Multi-Scale Organization of the Doumbouo-Fokoué Bauxites Ore Deposits ( West Cameroon ) : Implication to the Landscape Lowering

Landscape distribution, macroscopic, microscopic, mineral and geochemical characterizations were conducted on the Doumbouo-Fokoué bauxite ore deposit in order to estimate bauxites potential and its implication to general lowering of the relief. Fourteen bauxitic plateaus covering a surface area of 5.7 km were identified. Bauxitic pedons show deep weathered profiles (10.0 12.0 m) with thick bauxitic mantle (4.0 8.0 m). Saprolite and pisolith bauxitic facies own high aluminium (47.5% 49.5% Al2O3), relatively low iron (20.0% 22.0% Fe2O3) and low silica contents (1.8% 7.6% SiO2). Gibbsite is the dominant mineral (49% 68% of minerals detected by X-ray); meanwhile hematite, goethite and kaolinite occur in small amounts. Bauxitization corresponds to intense allitization with abundant accumulation of gibbsite and development of lateritic iron bearing ortho-bauxites. Bauxite ores yielded bauxite reserves of 9.2 million tons. They occur as old and residual bauxitic mantles representing remnants of the Miocene residual lateritic deposits in West Cameroon referring to the African surface of Valeton [1]. Its mean altitude (1532 1590 m als) below the African surface reveals general lowering of the relief.

In Cameroon, works on laterites in general, and lateritic bauxites in particular, started at the b eginning of the 20 th century [5][6][7][8][9][10][11].The majority of these studies were focussed on the characterization and the evaluation of the well known bauxites ores deposits in Minim-Martap and Ngaoundal-Ngaoundouro in the Adamaoua region and Fongo-Tongo and Bagam in the West region.They noted that Cameroon has the 6 th world bauxite reserves with approximately 1.5 billion tons.This amount seems to be underestimated since there still be many non explored bauxitic indices in Cameroon as indicated by the SABAP exploration licence [7] and the recent geological map of Cameroon [12].The Doumbouo-Fokoué area contains one of these non explored bauxitic indices of Cameroon.The main objective of this study is to characterize the bauxitic facies of Doumbouo-Fokoué at different scales in order (i) to estimate the bauxites potential in this area and (ii) to correlate bauxites landscape distribution and evolution in this region with the general lowering of the relief in the West Cameroon Highlands.Results may contribute to increase the Cameroon bauxite reserves, examine in detail bauxitization process in the West Cameroon Highlands and correlate these bauxite ore deposits to the well known bauxites ore deposits in Cameroon and Africa.

The Site Setting
The Doumbouo-Fokoué region is located between longitudes 10˚05' and 10˚08' East and latitudes 5˚21' and 5°27' North (Figure 1).Morphologically, it belongs to the Bamiléké plateau (small insert in Figure 1) which corresponds to the south-western part of the West Cameroon Highlands with mean altitude 1400 -1600 m asl.Very elongated interfluves with convex slopes are the dominant features in this region.
The Doumbouo-Fokoué region is deeply dissected by a dense hydrographic network which belongs to the Ménoua watershed.It is under sub-equatorial climate of high altitude characterized by a long rainy season from March to October and short dry season from November to February.The mean annual rainfall is 1755 mm and the mean annual temperature is 20˚C.
From the geological point of view, the Doumbouo-Fokoué region belongs to the Cameroon volcanic line (CVL) [13] that crosses the western part of the Cameroon territory.It includes Cenozoic volcanic complexes overlying on the Neo-proterozoic panafrican granito-gneissic units (Figure 1) [14].In this region, the bauxite ores deposits are developed exclusively from the Miocene aphyric or porphyric basalts [15].They are composed predominantly of laths of plagioclase (An 46-72 ), coarse to fine grains of forsterite (Fo 66-87 ), augite and diopside.Fe-Ti oxides occur as accessory minerals and Cr-Fe spinels as inclusions in forsterite.The groundmass consists of microlites of plagioclase and very fine interstitial grains of augite and diopside.The mean chemical composition of these basalts is as follow: 15.9% of Al 2 O 3 , 13.5% of Fe 2 O 3 and 44.6% of SiO 2 [15] (cf.Table 3).

