P.-D Ndjigui
email: lndjigui@yahoo.fr

M F B Badinane

B Nyeck

H P K Nandjip

P Bilong

Mineralogical and geochemical features of the coarse saprolite developed on orthogneiss in the SW of Yaoundé, South Cameroon

Keywords: Yaoundé, Orthogneiss, Coarse saprolite, Mineralogy, Geochemistry, Mass balance -15.43 -40.51 -18.34 -19

A petrological investigation was performed in the coarse saprolite on orthogneiss in Yaoundé (South Cameroon) using combined whole rock geochemical (XRF, ICP-MS) and mineralogical (XRD, SEM) techniques. The orthogneiss has high contents in SiO 2 (61.56 wt.%), Ba (916 ppm) and REE (209 ppm), moderate content in Al 2 O 3 (14.34 wt.%) and negative Eu anomaly (Eu/Eu * = 0.68). The weathering leads to the formation of three main constituents in the coarse saprolite: (i) the loose materials (~85vol.%) are basically clayey silty with relic structure. They are composed of kaolinite, quartz and goethite. The loose materials have high contents in SiO 2 (56 to 64.83 wt.%) and Al 2 O 3 (21.48 to 23.96 wt.%), and moderate contents in V (163 to 236 ppm), Ba (95 to 340 ppm) and Zr (160 to 313 ppm). The REE content is low (~ 49 to 169 ppm) relative to the parent rock with LREE-enrichment (LREE/HREE ~ 7 to 17). Positive Ce anomaly (Ce/Ce * ~ 3.35) is observed in the white veins and slight positive Eu anomalies (Eu/Eu * ~ 1.2 to 1.4) are noted in all loose samples. The kaolinite and goethite. The SEM investigation reveals that Ba and Pb are linked in Mnbearing phases and Ce-oxides appear as fine-grained intergrowth between Mn-bearing phases. The Mn-bearing phases are enriched in MnO (33.86 wt.%), BaO (4.30 wt.%), Co (1716 ppm), Pb (1315 ppm) and Ce (5202 ppm). Positive Ce and Eu anomalies are observed (Ce/Ce * ~ 15.60 and Eu/Eu * ~ 2). The mass balance calculations indicate the strong accumulation of Mn, Ni, Co, Zn, Sc, Cu, Ba, Pb, Y, Ga, Zr and REE. The Mn-bearing phases might be derived from the accumulation of silicate residues and Mn within the parent rock.

The transition from the parent rock to the coarse saprolite is marked by high leaching of several elements.

(La/Yb) N ratios (~ 0.8 to 1.5) indicate high REE-fractionation. The mass balance calculation reveals the depletion of several elements except Al, Ti, Sc, Y, Th, Sb and Hf; (ii) the iron duricrust (~10vol.%) is located at the bottom and the top of the horizon. The mineral assemblage is dominated by hematite and goethite. The upper iron duricrust has high contents in Fe 2 O 3 (45.60 wt.%) and Cr (1641 ppm), moderate contents in V (459 ppm) and Zn (143 ppm), and low REE content (47 ppm) with low LREE/HREE ratio (4.28). The upper iron duricrust is more enriched in Fe 2 O 3 (53.26 wt.%) than the lower one. Vanadium, Cr and Zr have high contents relative to other trace elements. The REE content is low (39 ppm) as well as the LREE/HREE ratio (2.94). The iron duricrust has negative Ce anomalies (Ce/Ce * ~ 0.66 to 0.69) and very low (La/Yb) N ratios (0.1 to 0.3). Several elements reported in the iron duricrust are highly leached except Fe, Cr, Zn, Sc, V, Pb, Zr, Cu and Th; and (iii) the Mn-rich materials (<5vol.%) are made up of birnessite, cryptomelane, and low quantities of quartz,

Introduction

Laterites are the main constituents of weathering mantles in the tropical zone (Mc Farlane, 1976). They are complex materials that derive from the weathering of various rock types [START_REF] Kamgang Beyala | Altération et bilans géochimiques des biotites des gneiss de Nkolbisson (NW de Yaoundé, Cameroun)[END_REF][START_REF] Ndjigui | Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinite in the Lomié ultramafic complex, South-East Cameroon[END_REF][START_REF] Etame | Phillipsite formation in nephelinitic rocks in response to hydrothermal alteration at Mount Etinde, Cameroon[END_REF].

Laterites are characterized by large iron duricrust [START_REF] Beauvais | Formation and transformation processes of iron duricrust systems in tropical humid environment[END_REF][START_REF] Bitom | Evolution and alteration in situ of a massive iron duricrust in Central Africa[END_REF][START_REF] Beauvais | Ferricrete biochemical degradation on the rainforest-savannas boundary of Central African Republic[END_REF]. Several works have reported the determination and dating of geochemical fractionation in laterites using U-Th radioactive series [START_REF] Boulad | Essai de détermination de l'âge et de la vitesse d'altération d'un sol ferrallitique camerounais à l'aide de la méthode du déséquilibre radioactif uranium/thorium[END_REF][START_REF] Mathieu | Short-lived U and Th isotope distribution in a tropical laterite derived from granite (Pitinga River Basin, Amazonia, Brazil): application to assessment of weathering rate[END_REF] or 238 U-235 U-230 Th nuclides [START_REF] Chabaux | Tracing and dating recent chemical transfers in weathering profiles by trace-element geochemistry and 238 U-234 U-230 Th disequilibria: the example of the Kaya lateritic toposequence (Burkina-Faso)[END_REF]. Chabaux and coworkers (2003) highlighted the chemical mobility in the lateritic profile controlled by the iron-cap dismantling. The REE-geochemistry is a useful complementary tool to elucidate chemical variations in geological systems because of their coherent and predictable behaviour [START_REF] Leybourne | Rapid development of negative Ce anomalies in surface waters and contrasting REE patterns in ground waters associated with Zn-Pb massive sulphide deposits[END_REF]. This behaviour combined with their sensitivity to changes in pH, redox conditions and adsorption/desorption reactions, make the rare-earth elements particularly useful as pedogenetic tracers in external geodynamic studies of the earth.

The geochemical characterization has for long been focused on the distribution of major and five or six trace elements in the whole fraction of each soil horizon along the lateritic profile in the Central African rainforest [START_REF] Ndjigui | Les produits d'altération du gneiss à biotite et amphibole dans la plaine côtière de Douala[END_REF][START_REF] Bitom | Evolution and alteration in situ of a massive iron duricrust in Central Africa[END_REF][START_REF] Nguetnkam | Etude pétrologique d'une formation latéritique sur granite en milieu tropical forestier sud-camerounais (Afrique centrale). Mise en évidence de son caractère polyphasé[END_REF]. Many works reported since the 90s included the mineralogy and geochemistry of REE in laterites [START_REF] Braun | Mobilization and redistribution of REEs and Th in syenitic lateritic profile: -a mass-balance study[END_REF][START_REF] Braun | Solid/liquid REE fractionation in the lateritic system of Goyoum, East Cameroon: the implication for the present dynamics of the soil covers of the humid tropical regions[END_REF][START_REF] Ndjigui | Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinite in the Lomié ultramafic complex, South-East Cameroon[END_REF][START_REF] Ndjigui | Negative cerium anomalies in the saprolite zone of serpentinite lateritic profiles in the Lomié ultramafic complex, South-East Cameroon[END_REF][START_REF] Kabeyene Beyala | Behaviour of REE and mass balance calculations in a lateritic profile over chlorite schists in South Cameroon[END_REF]. These works have shown that the redox conditions and the nature of the parent rock control the REE distribution in the weathering mantles. They also display an intense REE accumulation in laterites with strong Ce abundance. The lateritic profile is made up, from the bottom to top, of coarse saprolite, fine saprolite, nodular zone and loose clay horizon [START_REF] Ndjigui | Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinite in the Lomié ultramafic complex, South-East Cameroon[END_REF]. Each soil horizon is made up of numerous plates and patches with different petrophysical, mineralogical and geochemical characteristics. Previous works on the petrology of major constituents of horizon are very scarce. The Mn-oxide commonly observed is characterized by a fibrous growth texture. It contains Ba and, in few cases, K. This phase can be assigned to either hollandite (BaMn 8 O 16 ) or cryptomelane (KMn 8 O 16 ), which are both isostructural 2:2 tunnel manganates and can contain up to 16 wt.% Ba and 5 wt.% K, respectively, based on the formula [START_REF] Loges | Negative Ce anomalies in Mn oxides: the role of Ce 4+ mobility during water-mineral interaction[END_REF]. The behaviour of cerium is complex in the Mn-oxides; Ce has commonly positive anomaly [START_REF] Koppi | Rare earth element trends and cerium-uranium-manganese associations in weathered rock from Koongarra, Northern Territory, Australia[END_REF][START_REF] Ohnuki | Concurrent transformation of Ce(III) and formation of biogenic manganese oxides[END_REF] due to the stability of Ce 4+ at the oxide surface [START_REF] Ohta | REE(III) adsorption onto Mn dioxide (δ-MnO 2 ) and Fe oxyhydroxide: Ce(III) oxidation by (δ-MnO 2 )[END_REF][START_REF] Takahashi | Direct observation of tetravalent cerium in ferromanganese nodules and crusts by x-ray-absorption near-edge structure (XANES)[END_REF][START_REF] Takahashi | Chemical and structural control of the partitioning of Co, Ce, and Pb in marine ferromanganese oxides[END_REF]. The negative Ce anomaly is rarely observed [START_REF] Loges | Negative Ce anomalies in Mn oxides: the role of Ce 4+ mobility during water-mineral interaction[END_REF]. This is due to the strong complexation of Ce 4+ by siderophores or organic molecules [START_REF] Davranche | Adsorption of REE(III)-humate complexes onto MnO 2 , experimental evidence for cerium anomaly and lanthanide tetrad effect suppression[END_REF][START_REF] Davranche | Competitive binding of REE to humic acid and manganese oxide: impact of reaction kinetics on development of cerium anomaly and REE adsorption[END_REF][START_REF] Loges | Negative Ce anomalies in Mn oxides: the role of Ce 4+ mobility during water-mineral interaction[END_REF]. At the same time, experimental studies reveal the selective adsorption of REE on kaolinite [START_REF] Laufer | The adsorption of quadrivalent cerium by kaolinite[END_REF][START_REF] Coppin | Sorption of lanthanides on smectite and kaolinite[END_REF].

