₂O₃-Fe₂O₃ ternary diagram of Schellmann (Figure 8A) and the Laterites classification diagram of Bardossy and Aleva (Figure 8B) were used. In the studied area, TRB and WIP parameters decrease significantly (TRB: from 416 to 11mg.kg⁻¹; WIP: from 56 to 4%) whereas these same parameters increase (TRB: from 2 to 100mg.kg⁻¹; WIP: from 0 to 29%) in the weathering products developed on quartz veins. This may be attributed to contamination by alkaline and alkaline earths during lateral migrations since the weathering profiles derived from the quartz veins are located in lowlands (around 930 m high). The CIA values above 65% (CIA: 65 - 92%), and the silica exporting ratios largely above 1 (Ki: 2 - 9) in almost all the studied soils profiles attest an intense rocks weathering in this area. This intense rock weathering may result from oscillation periods of hot and humid tropical climate with abundant rainfall (> of annual rainfall) that has affected the Adamaoua plateau between Eocene and Holocene, keeping the subsoil into permanent humid conditions favorable to intense leaching in a well-drained milieu.

Thus, the weathering products observed in the upland between 949 and 974 masl (weathering products developed on granite, micaschist, and in some extend orthogneiss) will be progressively eroded to lowland where prevailed below 933 masl the weathering products developed on quartz veins. This statement may justified the alkaline and alkaline earths contamination of the weathering profiles derived from the quartz veins during lateral migrations. In fact, the weathering profile developed on quartz veins is located at the bottom of the catena and soil material present in this place is of allogenic origin (erosion of soils situated above this profile, probably micaschists).

The SiO₂-Al₂O₃-Fe₂O₃ ternary diagram of Schellmann (Figure 8A) shows that almost every weathering products are observed near the SiO₂ limit, between the kaolinization domain and the non-defined domain very close to the SiO₂ edge. Rocks weathering does not exceed the kaolinization stage. Moreover, in the Laterites classification diagram of Bardossy and Aleva (Figure 8B), almost all the Meїganga weathering products are located very closely to the Kaolinite-Hematite (Goethite) boundary, between the Kaolin and the Ferruginous Kaolin domains. Thus, kaolinization remains the major soil forming process, whatever the rock. This very early stage of lateritization known as kaolinization in Meїganga may result from progressive and continuous rejuvenation of these weathering mantles during arid periods that affected the Adamaoua plateau between Eocene and Holocene [53]. These arid periods seem to have favored an intense dissection and incision, followed by the important striping of materials by the major rivers draining this area (Sanaga, Benue, Logone).

5.2. Affinity between Major Oxides and Identified Useful Elements in Meїganga: Implications for Residual Mineralization Indices

Affinity between major elements and traces elements is a useful tool to better constrain the useful elements behavior during weathering, given that most of the less mobile major oxides are the main constituents of primary and newly formed secondary minerals, and useful elements are regularly incorporated within those minerals as substituting elements or impurities [54,55,56,57,58]. In the Meїganga weathering products, TiO₂, Al₂O₃, Fe₂O₃, and P₂O₅ are the major oxides known to have high affinity with identified useful elements, namely, Nb, Zr, V, Cr, Mo, U, W, Hf, and Sb. For instance, TiO₂ is closely related to Nb, Zr, V, and Cr; Al₂O₃ to Nb, Mo, U, W and Hf; Fe₂O₃ to Mo, Cr, V, Zr, Hf and Sb; and P₂O₅ to U, Hf and Sb (Figure 7). All the above major oxides controlled the presence of residual primary minerals (titanite, apatite, and zircon) or newly formed secondary minerals (phlogopite, hematite, goethite, kaolinite and smectite) in the Meїganga weathering products. The high affinity between these major oxides and identified useful elements may give evidence of their likely incorporation into the crystalline lattice of these primary or newly formed minerals [2,38,39,59,60,61,62], which can correspond to prospective mineralization indices in some weathering products. TiO₂, Al₂O₃, Fe₂O₃, and P₂O₅ oxides can be used as indicators for the identification of useful elements prospection at large scale, since their concentrations in weathering products are closely related to the associate useful elements contents.

In addition, the Meїganga weathering products derived from the quartzite veins have the highest numbers of identified useful elements: U, W, Zr, Cr, V, Hf, Sb, Nb, and Mo; followed by the weathering products from orthogneiss (V, Cr, Nb, Mo); micaschists (Mo, U, Cr); and granite (V, Nb). The weathering products with large number of useful elements proceed from weathering products (weathering products derived on quartzite veins) located at lowlands, while those with very low numbers of useful elements derived from the weathering products (weathering products derived on granite) located at the upper landscape. The increasing numbers of useful elements in lowlands may result partly from downward migration during erosion.