Analysis Methods
The study of the Doumbouo-Fokoué bauxite indices were refers to the bauxite reserves m il morphological de microscopic, mineral and ch oscopic analysis, non disturbed bauxiti s were determined using X-ray diffraction (X ts determination was performed using X

Calculation Methods
ents in bauxitic facies was conducted at four different scales: landscape, pedons, thin section and sample.
The landscape study apping.It was based on preliminary works which consisted at drawing and overlapping geological and morphological maps of the Doumbouo-Fokoué region using GIS (geographic information systems), with the resultants detailed geomorphologic map of the study area.Cartography of the bauxitic mantles was based on data collected during field works.Data correspond to bauxitic samples and their sampling sites coordinates which were afterwards projected onto the geomorphologic map using GIS to demarcate the bauxitic plateaus.
Pedons study is based on the field so scription method [16].It was performed on road cuts, quarries and pits.After what, bauxitic samples were collected for laboratory analyses.
Laboratory analyses concern emical analysis.
Concerning micr c samples were hardened using epoxy resin, sharpened and polished to obtain thin sections for microscope description.
Mineral RD) and thermo-gravimetric analysis (TGA).X-ray diffraction was performed on 0.2 mm bauxitic powder using BRUKER diffract-meter according to the following conditions: copper anode (Cu-Kα), wavelength, λ: 1.5418 Å, scanning speed: 0.002°/s, scan range: 5° -70°, drive axis 2θ.Thermo-gravimetric analysis consists in the evaluation of the weight loss of bauxitic powder during an increasing temperature (up to 900˚C).Minerals were identified and their relative abundance calculated using percentage of weight loss in a defined interval of temperature [17].
Chemical elemen -ray fluorescence on bauxitic powder crushed and sieved into 0.2 mm.300 mg of bauxitic powder were fused with 900 mg of lithium meta-borate (LiBO 2 ) during 1h at 980˚C, and dissolved into 1.55 M of nitric acid.Chemical elements are quantified using ICP-AE spectrometry for major elements and FI-ICP-M spectrometry for trace elements.
Enrichment-depletion of elem assessed using enrichment factors (EF).EF which is the ratio between the content of an element in any given bauxitic facies and that of the parent rock has been calculated using the relation: EF(X) = (Xi/Ri)/(Xs/Rs) [18].In this relation Xi and Ri are the concentrations of the element of interest and immobile element (Ti) and Xs and Rs are the concentrations of the same elements in the serves were also calculated using the follo

Fou
been identified in Doumince spreads out north of th parent rock.
Bauxite re wing formula: M = S × H × Da, where S is the total bauxitic plateaus surface, H the mean thickness of bauxitic mantles and Da the bulk densities of the bauxitic facies.