Larger amounts of cerium were adsorbed by natural kaolinite. Cerium may be adsorbed either as a monomeric species or as a polymeric hydroxyl cation [START_REF] Laufer | The adsorption of quadrivalent cerium by kaolinite[END_REF]. In this study, we present detailed mineralogical and geochemical data of three major constituents (loose materials, iron duricrust and Mn-bearing phases) of a coarse saprolite on orthogneiss. The high Ba, Pb and Ce contents in the Mn-bearing phases enabled to determine their stable forms using the microchemistry of the Mn-bearing phases. The last step of this study is the mass balance evaluation of major and trace elements from three main constituents of the coarse saprolite.

Geographical and geological setting

The study site is located in the SW of Yaoundé (11°25'-11°30'E and 3°45'-3°50'N; Fig. 1). The climate is humid tropical with four seasons marked by a mean annual temperature of 24°C and an average annual rainfall of 1495 mm [START_REF] Suchel | Les climats du Cameroun[END_REF]. The vegetation corresponds to a transitional zone between rainforest and savannah [START_REF] Letouzey | Notice explicative de la carte phytogéographique du Cameroun à l'échelle de 1/500 000[END_REF]. The morphology of the Yaoundé area (~750 m) is dominated by the smooth-rocky hills with large convex slopes relayed by large swampy valleys. The Yaoundé group constitutes a part of the Central African Mobile Zone (CAMZ) and is pan-African in age [START_REF] Toteu | Diachronous evolution of volcano-sedimentary basins north of the Congo craton: insights from U-Pb ion microphobe dating zircon from Poli, Lom and Yaoundé groups (Cameroon)[END_REF].

Micaschists, quartzites and gneiss occur and are intensively folded (Fig. 1B). The Yaoundé group is made up of two series: the Mbalmayo-Bengbis-Ayos and the Yaoundé series [START_REF] Maurizot | Etude et prospection minière du Sud-Ouest Cameroun[END_REF]. Paragneiss and orthogneiss are predominant in the Yaoundé series (Fig. 1C). The weathering leads to hillside ferrallitic soils and the swampy hydromorphic soils. The whole swamp is overlain by a grey clayey sandy material with depth varying from 0.5 to 2 m. It is essentially composed of kaolinite and residual quartz, Ti-oxides and zircon grains [START_REF] Braun | Present weathering rates in a humid tropical watershed: Nsimi, South Cameroon[END_REF].

Sampling and analytical techniques

Sampling techniques

The pit is situated at the hilltop. The weathering profile is 9 m thick and is made up, from the bottom to top, of a coarse saprolite, a fine saprolite, a lower nodular horizon, an iron duricrust horizon, an upper nodular horizon and a loose clayey horizon (Fig. 2). Eleven weathered samples from the coarse saprolite and two rock samples (fresh and slightly weathered) from the outcrop were collected for mineralogical and chemical analyses in the Geoscience Laboratories (Sudbury, Canada).

Analytical techniques

Thin sections of rocks were observed with an optical microscope (Euromex).

Mineralogical analyses were performed on whole rock powders. Powders were prepared by crushing the samples using an agate mortar. Mineral assemblage of rocks and soils was determined using the Panalytical X'Pert Pro. The controlling software is X'Pert data collector, version 2.2h. The analytical conditions are 40 kV and 45 mA. The scan range varies 5 to 85 degrees 2 theta and the step size is 0.01. The run time is 8 minutes and 30 seconds/scan. The scanning is continuous and the type of radiation is Co. The mineralogical composition is given in Table 1. Samples were analyzed for major and trace (including lanthanides) elements. They were crushed in an agate mortar and then pulverized in a planetary ball mortar made up of 99.8% Al 2 O 3 . After crushing, the loss on ignition (LOI) was determined. Firstly, the powders were oven-dried at 105°C under nitrogen in order to eliminate water; another sample fraction was heated at 1000°C under oxygen so as to remove the volatile components and oxidize iron.

After the determination of the LOI, the major element composition was determined by X-Ray Fluorescence (XRF) with a Panalytical Axios Advanced PW 4400 fluorescence spectrometer.

The international reference materials (INTL-09-05401, 09-04868 and 09-04869) and internal laboratory standards (ISHT-09-04204, 09-03903 and 09-03904) were used. Comparisons of measured and reference values are available upon request. The precision of analysis is 5 %.

The detection limits and results of major elements are presented in Table 2. Another fraction of the powder was prepared for the trace elements analysis by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) following the digestion using three acids (see e.g., [START_REF] Burnham | Trace element analysis of geological samples by inductively Coupled Plasma Mass Spectrometry at the Geoscience Laboratories: revised capabilities due to improvements to instrumentation. Summary of Field Work and Other Activities[END_REF][START_REF] Ndjigui | Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinite in the Lomié ultramafic complex, South-East Cameroon[END_REF]. The powders were then treated in an acid mixture (HCl and HClO 4 ) at 120°C in a closed container for one week, and then rinsed from their containers with dilute HNO 3 and dried. The residues were dissolved in an acid mixture (HCl and HClO 4 ) and oven-dried for a second time before they were then dissolved in an acid mixture (HNO 3 , HCl and HF) at 100°C. The dissolved samples were analyzed in a Perkin Elmer Elan 9000 ICP-MS instrument. The instrumental precision of almost all elements was 5% (2σ) for either all or five of the six compiled solutions where the elements were above the limit of quantification. Where the concentrations approached this limit (e.g., for Zr, Ba, La and Pr in the trace-element poor basalt standard BIR-1, or Eu in the rhyolite standard RGM-1), the error varies between 5 and 8.5% [START_REF] Burnham | Trace element analysis of geological samples by inductively Coupled Plasma Mass Spectrometry at the Geoscience Laboratories: revised capabilities due to improvements to instrumentation. Summary of Field Work and Other Activities[END_REF].

Density measurements were carried out in the Department of Earth Sciences of the University of Yaoundé 1 and the results are presented in Table 3. The bulk density (ρ w ) was obtained by the paraffin method. The grain densities (ρ g ) were obtained by the air picnometer method (two replicates). Porosity is calculated using the equation ɸ = [1-(ρ w /ρ g )]*100.

Scanning Electron Microscope (SEM) analysis were carried out at the Geoscience Laboratories using the BSE detector and ED X-ray spectrometer to (i) search for various Mn, Ba, Pb and Ce mineral species due to their high abundances in Mn-bearing phases; and (ii) provide qualitative X-ray data and BSE images of the individual phases. The sample was prepared in three steps: (i) mineral grains and representative pieces of the sample were handpicked; (ii) several grains were mounted in epoxy plug, polished and carbon coated prior to analysis by SEM; and (iii) several grains were crushed with an agate mortar and pestle, and a smear mount created for XRD analysis. SEM used is a Zeiss EVO 50 and the operating conditions were 20 kV acceleration voltage/1 nA beam current.