Figure 8. Weathering parameters used in the Meїganga weathering products, 8A. SiO₂-Al₂O₃-Fe₂O3, ternary diagrams illustrating the different stages of alteration (after [52]). 8B. Classification of laterites (after [63])

Table 5. Clarks of concentrations of the identified noble metals in the Meїganga weathering products and related residual mineralization indices

This downward migration may contribute to useful elements accumulation in lowlands, with expected mineralization of the corresponding weathering products [64,65,66]. It might be worth to note the extremely low or totally absent of these useful elements contents in the fresh rock (quartz veins) (V: 7 ppm; Cr: 44 ppm; Cu: 8 ppm; Zr: 2 ppm; Nb: 0 ppm; Mo: 1 ppm; Sb: 4 ppm; Hf: 0 ppm; W: 0 ppm; and U: 0 ppm) than those reported in the weathering products. This may be the consequence of the lowlands position of these weathering mantles that facilitates their migration and accumulation.

The presence of U in the weathering products derived from both quartz veins and micaschist may suggest that micaschists could be the single rock-source for U in Meїganga. Other useful elements (Cr, Mo, V, Nb, W and Hf) may have originated from orthogneiss, mica schist and granite, when Zr, and Sb seem to be inherent for the quartz veins.

Likewise, in the Meїganga weathering products, the Clarks of concentrations considered as enrichment factor for a given element compared to its concentration in the Earth crust (Clark) (Table 5), were used to assess the useful elements enrichment in the Meїganga weathering products, and thus their expected mineralization indices. So, the Clarke of concentrations of useful elements above 1 for: Sb (60.00), W (1.60), Hf (1.33), U (1.48), Mo (1.67) and Nb (1.10) in soil profile on the quartzite veins; for U (1.11 - 1.48), Cr (1.49 - 1.56) and Mo (2.50 - 4.17) in soil profiles on micaschist; for Cr (1.10 - 1.21) and Mo (1.67 - 3.33) in soil profiles on orthogneiss; and for Nb (1.05 - 1.55) and V (1.23 - 1.27) in soil profile on granite suggest their accumulation these soil profiles. Thus in Meiganga, these soil profiles may be compared to the potential domains for these useful elements to be concentrated [67]. Therefore, in the Meїganga weathering mantles, mineralization indices could be assigned to the weathered horizon C, BC and B. Mineralization indices in weathering products from weathered horizons C and BC may suggest that part of these useful elements accumulated during the first stages of weathering. In contrary, mineralization indices in weathering products from mineral horizon B may indicate that metals were trapped preferentially into the newly formed secondary minerals. However, in Meїganga, very low number of expected mineralization indices were reported in the weathering mantles derived from granite and orthogneiss. Considering their location in the upper landscape, this could be explained by intense erosions that affected the Adamaoua plateau, favoring the exportation of these metals from the upper landscapes to the lowlands [64,65,66]. Nevertheless, this relative accumulation of exported metals in lowlands remains transient. Their final destination seem to be the alluvial deposits along the rivers terraces in which important concentrations of these metals were reported [16,17].

6. Conclusion

In Meїganga, morphological, mineralogical and geochemical analyses of seven weathering profiles were used to understand the weathering process that may control the occurrence of mineral and chemical elements in weathering products, and eventually expected residual mineralization indices in this area. Based on the above weathering products; mineral associations; chemical elements mobility; correlation between major, trace and REEs; and affinities between major oxides and some metals lead to the following conclusions.

(i)Rocks have undergone chemical weathering (hydrolysis) under a relatively well-drained milieu that favored crystallization of 1:1 clay minerals preferably in a hot and humid tropical climate conditions inducing low lateritization process and the development of weathered A/C or A/B/C soil profiles globally not exceeding 4 m thick.

(ii)Very early stage of lateritization known as kaolinization remains the major soil forming process, whatever the rock type. Nevertheless in Meїganga, weathering mantles are on progressive and continuous rejuvenation. The upper weathering products are progressively eroded and accumulate in the lowland position.

(iii) In the weathering products, almost all the REEs and a large number of trace elements (Ga, W, Y, Sn, Hf, Nb, Cu, Sc, V, Zn, Cr, Sb, Pb, Ni, Co, Li, Mo, Th, Rb, Cs, U, Zr and Ba) are regularly incorporated into newly formed secondary minerals and/or retained in resistant primary minerals that act like repositories, and may induce their accumulation.

(iv) In the Meїganga soil profiles, major elements that may be considered as indicators for mineralization are: TiO₂ for Nb, Zr, V, and Cr; Al₂O₃ for Nb, Mo, U, W and Hf; Fe₂O₃ for Mo, Cr, V, Zr, Hf and Sb; and P₂O₅ for U, Hf and Sb Fe₂O₃. TiO₂, Al₂O₃, Fe₂O₃, and P₂O₅ oxides have been admitted to be the main indicators for the useful elements prospection at large scale in this area, because their concentrations are closely related to the corresponding useful elements.

(v) Still in the Meїganga soil profiles, mineralization indices could be assigned to W, V, Hf, Nb, Mo, Cr, Sb and U.

(vi) weathering products with large numbers of useful elements proceed from those located at the lowlands (quartz veins), while those with very low numbers derived from the weathering products developed on rocks located at the upper landscape (micaschist, orthogneiss and granite).

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