Landsca in Doumbouo-Fokoué
rteen bauxitic plateaus have bouo-Fokoué (Figure 2).All are made up of thick bauxitic mantles stretching out for kilometers at the summit and upper slopes of interfluves.They have been subdivided into two main bauxitic provinces: the Doumbouo and Fokoué bauxitic provinces.
The Doumbouo bauxitic prov e locality and covers about three quarters of the area.In this area, the bauxitic mantles cover a surface area of 5.1 km 2 , which represents about 89% of the bauxitic surface.It is thus the most abundant bauxite ores deposits in Doumbouo-Fokoué.It consists of ten bauxitic plateaus called bowal that are organized into large interfluves with summit capped with bauxitic mantles and surrounded by steep slopes.They are thus characteristic of regions with contrasted climate.They are: Tsingla, Sa'a, Ngwa, Letagli, Doumbouo, Letet1, Letet2, Tougong, Honto and Meka plateaus.The Tsingla plateau shows a forked bauxitic mantle belt of about 2.5 km long (Figure 2).The Sa'a plateau consists of a branching bauxitic mantle with about 3.5 km long.The Ngwa plateau corresponds to a small bauxitic mantle strip (less than 1km long) near the Sa'a plateau.The Doumbouo plateau shows a bauxitic mantle belt with more than 4 km long in the NE area.The Letet1, Letet2 and Letagli plateaus occur in the centre of the area and consist of pockets of bauxitic mantles not exceeding 1km wide.The Honto, Tougong and Meka plateaus correspond to the bauxitic mantle belts stretching out on 3 km, 1.5 km and 2 km respectively.
The Fokoué province is located in the southern part of the region and covers only a surface area of 0.6 km 2 (Figure 2).It is thus the least abundant bauxite ores deposits the Mintso3 bauxitic view.The main result plateau.Macroscopic organization and morphological characteristics of these bauxitic pedons are presented in Figure 3 and Table 1 respectively.
In Doumbouo-Fokoué, all bauxit slightly sloped crests (1% -2%), between 1532 and 1590 m asl.They correspond to deep weathered soil profiles (10.0 to 12.0 m thick and more) with very thick bauxitic B horizon (4.0 to 8.0 m) lying over a saprolite C horizon (Figure 3).The uppermost and very thick bauxitic B horizon corresponds to the bauxitic mantle and is subdivided into B 11 , B 12 , B 13 and B 14 sub-horizons.The B 11 sub-horizon varies from 70 to 240 cm thick and is made up of bauxitic nodules and fragments embedded into a loose fine earth.The loose fine earth is dark red (2.5YR3/6), clayey sand, fragile, fine granular and friable.The bauxitic nodules and fragments represent 40% to 45% in volume of the sub-horizon.They are made up of red (10R4/6), very hard and more or less differentiated bauxitic matrix.The B 12 sub-horizon has 200 to 650 cm thick.It is a continuous bauxitic duricrusts consisting of non differentiated, very hard and red (10R4/6) bauxitic matrix interrupted with yellow red (5YR4/8) and fairly hard streaks.Its lower boundary is distinct and irregular when lying on B 13 sub-horizon, diffuse and regular on the contrary when lying on the saprolite C horizon.The B 13 sub-horizon is poorly represented (only in PZ1 and PS2 bauxitic pedons) (Figure 3 and Table 1), thin (less than 200 cm) and made up of discontinuous bauxitic fragments embedded into fine earth.The fine earth is clayey, pale red (2.5YR4/8), fragile, coarse granular and firm.The bauxitic fragments do not exceed 30% in volume.They show yellow red (7.5YR6/6), fairly hard and non differentiated bauxitic matrix.The B 14 sub-horizon is also poorly represented (only in PF pedon) (Figure 3 and Table 1).It always lies over the B 12 sub-horizon when exists and corresponds to a 400cm thick and continuous bauxitic duricrusts made up of very hard, dark red (2.5YR3/6) and interstitial bauxitic matrix surrounding numerous yellow red (7.5YR6/8) bauxitic nodules.