Results

Petrology of orthogneiss

The orthogneiss is dark, microbedded and has an augen structure. The rock, with heterogranular grano-lepidoblastic texture, is essentially made up of quartz, biotite, green hornblende, garnet, orthoclase, and accessory microcline, opaque minerals and zircon. The XRD spectra reveal the presence of micas (Table 1). The fresh rock is characterized by high SiO 2 , and moderate Al 2 O 3 , Fe 2 O 3 , MgO, CaO, K 2 O and Na 2 O contents (Table 2). The slightly weathered sample is characterized by an increase in SiO 2 , Al 2 O 3 , K 2 O, TiO 2 and P 2 O 5 contents (Table 2). The bulk density is 2; it is a less porous rock (27.6%, Table 3). The trace element contents are variable (Table 4). In the fresh rock, elements whose concentrations are more than 100 ppm include Cr, Zn, V, Ba, Zr, Rb and Sr (Table 4). The second category, made up of elements with low concentrations (20 to 90 ppm), include Ni, Cu, Y, Co, Li and Ga. The third category is made up of those whose concentrations are below 19 ppm (Table 4).

The chondrite-normalized [START_REF] Mcdonough | The composition of the Earth[END_REF] multi-element patterns reveal that orthogneiss and their weathered products are depleted in Co, Ni and Cr contrary to several trace elements (Fig. 3). The total REE content is 209 ppm (Table 4). The elements whose concentrations are high (>40 ppm) include La (43.04 ppm), Ce (87.02 ppm) and Nd (40.79 ppm) (Table 5). The orthogneiss is more enriched in LREE (LREE/HREE ~ 10). The chondrite-normalized REE spectra reveal: (i) strong LREE-enrichment; (ii) slight HREEenrichment; and (iii) moderate negative Eu anomaly (Eu/Eu * = 0.68) (Fig. 4). The (La/Yb) N ratios are very high (13.19).

Petrology of the coarse saprolite

Morphology, mineralogy and distribution of major elements

The coarse saprolite is 2.50 m thick (Fig. 2), with a microbedded structure inherited from the parent rock. It is mainly violet red, silty and interspersed with plates of variable colour and texture.

The bottom is made up of a brown material and white veins. The brown material (~80 vol.%) is organized in horizontal, mm-sized, red, grey yellow or white layers. The mineral assemblage is comprised of kaolinite, quartz, goethite, anatase and accessory hematite (Table 1). This material shows high contents in Al 2 O 3 (22.48 wt.%) and TiO 2 (1.36 wt.%) relative to the orthogneiss (Table 2). The Fe 2 O 3 , MnO, MgO, CaO, Na 2 O and K 2 O contents decrease strongly from the parent rock to the brown material (Table 2). The white veins occur inside the brown material. They are cm-thick, sandy clayey and oriented parallel to the brown material. Kaolinite and quartz are the dominated minerals (Table 1). Slight increase of contents in SiO 2 (64.83 wt.%), Al 2 O 3 (22.76 wt.%) and TiO 2 (1.46 wt.%) is observed relative to the orthogneiss (Table 2).

The whole fraction of the bottom shows a similar mineralogy and geochemistry like the brown material (Tables 12).

The upper part is mainly yellowish brown, clayey silty and microbedded. The whole fraction has practically the same mineralogy and geochemistry like the brown material (Tables 12). It embeds numerous plates like dark red iron duricrust, black plates, yellowish brown plates and dusky red iron duricrust.

The dark red iron duricrust is fusiform (2 to 3 m long and 0.5 m wide), and includes piled up concentric and cm-layers. It is surrounded by the yellowish brown plate (Fig. 2). The mineral assemblage is made up of hematite, goethite, and few amounts of kaolinite, quartz and anatase (Table 1). The Fe 2 O 3 content is 53.26 wt.% and those of SiO 2 and Al 2 O 3 are 26.7 and 10.04 wt.%, respectively.

The black phases (~5vol.%) form cm-to dm-sized lobe-shape. They are porous and very hardened materials. They are made up of birnessite, cryptomelane, with accessory quartz, kaolinite and goethite (Table 1). The X-ray diffraction data of selected grains confirms the presence of the birnessite and cryptomelane groups (Fig. 5). The Mn-bearing phases are very enriched in Mn (33.86 wt.%), and depleted in SiO 2 (27.97 wt.%) and Al 2 O 3 (12.16 wt.%). The Fe 2 O 3 content is quite high (10.08 wt.%) as well as those of other major elements (Table 2).

The yellowish brown plates are disseminated with variable shapes and sizes. They are composed of kaolinite, quartz, goethite and few amounts of rutile and anatase (Table 1). The chemical composition is very close to the uppermost part whole fraction one (Table 2).

The upper boundary of the coarse saprolite is also marked by the presence of dusky red iron duricrust. It is made up of flattened and lobular, interconnected and millimetric plates. The X-ray diffraction data show that hematite and goethite are dominantly associated with few amounts of quartz, kaolinite and anatase (Table 1). The dusky red iron duricrust have high content in Fe 2 O 3 (45.60 wt.%) and moderate contents in SiO 2 (30.79 wt.%) and Al 2 O 3 (11.90 wt.%).

The bulk density values range between 0.9 and 2.53 (Table 3). The high values are measured in the lower iron duricrust (2.53) and in the Mn-bearing phases (2.36). The upper iron duricrust is less dense (1.66). The loose materials have low density (0.93-1.42) and moderate porosity (35-50%).

Distribution of trace elements

Trace element behaviour was subdivided into five groups according to the periodic table of elements: a) -alkaline (Li, Rb, Cs) and alkali-earth (Be, Sr, Ba) contents are very low in the loose samples and iron duricrust compared with the parent rock (Table 4). Lithium, Rb, Sr and Ba contents are high in Mn-bearing phases (Table 4); b) -scandium, V, Cr, Co, Ni and Cu contents are low, except Cr (1641 ppm) and V (459 ppm) in the upper iron duricrust and Co (1716 ppm) in the Mn-bearing phases (Table 4).

Other elements also show high contents in Mn-bearing phases (Table 4); c) -yttrium, Nb, Mo, Hf and W show low concentrations except Zr (Table 4). The Zr contents vary from 83 to 316 ppm; the lowest content occurs in the upper iron duricrust and the highest one is observed in the white veins. Other trace elements of this series have high contents in the Mn-bearing phases (Table 4); d) -other metals (Ga, Zn, Cd, In, Sn, Sb, Tl and Pb) show a similar behaviour like those of the previous groups (Table 4). Elements which show high contents include Pb (1315 ppm) and Zn (242 ppm) in the Mn-bearing phases; e) -thorium contents range between 5 and 42 ppm. The upper iron duricrust and white veins have highest Th contents, 42.50 and 32.66 ppm, respectively. The lowest Th contents are measured in the Mn-bearing phases (5.8 ppm). A slight increase of uranium contents is observed from the parent rock (1.52 ppm) to the weathered samples (ranging from 1.75 to 3.76 ppm) (Table 4).

Correlations

The SiO 2 /Al 2 O 3 and Fe 2 O 3 /K 2 O ratios show that Si and Fe are most abundant than Al and K (Table 2). The Th/U and Th/Co ratios show that orthogneiss and their weathered products are more enriched in Th than U and Co (Table 4). The parent rock-normalized multielement patterns show strong negative Rb anomalies (Fig. 6). The Harker diagrams show that SiO 2 has positive correlations with Al 2 O 3 , TiO 2 , P 2 O 5 , Y and U (Fig. 7). However, correlations of SiO 2 with Fe 2 O 3 , Zr, Cr and V are negative. K 2 O, Rb and Th have no significant correlations with SiO 2 (Fig. 7). The binary diagrams show that Fe 2 O 3 has a similar trend with ferromagnesian trace elements (Fig. 8). Figure 9 shows (i) strong positive correlations of Zr with Hf, Nb and U; (ii) negative correlations of Zr with Y and Mo; and (iii) any correlation between Zr and Th.

Behaviour of rare-earth elements

The total REE content varies between 39 and 5633 ppm (Table 5). The high value is measured in the Mn-phases. The REE content is higher than 100 ppm in two samples and varies from 49 to 96 ppm in several loose samples (Table 5). Both iron duricrust samples have very low REE content, ranging from 39 to 47 ppm. All the weathered samples highlight LREE-enrichment (LREE/HREE ~ 3 to 86). LREE have strong positive correlations with HREE (Fig. 10).

Overall, the high concentrated REE include La, Ce, Pr, Nd, Sm, Gd and Dy (Table 5).

The binary diagrams reveal that Zr and Pb have strong positive correlations with Ce (Fig. 11A-B). Cerium reveals positive correlation with LREE and REE (Fig. 11C-D). The Ce content influences the behaviour of the bulk LREE and REE. The sum of three lanthanide (La, Ce and Nd) contents is moderate to high, ranging from 30 to 5408 ppm. Lanthanum, cerium and neodymium are the most abundant REE. The binary diagram (La+Ce+Nd) vs. REE shows strong positive correlation of three LREE (La, Ce and Nd) with REE (Fig. 11E). Europium shows a moderate positive correlation with Sr (Fig. 11F).

The parent rock-normalized patterns reveal (Fig. 12A; Table 5) [START_REF] Mcdonough | The composition of the Earth[END_REF] patterns reveal the similar trend like the parent rock-normalized spectra (Fig. 12 B). The (La/Yb) N ratios range between 0.12 and 1.63 (Table 5).

SEM analysis of Mn-phases

The SEM analysis is initiated to understand the distribution of several elements with high concentrations like Mn, Pb, Ba and Ce inside the Mn-rich materials. Under SEM, the Mn-rich material has concentric structure . Barium is detected in all Mn-bearing phases that were analyzed from numerous pieces of the sample. Kaolinite is also detected in the Mn-bearing phases with Al/Si ratio ~ 1 (Fig. 13). In addition, the atomic ratio of Ba/K is always >1 suggesting that the cryptomelane component could be termed ''hollandite''. The ''typical'' birnessite type composition using the SEM is not identified; the birnessite structure in this case also contains significant Ba, where the ratio of Ba/K also exceeds 1. Cyclical growth patterns are evident in the sample (Fig. 13). The SEM investigation also reveals a much less abundant Pb bearing Mn-phase, perhaps the corandite member of the cryptomelane group. Ce-oxide phase is also relatively common in this sample. In some cases, it forms a rim around the sample or is contained between the apparent growth rings (Fig. 14). There appears to be a fine grained intergrowth between the Mn-bearing phase and an aluminosilicate mineral with a stoichiometry that is consistent with kaolinite (Figs. 1415).

Mass balance calculation

The mass balance calculation is a method that enables one to confirm the mobility of chemical elements during weathering. This is done by the estimation of losses or gains of matter expressed either in per cent or kg/m 3 of the weathered parent material. The method currently used takes into account the concentrations, the bulk density and the volume [START_REF] Brimhall | Constitutive mass-balance relations between chemical composition, volume, density, porosity and strain in metasomatic hydrochemical systems: results on weathering and pedogenesis[END_REF]Colin and Ambrosi, 1993;[START_REF] Mungall | Severe leaching of trachyte glass without devitrification, Terceira, Azores[END_REF][START_REF] Beauvais | Geochemical balance of lateritization processes and climatic signatures in weathering profiles overlain by ferricretes in Central Africa[END_REF][START_REF] Cornu | Evidence of titanium mobility in soil profiles. Manaus, Central Amazonia[END_REF][START_REF] Moroni | The Serra Pelada Au-PGE deposit, Serra dos Cerajás (Pará State, Brazil): geological and geochemical indications for a composite mineralising process[END_REF][START_REF] Tollari | Trace element concentrations in apatites from the Sept-Îles intrusive suite, Canada -Implications for the genesis of nelsonites[END_REF]. This method enables one to estimate the quantity of each element which is depleted or accumulated (Colin et al., 1993). This approach can be summarized by the following equations:

The first equation is that of [START_REF] Millot | Transformations iso-volumiques dans les phénomènes de latéritisation et de bauxitisation[END_REF], improved by [START_REF] Brimhall | Constitutive mass-balance relations between chemical composition, volume, density, porosity and strain in metasomatic hydrochemical systems: results on weathering and pedogenesis[END_REF]:

K jm = {((C jw x ρ w )/(C jp x ρ p )) -1} x 100 (1)
where K jm is the enrichment or depletion factor, C jw is the concentration of the element j in the weathered material w, C jp is the concentration of the element j in the fresh rock p, ρ jw is the bulk density of the weathered material w, and ρ jp is the bulk density of the parent rock p.

The total mass m jw of each chemical element is obtained through the application of the following equation by Colin et al. (1993):

m jw = C jp ρ p V p K jm (2)
where m jw is in kg, C jp is the concentration of the element j in the parent rock in mg/kg, ρ p is the bulk density of the parent rock in 10 3 kg/m 3 , V p is the volume of the parent rock in m 3 (Vp = 1 m 3 ), K jm is the enrichment or depletion factor of the element j in the weathered material w.

Equation #2 gives the mass m (in kg or g) of each element that is transferred per unit volume of the parent rock during the weathering.

The application of the previous equations leads to results presented in Tables 6, 7, 8, 9, 10 and 11. The mass balance calculations reveal a clear disparity in the behaviour of elements in the coarse saprolite.

Relative element mobility

The relative mobility of major elements enables one to classify them into four categories (Table 7): (i) the strongly depleted elements (depletion rate > 90%) such as Mn, alkaline and alkali-earths, and P; (ii) the second category is made up of moderately leached elements (leaching rate ~ 50%) such as Fe and Si; (iii) the third category is made up of weakly leached elements such as Al and Ti; and (iv) the fourth category is that of moderately leached elements to strongly accumulated elements in several materials notably Fe in the iron duricrust, Mn in the Mn-bearing phases and Ti in some loose samples from the top of the coarse saprolite.

Trace elements display a high mobility compared to the major elements. The notable accumulation is found within Mn-bearing phases with more than ten elements (Table 8).

Three or four trace elements are accumulated in most samples from the upper part of the coarse saprolite. In all, trace elements are leached from this horizon (Table 8).

The behaviour of REE is very close to that of trace elements. Lanthanides are strongly depleted (rate > 60%) except in the Mn-bearing phases (Table 9).

Mass balance calculation

The weathering of 1 m all the weathered samples (Table 11).

Discussion

Petrology of orthogneiss

The mineralogy and geochemistry indicate that orthogneiss are derived from plutonic rocks. These plutonic rocks might be heterogeneous and characterized by a very variable chemistry. The increase of porosity from the fresh rock to the slightly weathered one might be the result of the dissolution of primary minerals. In the slightly weathered rock, the slight increase in K 2 O is due to adsorption and the increase in Al 2 O 3 is due to the silicate dissolution. The total moderate REE content might be due to the low proportion of primary REE-bearing minerals (allanite, monazite, xenotime and apatite) in the gneissic formations of South Cameroon [START_REF] Braun | Solid/liquid REE fractionation in the lateritic system of Goyoum, East Cameroon: the implication for the present dynamics of the soil covers of the humid tropical regions[END_REF]. The negative Eu anomaly could be due to the partial dissolution of feldspars [START_REF] Gromet | Rare earth element distributions among minerals in a granodiorite and their petrogenetic implications[END_REF][START_REF] Panahi | Behavior of major and trace elements (including REE) during Paleoproterozoic pedogenesis and diagenetic alteration of an Archaean granite near Ville Marie, Québec, Canada[END_REF].

Petrology of the coarse saprolite

Morphology, mineralogy and distribution of major elements

The presence of several phases in the coarse saprolite reveals that the high degree of weathering favours the thinning out of the large microbedded layers observed at the bottom.

The mineralogical composition reveals two mineral phases: (i) the first one is composed of kaolinite, goethite, quartz, gibbsite, hematite and anatase. Kaolinite, goethite and gibbsite are characteristic of the well-drained environments in the rainforest region [START_REF] Nguetnkam | Etude pétrologique d'une formation latéritique sur granite en milieu tropical forestier sud-camerounais (Afrique centrale). Mise en évidence de son caractère polyphasé[END_REF]. The high quartz proportion is related to acid nature of the parent rock, as well as its slow dissolution during weathering. Goethite might be related to the high porosity that seemingly facilitates the stagnation of water in cavities. It has already been strongly documented that goethite is the main Fe-hydroxide that crystallizes in the saprolite zone [START_REF] Tardy | Pétrologie des latérites et des sols tropicaux[END_REF][START_REF] Delvigne | Atlas of micromorphology of mineral alteration and weathering[END_REF]. Hematite results from the dehydration of goethite [START_REF] Tardy | Pétrologie des latérites et des sols tropicaux[END_REF]; and (ii) the second mineral phase is mainly manganiferous. It is characteristic of Mn-rich materials which might derive directly from the accumulation of silicate residues and Mn within the parent rock [START_REF] Bourgauth | Genesis and characterization of manganiferous soils in the Eastern Piedmont, USA[END_REF]. The presence of the concentric structures could result from successive centripetal reorganizations [START_REF] Beauvais | Nodules and pisolites from manganiferous weathering profiles under lateritic influences. Examples from Ivory Coast and Gabon[END_REF].

The high SiO 2 contents might be related to the low degree of quartz dissolution (predominant mineral in these samples) in the rainforest zone [START_REF] Nandjip | Contribution à l'étude pétrologique d'une isaltérite développée sur orthogneiss au SW de Yaoundé : recherche des indices de cérium dans les matériaux ferruginisés[END_REF]. The sharp drop in the concentrations of alkaline and alkali-earth elements might be linked to a rapid dissolution of carrier minerals. The increase in aluminium contents might be correlated with an intense weathering and formation of kaolinite. High Mn contents have been noted in the lower horizons of Ni-laterites in New Caledonia [START_REF] Golightly | Geology of Soroako nickeliferous laterite deposits[END_REF][START_REF] Llorca | Supergene cobalt minerals from New Caledonia[END_REF][START_REF] Traoré | Serpentinisation hydrothermale et altération latéritique des roches ultrabasiques en milieu tropical: évolution géochimique et minéralogique de la minéralisation en platine de la rivière des Pirogues (Nouvelle-Calédonie)[END_REF]. The positive correlations of SiO 2 with Al 2 O 3 , TiO 2 , P 2 O 5 , Y and U confirm that Si and Al are the main constituents of silicate residues. At the same time, Ti, P, Y and U might be associated with the relic minerals. The negative correlation between SiO 2 and Fe 2 O 3 is due to the weathering of ferromagnesian primary minerals (e.g., biotite, green hornblende) and the formation of goethite and hematite.