Two main bauxitic facies were identified in Dou bouo-Fokoué: the saprolite and the pisolith bauxitic facies.The saprolite bauxitic facies is the most abundant bauxitic facies in Doumbouo-Fokoué.It is observed in all bauxitic pedons where it corresponds to the bauxitic duricrusts in B 11 , B 12 and B 13 sub-horizons (Figure 3 and Table 1).The pisolith bauxitic facies is the least abundant and corresponds to the continuous bauxitic duricrusts in B 14 sub-horizon.
Optic microscopy reveals that the saprolite bauxitic facies is a reddish brown ay-and crystal-rich plasmas (Figure 4(c)).The clay-rich plasma has insepic and plasmic fabric and corresponds to a brown red ferruginous matrix dissected by a network of micro-cracks.Locally, this plasma is superimposed on the crystal-rich plasma.The crystal-rich plasma has a crystallic b-fabric with whitish grey well developed crystals of gibbsite infilling micro-cracks (Figure 4(c)) or present as individual domains under the clay-rich plasma.This s-matrix is very porous, with packing and planar voids.The pedic features refer to the micro-particles with undulic to isotic b-fabric and the well developed banded cutans with alternate layers of ferrans and gibbsitans.In the pisolith bauxitic facies, two bauxitic domains were identified: the interstitial bauxitic matrix and the bauxitic nodules (Figure 4(d)).The interstitial bauxitic matrix represents less than 5% in volume.It shows the same microscopic organization like in the saprolite bauxitic facies, with porphyric and very porous s-matrix.The bauxitic nodules are very abundant (around 45% in volume) and have spherical to sub-spherical shape, with mean diameter varying from 1 mm to 1.5 cm.
Their internal matrix also shows the same microscopic organization like in the saprolite bauxitic facie riking features of these bauxitic nodules remain the presence of continuous, well differentiated, dark red and thin (0.08 to 0.125 mm) bauxitic cortex (Figure 4(d)).They are thus bauxitic pisoliths.
From a mineral point of view, the saprolite bauxitic facies contains predominantly able 2).Gibbsite is the most abundant mineral, and represents 68.1% of minerals detected by X-ray.In XRD diagram (Figure 5), its major peak at 4.84 Å (not shown) suggests a well crystallized mineral.Hematite is also significant and represents 31.9% of minerals detected by X-ray.Copyright © 2012 SciRes.OJG    2), u 7 % of SiO espectively) and titanium (2. and 2.00% of TiO 2 respectively).In the saprolite bauxitic ts major peak is li b t not by XRD due to their very low concentrations.In the pisolith bauxitic facies, mineral association includes gibbsite, goethite and kaolinite (Table 2).Gibbsite is still the most abundant mineral, though amounting only 48.6% of minerals detected by X-ray.Its major peak also appears at 4.84 Å (not shown).Goethite is the second most abundant mineral in this