Trace elements

The behaviour of trace elements is also dependent on the nature of weathered phases.

The Mn-phases are characterized by their high contents in more than ten trace elements including Ni, Co, Zn, Cu, Ba, Pb, Y, Li, Cd, Mo and Tl. These highly concentrated elements might have been adsorbed by precipitation in the interlayer spaces of Mn-oxides [START_REF] Brindley | Crystal structures of clay minerals and their x-ray identification[END_REF][START_REF] Mishra | Contrasting REE signatures on manganese ores of iron ore group in North Orissa, India[END_REF]. The high Cd contents are related to its fixation by adsorption on kaolinite [START_REF] Vasconcelos | EXAFS analysis of cadmium (II) adsorption to kaolinite[END_REF]. The highest Cr contents observed in the upper iron duricrust result from relative Cr accumulation as mineral species that were not identified by X-ray diffraction or from their incorporation in the lattice of goethite [START_REF] Singh | Incorporation of Cr, Mn and Ni into goethite (α-FeOOH): mechanism from extended X-ray absorption fine structure spectroscopy[END_REF]. The negative Nb anomalies confirm the high mobilization of Nb during weathering. The correlation between Fe 2 O 3 and ferromagnesian trace elements reveals that ferromagnesian trace elements could be fixed by adsorption on goethite. Both positive correlations (Zr vs. Hf, Nb vs. U) show that Zr, Hf, Nb and U could be hosted by the same minerals such as zircon. The flat signature between Zr and Th shows that zircon is not probably the main Th-bearer.

Rare-earth elements

The low REE content in the loose materials and iron duricrust are characteristic of the weathered materials developed on acidic parent rock [START_REF] Kabeyene Beyala | Behaviour of REE and mass balance calculations in a lateritic profile over chlorite schists in South Cameroon[END_REF]. This might be due to the rapid dissolution of REE-bearers [START_REF] Braun | Solid/liquid REE fractionation in the lateritic system of Goyoum, East Cameroon: the implication for the present dynamics of the soil covers of the humid tropical regions[END_REF].

Meanwhile, the LREE-enrichment is inherited from the parent rock; this could be also due to the formation of secondary LREE-bearers such as rhabdophane (LREEPO 4 .nH 2 O) [START_REF] Braun | Solid/liquid REE fractionation in the lateritic system of Goyoum, East Cameroon: the implication for the present dynamics of the soil covers of the humid tropical regions[END_REF]. Braun and co-workers (1998) Mn-hydroxides [START_REF] Huang | Geochemical characteristics and behaviors of rare earth elements in process of vertisol development[END_REF][START_REF] Laveuf | A review on the potentiality of rare earth elements to trace pedogenetic processes[END_REF] this study). The strong positive Ce anomaly results from the lower mobility of Ce 4+ than REE 3+ [START_REF] Yoshida | Adsorption of rare earth elements by γ-Al 2 O 3 and Pseudomonas fluorescens cells in the presence of desferrioxamine B: implication of siderophores for the Ce anomaly[END_REF][START_REF] Tanaka | A specific Ce oxidation process during sorption of rare earth elements on biogenic Mn oxide produced by Acremonium sp. Strain KR21-2[END_REF]. This positive Ce anomaly is commonly observed in manganese oxides [START_REF] Koppi | Rare earth element trends and cerium-uranium-manganese associations in weathered rock from Koongarra, Northern Territory, Australia[END_REF][START_REF] Ohnuki | Concurrent transformation of Ce(III) and formation of biogenic manganese oxides[END_REF]Feng, 2010). Also, positive Ce anomaly in the kaolinitic white veins indicates that Ce might have been retained by adsorption into the intracellular crystals of kaolinite [START_REF] Coppin | Sorption of lanthanides on smectite and kaolinite[END_REF][START_REF] Vasconcelos | EXAFS analysis of cadmium (II) adsorption to kaolinite[END_REF]. Laufer and co-workers (1984) have shown that Ce (IV) forms several hydroxyl complexes such as [Ce(OH)] 3+ and [Ce(OH) 2 ] 2+ . The number of hydroxyls which are coordinated to Ce depends on the pH and the age of the solution [START_REF] Laufer | The adsorption of quadrivalent cerium by kaolinite[END_REF]. The negative Ce anomalies in several samples show that cerium exists in trivalent form as other REE 3+ [START_REF] Marsh | REE fractionation and Ce anomalies in weathered Karoo dolerite[END_REF][START_REF] Leybourne | Rapid development of negative Ce anomalies in surface waters and contrasting REE patterns in ground waters associated with Zn-Pb massive sulphide deposits[END_REF][START_REF] Ndjigui | Negative cerium anomalies in the saprolite zone of serpentinite lateritic profiles in the Lomié ultramafic complex, South-East Cameroon[END_REF]. The positive Eu anomalies might be due to either the dissolution of feldspars or to other supergene mechanism. Europium might partially substitute Ca 2+ and Sr 2+ in feldspars [START_REF] Gromet | Rare earth element distributions among minerals in a granodiorite and their petrogenetic implications[END_REF][START_REF] Panahi | Behavior of major and trace elements (including REE) during Paleoproterozoic pedogenesis and diagenetic alteration of an Archaean granite near Ville Marie, Québec, Canada[END_REF]. The proportion of relic primary minerals (e.g.: feldspars, zircon) and secondary minerals (e.g.: kaolinite, goethite) control the major and trace element distribution [START_REF] Dequencey | Lanthanide and trace element mobilization in a lateritic toposequence: inferences from the Kaya laterite in Burkina Faso[END_REF].

Mass balance calculation

The high leaching of major elements might be related to the primary mineral weathering. The dissolution of feldspars in alkaline granites of NW Spain leads to considerable losses in Si, Na, Ca, K, Rb, Cs, Ba, U and P [START_REF] Galán | Residence and fractionation of rare earth elements during kaolinization of alkaline peraluminous granites in NW Spain[END_REF]. The accumulation of Al, Ti and Fe is reported in the saprolite developed on granodioritic gneiss of Southern India [START_REF] Tripathi | Geochemistry and origin of ferruginous nodules in weathered granodioritic gneisses, Mysore Plateau, Southern India[END_REF] or on serpentinite in the South-East Cameroon [START_REF] Ndjigui | Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinite in the Lomié ultramafic complex, South-East Cameroon[END_REF][START_REF] Dzemua | Micromorphology, mineralogy and geochemistry of lateritic weathering over serpentinite in south-east Cameroon[END_REF]. The Al accumulation might be due to the fact that aluminium is included within kaolinite. Those of Fe in the iron duricrust suggest an autochthonous origin by leaching of the uppermost ferruginous horizon [START_REF] Dequencey | Chemical mobilizations in laterites: Evidence from trace elements and 238 U-234 U-230 Th disequilibria[END_REF]. The mobility of trace elements is intimately related to the weathering. The high REEdepletion might be linked to the dissolution of REE-bearers [START_REF] Boulangé | Rare earth element mobility during conversion of nepheline syenite into lateritic bauxite at Passa Quatro. Minais Gerais, Brazil[END_REF].

Conclusion

This study enables us to understand the mobilization and redistribution of major and trace elements inside the variegated coarse saprolite represented by three main constituents:

1) -loose materials have simple mineral assemblage (quartz, kaolinite and goethite) and slight positive Eu anomalies. However, strong positive Ce anomaly is observed in the kaolinitic white veins;

2) -iron duricrsut is hematitic and goethitic with negative Ce and positive Eu anomalies;

3) -Mn-bearing phases are dominated by birnessite and cryptomelane groups. They show: (i) higher Mn and trace element contents than other weathered samples; (ii) high REE abundance; and (iii) strong positive Ce and Eu anomalies. Ce-oxides appear as fine grained in the Mn-bearing phases.

Weathering helps in accumulation of Al and Ti in the loose samples and numerous elements (Fe, Mn, Cr, Ni, Co, Zn, Sc, Cu, V, Ba, Pb, Y, Ga, Zr, Th and Tl) in the iron duricrust and Mn-bearing phases. This study shows that the nature and the proportion of weathered phases have a prominent influence on the horizon mineralogical and geochemical budget. 4).