facies, and represents 27.5% of minerals detected by X-ray.Its major peak at 4.16 Å confirms the significant Al substitution (6.5% of AlOOH).Kaolinite is also present (23.9% of minerals detected by X-ray).Its major peak at 7.15 Å (not shown) suggests that this mineral is well crystallized.The low gibbsite content and the total lack of hematite of the pisolith bauxite facies are the main features segregating the two bauxitic facies.Aluminium is the most abundant chemical element in these bauxitic facies (Table 3), with 49.49% and 47.49% of Al 2 O 3 respectively in the saprolite and the pisolith bauxitic facies.It is followed by relatively high content of iron (20.24% and 22.33% of Fe 2 O 3 respectively) and low to very low content of silica (1.81% and facies, the most abundant trace element is Zr with 1111 mg•kg -1 , followed by Cr (616 mg•kg -1 ) and Nb (105 mg•kg -1 ).In the pisolith bauxitic facies, the most abundant is unlike Cr with 1916 mg•kg -1 , followed by Zr (666 mg•kg -1 ), V (224 mg•kg -1 ), Zn (80 mg•kg -1 ) and Nb (35 mg•kg -1 ).In comparison with the parent rock (basalt) (Table 3), one notes a drop of the silica contents (44.61 to 1.81% of SiO 2 ) and a total exportation of alkali and alkaline earths in bauxitic facies.Inversely, aluminium, iron and some trace elements contents increase very significantly (Al 2 O 3 : 15.91% to 49.49%; Fe 2 O 3 : 13.52% to 22.33%; Cr: 9 to 1916 mg•kg -1 ; Zr: 370 to 1111 mg•kg -1 and Nb: 82 to 105 mg•kg -1 ).The iso-titanium geochemical balance calculation based on the enrichment factor (EF) evaluation between fresh rock and the bauxitic facies (Table 3) shows that the most enriched element in these bauxitic facies is Cr with EF equal to 96 and 435 in the saprolite and the pisolith bauxitic facies respectively, followed by Al (EF: 4.37 and 6.10 respectively), Zr (EF: 4.22 and 3.68 respectively), Fe (EF: 2.10 and 3.38 respec-.66 2 r 91% Copyright © 2012 SciRes.OJG tively) and Nb (EF: 1.80 and 0.87 respectively) (Table 3).47.49% of Al 2 O 3 re contents (1.81% to 7.66% of SiO 2 ).This ev bauxite ore in Doumbouo-Fokoué [2].In bauxitic mantles in this region cover a surface area of approximately 5.7 km 2 ; with a mean thickness of 6.30 m and bulk densities of 2.28 g•cm -3 and 2.09 g•cm -3 respectively in each facies.In some respects, these bauxitic mantles constitute approximately 9.2 millions tons of bauxite reserves, and thus can be classified as an important bauxite ore deposit.Since it is located close (less than 20 km) to the Fongo-Tongo bauxite ore deposit it may constitute a real additional income.The bauxitic facies in Doumbouo-Fokoué shows macroscopically white grey micrometric punctuations which represent completely weathered plagioclases.Optic microscopy reveals that plagioclases and forsterites are replaced with respect to their volume and shape (pseudomorphosis) by gibbsite and iron bearing minerals respectively.These parent rock remaining features indicate that the Doumbouo-Fokoué bauxite preserved saprolitic nature and then corresponds to ortho-bauxites [19].The [Fe -Clay -(Al + Ti)] minerals diagram (not shown) and relatively high Fe contents (20.24% and 22.33% of Fe 2 O 3 respectively) note that they belong to iron rich bauxite [1].Thus, the bauxite ore deposits in Doumbouo-Fokoué can be classified as the iron bearing lateritic ortho-bauxites.