Figure captions

Fig. 4. Chondrite-normalized (McDonough and Sun, 1995) rare-earth element patterns for orthogneiss.

Fig. 5. X-ray diffraction spectra for Mn-bearing phase of several grains from the black plates, indicating good peaks for birnessite and cryptomelane groups, as well as quartz.

Fig. 6. Parent rock-normalized multi-element patterns for the coarse saprolite (for acronyms, see Table 4). Chondrite-normalized [START_REF] Mcdonough | The composition of the Earth[END_REF] rare-earth element patterns for the coarse saprolite (for acronyms, see Table 5).

2 -2 2 2 2 1 4 2 -1 -2 -3 Be -1 -2 -1 -1 -1 -1 -1 -1 -1 -1 -1 Mo -6 -6 -6 -5 -6 11 -6 -6 -6 -5 -1 Sn -1 -2 -1 -1 0 0 -1 -1 0 -2 -4 Tl -1 1 -1 -1 -1 79 -1 -1 -1 -1 - W -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 Cs -7 -7 -7 -7 -7 -7 -7 -7 -7 -7 - 7
-2 -2 -2 -2 -1 18 -2 -2 -2 -2 -2 Gd -8 -9 -9 -8 -7 24 -10 -9 -7 -7 -8 Tb -1 -1 -1 -1 -1 6 -1 -1 -1 -1 -1 Dy -6 -7 -7 -6 -3 31 -7 -7 -6 -5 -6 Ho -1 -1 -1 -1 -1 5 -1 -1 -1 -1 -1 Er -3 -4 -4 -4 -1 17 -4 -4 -3 -3 -3 Tm -1 -1 -1 -1 0 3 -1 -

  : (i) similar behaviour of REE except in the Mn-bearing phases; (ii) slight LREE-depletion; (iii) strong positive Ce and Eu anomalies in Mn-bearing phases (Ce/Ce * = 15.57 and Eu/Eu * = 2.01); (iv) positive Ce anomaly in white veins (Ce/Ce * = 3.35); and (v) moderate negative Ce (Ce/Ce * ~ 0.66 to 0.86) and weak positive Eu anomalies (Eu/Eu * ~ 1.18 to 1.40) in the loose materials and in the iron duricrust. The chondrite-normalized

  have reported that xenotime is the major HREEbearer in gneiss; the low HREE content in the coarse saprolite is controlled by the dissolution of xenotime. The REE content in the Mn-bearing phases is very high (5632 ppm) as compared to those of the parent rock (209 ppm) and other weathered samples (ΣREE is 39-169 ppm).The formation and distribution of secondary minerals have an effect on the REE distribution(Harlavan et al., 2009). The positive correlation of HREE with LREE confirms the coherent behaviour of REE in the coarse saprolite. The positive correlation of the sum of three lanthanide (La, Ce and Nd) contents with the total REE content confirms the prevalence of La, Ce and Nd inside REE. The high Ce content (5202 ppm) in the Mn-bearing phases is due to the stronger sorption of Ce 4+ or to the probable Ce precipitation as cerianite (CeO 2 ) onto

Fig. 1 .Fig. 2 .

 12 Fig. 1. Location and geological map of Yaoundé. A. Location of Yaoundé; B. Geological map

Fig. 3 .

 3 Fig. 3. Chondrite-normalized (McDonough and Sun, 1995) multi-element patterns for

Fig. 7 .

 7 Fig. 7. Harker diagrams of selected major and trace elements.

Fig. 8 .

 8 Fig. 8. Scattergrams of Fe 2 O 3 with selected ferromagnesian trace elements: A. Fe 2 O 3 vs. Cr;

Fig. 11 .

 11 Fig. 11. Scattergrams of selected rare-earth elements with Zr, Pb and rare-earth elements: A.

Fig. 13 .

 13 Fig. 13. BSE image of a grain displaying cyclic growth patterns in sample. The EDS spectra

Fig. 14 .

 14 Fig. 14. BSE image of the Mn-bearing phase with a blow-out picture of Ce-oxide marginal to

Fig. 15 .

 15 Fig. 15. BSE image displaying the intergrowth of the Mn-bearing phase with the

  Figure 6

  Figure12

  Fig. 1

  Table 10); (ix) relative accumulation of Zn, Sc, V, Pb, Th, U, Cr, Zn, Cu and Zr in the iron duricrust (Table 10); (xi) accumulation of Ba, Co, Pb, Ni, Zn, Sc, Cu, V, Y, Ga, U, Zr, Sb, Hf, Cd, Mo and Tl in the Mn-bearing phases (Table 10); (xii) high REE-enrichment in the Mn-bearing phases particularly Ce (Table 11); and (xiii) high REE-depletion in almost

3

of orthogneiss into coarse saprolite indicates: (i) strong depletion of an important quantity of Si (400 to 726 kg); (ii) low accumulation of Al in the whole fraction from the upper part (17.85 kg), in the yellowish brown plates (24.45 and 14.05 kg); (iii) more pronounced depletion of Fe than Al (Table

9

). However, Fe is strongly accumulated in the Mn-bearing phases (95.98 kg), in the iron duricrust from the bottom (1205 kg) and from the top (615.05 kg); (iv) high Mn accumulation in the Mn-bearing phases (796.48 kg) and leaching of Mn in several samples (Table

9

); (v) strong depletion of alkaline and alkali-earth elements (Table

9

); (vi) moderate Ti and P depletion; (vii) accumulation of Ti in the upper whole fraction

(1.31 kg)

, in the lower iron duricrust (1.44 kg), and in loose materials (Table

9

); (viii) relative accumulation of Pb, Zr, Th, Hf, Sc, V and U in several loose samples (

Table 1

 1 Mineralogical composition of the parent rock and the main constituents of the coarse saprolite.

		Parent rock		Coarse saprolite
		Fresh	Sligthly	Bottom			Top
		rock	weathered	Whole	Brown	White	Whole	Dark red	Black		Yellowish brown plates		Dusky red
			rock	fraction	material	veins	fraction	iron	phases					iron
								duricrust						duricrust
										(a)	(b)	(c)	(d)
	Depth (m)	-	-	-	+ 0.10	+ 0.40	-	+1.70	+1.7 0	+1.70	+1.80	+2.10	+2.40	+2.50
	Ref. code

Table 2

 2 Major element composition of the parent rock and the main constituents of the coarse saprolite (%wt of oxides).

		d.l. Parent rock		Coarse saprolite
			Fresh	Sligthly	Bottom			Top
			material	weathered	Whole	Brown	White	Whole	Dark red	Mn-		Yellowish brown plates		Dusky red
				material	fraction	material	veins	fraction	iron	phases					iron
									duricrust						duricrust
											(a)	(b)	(c)	(d)
	Depth (m)	-	-	-	-	+ 0.10	+ 0.40	-	+1.70	+1.7 0	+1.70	+1.80	+2.10	+2.40	+2.50
	Ref. code	-	CE 01	CE 02	CE 04	CE 07	CE 08	CE 10	CE 09	CE 13	CE 11	CE 12	CE 06	CE 14	CE 05
	SiO 2	0.01	61.56	61.84	61.80	61.25	64.83	58.03	26.70	27.97	56.10	57.01	59.05	60.30	30.79
	Al 2 O 3	0.01	14.34	15.63	22.34	22.48	22.76	22.18	10.04	12.16	22.02	22.90	23.96	21.48	11.90
	Fe 2 O 3	0.01	7.06	6.66	5.12	5.41	2.26	7.68	53.26	10.08	8.63	6.46	4.80	6.74	45.60
	MnO	0.01	0.13	0.11	0.01	0.01	0.01	0.01	0.01	33.86	0.01	0.01	0.01	0.01	0.01
	MgO	0.01	4.40	2.92	<dl	<dl	<dl	0.01	0.04	0.15	0.09	0.08	<dl	0.01	0.04
	CaO	0.01	4.71	3.64	0.03	0.02	0.02	0.02	0.02	0.03	0.02	0.02	0.02	0.02	0.02
	Na 2 O	0.01	2.72	2.58	<dl	<dl	<dl	0.02	<dl	0.11	0.02	0.02	<dl	0.02	<dl
	K 2 O	0.01	2.81	3.15	0.04	0.03	0.04	0.03	0.01	0.32	0.02	0.02	0.04	0.05	0.01
	TiO 2	0.01	0.91	1.07	1.35	1.36	1.46	1.42	0.78	0.16	1.50	1.46	1.52	1.02	0.67
	P 2 O 5	0.01	0.23	0.28	0.09	0.09	0.08	0.07	0.03	0.03	0.05	0.06	0.10	0.09	0.10
	LOI	0.05	0.43	1.64	9.37	9.38	9.23	9.97	8.93	12.41	10.45	10.64	10.17	9.60	10.24
	Total	-	99.30	99.52	100.15	100.03	100.69	99.44	99.82	97.28	98.91	98.68	99.67	97.28	99.38
	SiO 2 /Al 2 O 3	-	4.29	3.96	2.77	2.72	2.85	2.62	2.66	2.30	2.55	2.49	2.46	2.81	2.59
	Fe 2 O 3 /K 2 O	-	2.51	2.11	1.28	180.33	56.50	256.00	5326.00	31.50	431.50	323.00	120.00	134.80	4560.00
	d.l.: detection limits.
	Note that the BaO content is 4.30 wt.% in the Mn-phases.
	Ref. code: reference code.

Table 3

 3 Bulk density and porosity (in %) of the parent rock and the main constituents of the coarse saprolite. Trace element composition of the parent rock and the main constituents of the coarse saprolite (ppm).

		Parent rock		Coarse saprolite
		Fresh	Sligthly	Bottom			Top
		rock	weathered	Whole	Brown	White	Whole	Dark red	Mn-phases		Yellowish brown plates	Dusky red
			rock	fraction	material	veins	fraction	iron					iron
								duricrust					duricrust
										(a)	(b)	(c)	(d)
	Depth (m)	-	-	-	+ 0.10	+ 0.40	-	+1.70	+1.7 0	+1.70	+1.80	+2.10	+2.40	+2.50
	Ref. code	CE 01	CE 02	CE 04	CE 07	CE 08	CE 10	CE 09	CE 13	CE 11	CE 12	CE 06	CE 14	CE 05
	Bulk density	2.01	2.06	1.28	0.93	1.10	1.38	2.53	2.36	1.42	1.32	1.14	1.29	1.66
	Porosity	27.69	25.09	50.38	35.22	46.41	45.01	22.86	27.00	47.00	49.00	51.81	50.00	36.72

Table 5

 5 Rare-earth element composition of the parent rock and the main constituents of the coarse saprolite (ppm). Ce orthogneissl /Ce chondrite )/(La orthogneiss /La chondrite ) 0.5 (Pr orthogneiss /Pr chondrite ) 0.5 . Eu anomaly = (Eu/Eu * ) (1) = (Eu orthogniess /Eu chondrite )/(Sm orthogniess /Sm chondrite ) 0.5 (Gd orthogneiss /Gd c hondrite ) 0.5 . Ce anomaly = (Ce/Ce * )(2) = (Ce weathered sample Ce orthogneiss )/(La weathered sample /La orthogneiss ) 0.5 (Pr weathered sample /Pr orthogneiss ) 0.5 . Eu anomaly = (Eu/Eu * ) (2) = (Eu weathered sampe /Eu orthogneiss )/(Sm weathered sample /Sm orthogneiss ) 0.5 (Gd weathered sample /Gd orthogneiss ) 0.5 . For rock samples: (La/Yb) N = (La orthogneiss /La chondrite )/(Yb orthogneiss /Yb chondrite ). For weathered samples: (La/Yb) N = (La weathered sample /La orthogneiss )/(Yb weathered sample /Yb orthogneiss ).

	REE	d.l. Parent rock		Coarse saprolite
		Fresh	Sligthly	Bottom			Top
		rock	weathered	Whole	Brown	White	Whole	Dark red	Mn-		Yellowish brown plates	Dusky red
				rock	fraction	material	veins	fraction	iron	phases				iron
									duricrust					duricrust
											(a)	(b)	(c)	(d)
			CE 01	CE 02	CE 04	CE 07	CE 08	CE 10	CE 09	CE 13	CE 11	CE 12	CE 06	CE 14	CE 05
	La	0.04	43.04	44.92	23.95	24.63	20.61	17.47	4.57	166.89	10.91	14.56	25.76	22.48	8.34
	Ce	0.12	87.02	86.86	35.92	33.31	117.58	27.00	9.15	5,202	17.15	21.77	39.54	33.40	12.62
	Pr	0.01	10.54	10.55	4.41	4.39	3.58	3.72	2.27	40.06	2.48	3.15	5.33	4.47	2.63
	Nd	0.06	40.79	41.32	16.79	16.86	13.58	14.20	9.40	123.75	9.61	12.66	20.81	17.42	10.87
	Sm	0.01	7.86	7.60	4.08	4.20	3.34	3.39	3.04	26.68	2.42	3.20	5.07	4.46	3.28
	Eu	0.03	1.56	1.56	1.08	1.11	0.90	0.88	0.70	9.092	0.64	0.88	1.30	1.25	0.80
	Gd	0.01	6.21	6.15	3.88	4.16	3.49	3.06	2.26	15.45	2.09	2.91	4.62	4.44	2.50
	Tb	0.002	0.88	0.89	0.56	0.61	0.53	0.44	0.45	3.21	0.32	0.43	0.67	0.64	0.43
	Dy	0.01	4.91	5.25	3.12	3.24	2.84	2.48	2.79	17.18	1.82	2.37	3.59	3.62	2.58
	Ho	0.003	0.91	0.98	0.51	0.52	0.49	0.41	0.49	3.08	0.30	0.37	0.61	0.61	0.44
	Er	0.01	2.50	2.66	1.20	1.21	1.17	1.00	1.48	9.45	0.74	0.90	1.47	1.42	1.24
	Tm	0.002	0.35	0.37	0.14	0.14	0.14	0.13	0.25	1.63	0.10	0.12	0.18	0.17	0.20
	Yb	0.01	2.22	2.29	0.80	0.78	0.83	0.86	1.95	12.40	0.70	0.74	1.05	0.96	1.42
	Lu	0.002	0.32	0.33	0.10	0.09	0.11	0.11	0.26	1.75	0.09	0.09	0.13	0.12	0.19
	ΣREE	-	209.10	211.73	96.56	95.24	169.17	75.16	39.06	5632.61	49.38	64.15	110.13	95.45	47.53
	ΣLREE	-	190.81	192.81	86.23	84.51	159.59	66.66	29.14	5568.48	43.22	56.22	97.81	83.48	38.53
	ΣHREE	-	18.29	18.92	10.34	10.73	9.59	8.50	9.92	64.13	6.16	7.92	12.32	11.10	8.99
	a.	-	10.44	10.19	8.34	7.87	16.65	7.84	2.94	86.83	7.01	7.09	7.94	6.97	4.28
	Ce/Ce * (1)	-	0.99	0.97	-	-	-	-	-	-	-	-	-	-	-
	Eu/Eu * (1)	-	0.68	0.69	-	-	-	-	-	-	-	-	-	-	-
	Ce/Ce * (2)	-	-	-	0.86	0.78	3.35	0.82	0.69	15.57	0.81	0.79	0.83	0.82	0.66
	Eu/Eu * (2)	-	-	-	1.22	1.19	1.18	1.23	1.20	2.01	1.40	1.30	1.20	1.25	1.25
	(La/Yb) N	-	13.19	13.35	1.51	1.63	1.29	1.05	0.12	0.69	0.80	1.02	1.26	1.21	0.30
	d.l.: detection limits.
	a. = LREE/ HREE.
	Ce anomaly = (Ce/Ce * )(1) = (

Table 6

 6 Geochemical balance evaluation of major elements in the coarse saprolite by isovolumetric method (%).

		Coarse saprolite
		Bottom			Top
		Whole	Brown	White	Whole	Dark red	Mn-		Yellowish brown plates	Dusky red
		fraction	material	veins	fraction	iron	phases				iron
						duricrust					duricrust
								(a)	(b)	(c)	(d)
	Depth (m)	-	+ 0.10	+ 0.40	-	+1.70	+1.70	+1.70	+1.80	+2.10	+2.40	+2.50
	Ref. code	CE 04	CE 07	CE 08	CE 10	CE 09	CE 13	CE 11	CE 12	CE 06	CE 14	CE 05
	Si	-36.07	-53.96	-42.37	-35.28	-45.28	-46.65	-35.62	-39.18	-45.60	-37.13	-58.69
	Al	-0.79	-27.47	-13.14	6.19	-11.87	-0.44	8.48	4.87	-5.24	-3.87	-31.47
	Fe	-53.82	-64.54	-82.48	-25.31	849.56	67.64	-13.64	-39.91	-61.44	-38.73	433.42
	Mn	-95.10	-96.44	-95.79	-94.72	-90.32	30482	-94.57	-94.95	-95.64	-95.06	-93.65
	Mg	-	-	-	-99.84	-98.86	-96.00	-98.55	-98.81	-	-99.85	-99.25
	Ca	-99.59	-99.80	-99.77	-99.71	-99.47	-99.25	-99.70	-99.72	-99.76	-99.73	-99.65
	Na	-	-	-	-99.50	-	-95.25	-99.48	-99.52	-	-99.53	-
	K	-99.09	-99.51	-99.22	-99.27	-99.55	-86.63	-99.50	-99.53	-99.19	-98.86	-99.71
	Ti	-5.53	-30.85	-12.20	7.13	7.89	-79.36	16.45	5.36	-5.26	-28.06	-39.19
	P	-75.08	-81.89	-80.96	-79.10	-83.58	-84.69	-84.64	-82.87	-75.34	-74.89	-64.09
	LOI	1288	909	1075	1492	2541	3281	1617	1525	1241	1333