Discussions
In Doumbouo-Fokoué bauxitic facies, very abundant silica leaching (EF: 0.06 and 0.35), total exportation of alkali and alkaline earths (EF close to 0.00) and important aluminium enrichment (EF: 4.37 and 6.10) as well as abundant accumulation of gibbsite suggest that bauxitization in this environment may co hydrolysis leading to allitization [20].Allitization, which refers to lateritization process, may be related to abun-

Bauxitic facies
Major el dant rainfall (1755 mm of mean annual rainfall), dense and dendritic hydrographic network and steep slopes which favoured a highly leaching milieu [21].Thus, lateritization in Doumbouo-Fokoué favoured bauxitization through aluminium mobilization [22] with individualization of the iron bearing lateritic ortho-bauxites which develop thick bauxitic mantles.In these bauxitic mantles, the saprolite bauxitic facies is located at the bottom, below the pisolith bauxitic facies.This vertical organization of the bauxitic facies suggests that the pisolith bauxitic facies likely derive from the saprolite bauxitic facies during geochemical and mineral individualization of bauxitic pisoliths with formation of bauxitic cortex [23], transformation of hematite into goethite and part of gibbsite into kaolinite [24].

Implication o
In the West and Central Africa, many works focuse ical organization and landscape distribution o bauxitic mantles and their implication to the landscape evolution [1,4,8,9,23,[25][26][27].They note that these bauxitic mantles are regularly located at the summit of interfluves where they form an old and residual surface dominated by a saprolite bauxitic facies and named African surface by Valeton [1].These initial bauxitic mantles were probably formed during early to low Miocene under contrasted climate [28] in an oxidizing-leaching milieu which favoured crystallization of hematite instead of goethite [29].Thus, the initial relief in DoumbouoFokoué likely correspond to this old and continuous bauxitic mantle surface slightly slopped southwards which consisted of hematite-rich saprolite bauxitic facies (Figure 6(a)).
Since the upper Miocene, the old and continuous bauxiti ocess [25] resulting into the separation of individual  er Pangaea," In: M. J. J. Lateritic Bauxites, "Deogy," Elsevier, Amsterdam, radatio [31] of the region which belongs to the Cameroon volanic line.It firs c activities along the Cameroon volcanic line [15] that contributed to the dislocation of this African surface and its abandonment to intense erosion processes [32].It also refers to the transformation at topsoil of the saprolite bauxitic facies into the pisolith bauxitic facies during individualisation of pisoliths [33], originating a thick bauxitic mantle with pisolithic facies overlying the saprolite bauxitic facies (Figure 6(a)).It also refers to regressive erosion favoured by the dense hydrographic network which strongly incised the African surface into large and elongated bauxitic plateaus surrounded by deep and profound valleys (Figure 6(b)).It also concerns deferruginization at topsoil of the bauxitic mantles where iron bearing secondary minerals are preferably dissolved in a highly leaching milieu [21] as indicated by the drop of Fe 2 O 3 contents (not shown) in fine earth [34,35].This originated at topsoil a gravelled light soil horizon consisting of bauxitic pisoliths and fragments embedded into clayey fine earth [23,36].The gravelled light soil horizon develops downward at the expense of the bauxitic mantles.At the same time at topsoil, it is subject to surface erosion [32].Both processes act together and cause significant reduction of the bauxitic mantles thickness [34,35,37].Lastly, the bauxitic mantle degradation in Doumbouo-Fokoué refers to mechanical erosion that causes around the plateaus dismantling of the bauxitic mantles into bauxitic blocks which move down-slopes, inducing bauxitic mantles to reduce in size [8].All these degradation processes acted together at different scales

Conclusions
In Doumbouo-Fokoué, important bauxite ore deposits were developed from basaltic rocks during early to lower Miocene under contrasted climates.They correspond to the iron bearing lateritic ortho-bauxite ore deposits with 9. [3] E. Lee Bray, "US Geological Survey, Mineral Commodity Summaries," United States Government Printing Office, Washington, 2009.

Figure 6 .
Figure 6.Landscape lowering of the Doumbouo-Fokoué bauxitic mantles.bauxiticplateaus.This deg n process was favoured by humid equatorial climate[30] and tectonic instability tly refers to the late Miocene tectonic undulated bauxitic plateaus completely overlapped to the Bamiléké plateau and then, contributed to general levelling of the relief in this region (Figure6(c)).These demonstrations allow relating this bauxitic mantle to the other bauxitic mantles of the African surface (Adamaoua and Bangam in Cameroon, Guinea, Burkina Faso, Ivory Coast and Nigeria) and then to explain the general lowering of this African surface as noted in the Central and West Africa[1,25].
2 millions tons of bauxite reserves.These lateritic bauxites are differentiated into pisolith bauxitic facies locally overlying sa and con ribut to lo r relie in this area.So sinc the upper Miocene, evolution of the landscape attributed to the bauxitic mantles development in Doumbouo-Fokoué differentiates three major stapes:  Development of the old and residual bauxitic surface (African surface) during the early to lower Miocene by intense bauxitization process under contrasted climates;  Intense degradation of the African surface favoured by regressive erosion, deferruginization and mechanical dismantling of the bauxitic mantle under humid sub-equatorial climate;  General lowering of the relief under the control of the above degradation processes and the joining up of this relief with the Bamiléké plateau.Thus, bauxitic mantles in Doumbouo-Fokoué, through their landscape distribution, their vertical organization and bauxitic facies evolution, have contributed significantly to the general levelling of the Bamiléké plateau in the West Cameroon Highlands.1990, pp. 27, 6 l'Inde et l'Australie," Thèse Université Louis mawa," Ph.D. Thesis, Erlangen Unibauxites du Mali.Géochimie et ol.107, No. *, 1993, pp.277-280.the Mechanism .Melfi and I. Valeton, "Climats et Paléo-24.