Table 7

 7 Geochemical balance evaluation of trace elements in the coarse saprolite by isovolumetric method (%).

		Coarse saprolite
		Bottom			Top
		Whole	Brown	White	Whole	Dark red	Mn-phases		Yellowish brown plates	Dusky red
		fraction	material	veins	fraction	iron					iron
						duricrust					duricrust
								(a)	(b)	(c)	(d)
	Depth (m)	-	+ 0.10	+ 0.40	-	+1.70	+1.70	+1.70	+1.80	+2.10	+2.40	+2.50
	Ref. code	CE 04	CE 07	CE 08	CE 10	CE 09	CE 13	CE 11	CE 12	CE 06	CE 14	CE 05
	Cr	-28.78	-43.40	-46.13	-8.81	-13.90	-55.38	-12.81	-25.61	-23.43	-44.72	390.50
	Ni	-84.17	-88.94	-89.44	-78.96	-50.84	10.88	-78.01	-80.96	-82.15	-83.97	-82.21
	Co	-92.89	-95.41	-95.99	-93.32	-66.46	7275	-92.79	-94.71	-95.29	-91.92	-83.77
	Zn	-75.74	-83.26	-80.72	-64.69	85.81	170.61	-58.28	-68.10	-71.37	-67.60	12.48
	Sc	-13.89	-34.01	-20.71	3.38	141.73	86.79	15.60	0.75	-21.37	-9.93	128.99
	Cu	-68.74	-74.27	-70.56	-33.49	-24.74	923.61	-19.02	-46.16	-64.61	-41.69	77.03
	V	0.13	-32.53	-22.68	20.52	163.50	9.93	29.25	5.83	-1.74	-22.36	181.00
	Ba	-77.78	-83.66	-81.38	-86.38	-97.44	6526	-92.67	-88.66	-80.16	-76.14	-90.42
	Pb	7.80	-22.89	-27.75	-5.77	48.08	9991	-21.04	-3.42	1.20	20.81	35.49
	Y	-71.29	-80.25	-76.55	-77.29	-69.65	60.84	-84.29	-80.87	-70.55	-67.23	-78.77
	Ga	-5.15	-31.94	-12.34	0.99	-9.80	45.75	0.71	-6.38	-3.34	-7.05	-26.11
	Th	-6.13	-40.60	66.74	-0.43	23.62	-36.47	-23.03	-20.90	-4.07	-12.31	227.44
	U	-7.61	-33.21	-5.65	20.23	120.09	35.47	31.17	11.32	-16.07	-19.68	105.02
	Zr	25.43	-21.48	31.01	25.35	40.17	14.74	56.81	36.82	-14.50	-22.21	-48.07
	Li	-88.91	-92.12	-87.73	-90.59	-93.01	-72.60	-91.10	-90.76	-89.29	-90.97	-94.80
	Sb	961.36	594.03	1177	587	1368	5966	-	-	1034	327.86	1,414
	Nb	-18.66	-38.47	-13.89	-4.97	-16.75	-32.95	-3.83	-22.15	-19.49	-25.85	-48.69
	Hf	20.58	-22.25	31.73	20.81	32.75	15.17	47.86	29.55	-17.71	-22.32	-45.62
	Be

Table 8

 8 Geochemical balance evaluation of rare-earth elements in the coarse saprolite by isovolumetric method (%).

		Coarse saprolite
		Bottom			Top
		Whole	Brown	White	Whole	Dark red	Mn-		Yellowish brown plates	Dusky red
		fraction	material	veins	fraction	iron	phases				iron
						duricrust					duricrust
								(a)	(b)	(c)	(d)
	Depth (m)	-	+ 0.10	+ 0.40	-	+1.70	+1.70	+1.70	+1.80	+2.10	+2.40	+2.50
	Ref. code	CE 04	CE 07	CE 08	CE 10	CE 09	CE 13	CE 11	CE 12	CE 06	CE 14	CE 05

Table 9

 9 Mass balance calculations of major elements in the coarse saprolite by isovolumetric method (kg/m 3 ).Table 10Mass balance calculations of trace elements in the coarse saprolite by isovolumetric method (g/m 3 ).

		Coarse saprolite
		Bottom			Top
		Whole	Brown	White	Whole	Dark red	Mn-phases		Yellowish brown plates		Dusky red
		fraction	material	veins	fraction	iron						iron
						duricrust						duricrust
								(a)	(b)	(c)	(d)
	Depth (m)	-	+ 0.10	+ 0.40	-	+1.70	+1.70	+1.70	+1.80	+2.10	+2.40	+2.50
	Ref. code	CE 04	CE 07	CE 08	CE 10	CE 09	CE 13	CE 11	CE 12	CE 06	CE 14	CE 05
	Si	-446.32	-667.73	-524.23	-436.54	-560.33	-577.26	-440.74	-484.82	-564.19	-459.49	-726.24
	Al	-2.28	-79.17	-37.87	17.85	-34.22	-1.26	24.45	14.05	-15.09	-11.14	-90.69
	Fe	-76.37	-91.59	-117.05	-35.92	1205	95.98	-19.36	-56.63	-	-54.96	615.05
	Mn	-2.49	-2.52	-2.50	-2.48	-2.36	796.48	-2.47	-2.48	-2.50	-2.48	-2.45
	Mg	-	-	-	-88.30	-87.43	-84.90	-87.16	-87.38	-88.44	-88.31	-87.78
	Ca	-94.29	-94.49	-94.45	-94.40	-94.17	-93.96	-94.39	-94.41	-94.44	-94.41	-94.34
	Na	-	-	-	-54.40	-	-52.08	-54.39	-54.41	-	-54.41	-
	K	-55.97	-56.20	-56.04	-56.07	-56.23	-48.93	-56.20	-56.22	-56.03	-55.84	-56.32
	Ti	-1.01	-5.64	-2.23	1.31	1.44	-14.52	+3.01	+0.98	-0.96	-5.13	-7.17
	P	-3.47	-3.79	-3.74	-3.66	-3.86	-3.92	-3.91	-3.83	-3.48	-3.46	-2.96
	LOI	11.29	78.59	92.89	128.94	217.29	284.23	139.75	131.81	107.30	115.20	161.34

Table 11

 11 Mass balance calculations of rare-earth elements in the coarse saprolite by isovolumetric method (g/m 3 ).

	Rb	-228	-229	-228	-227	-229	-227	-229	-229	-227	-228	-229
	Sr	-705	-723	-719	-725	-756	-736	-740	-731	-706	-694	-735

  La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

			CE 01 CE 04	CE 02 CE 07	CE 04 CE 08 CE 07	CE 08 CE 10	CE 10 CE 09	CE 09 CE 13
			CE 13 CE 11	CE 11 CE 12	CE 12 CE 06 CE 06 CE 01	CE 14 CE 02 CE 14	CE 05 CE 05
			100000,000 10 5 10000,000
	Yb Lu	Sample/chondrite Sample/parent rock	-3 -1 0,100 1,000 10,000 100,000 1000,000 10000,000 10 -1 1 10 10 2 10 3 10 4 1000,000 0,100 1,000 10,000 100,000	Sample/chondrite	-4 -1		-4 -1		-3 0		0 0		25 3		-3 -1	1 -3 -1	-1 -3 0	0 -3 0	0 -2 0
			0,010 10 -2
			0,001 10 -3 0,010						Figure 4
			0,001 10 -3	Co		Ni	Cr	V	Sr	Rb Ba	Pb	Zr	Y	Nb Hf	Th	U
				Co Ni Cr V Sr Rb Ba Pb Zr Y Nb Hf Th U
									Figure 3

  La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

	(A)	CE 01	CE 04	CE 07	CE 08	CE 10	CE 09
		CE 13	CE 11	CE 12	CE 06	CE 14	CE 05
		100,000 10 2
	Sample/orthogneiss	1,000 10,000 10 1 1
		10 -1
		CE 04	CE 07	CE 08	CE 10	CE 09	CE 13
		CE 11	CE 12	CE 06	CE 14	CE 05
		10000
	Sample/chondrite	100 1000
		10
		1
		La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

------

Acknowledgements

This work was partially supported by the Geoscience Laboratories (Sudbury, Canada) from the stages of sample preparation to laboratory analysis; we wish to thank Clement Merilla, John Hechler and Dave Crabtree for the SEM observations. The authors gratefully acknowledge the editor and two anonymous reviewers for their detailed comments that have considerably improved the manuscript.

+++: very abundant; ++: abundant; +: poorly represented; ε: trace; -: not identified.

(2)