Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinites in the Lomié ultramafic complex, South-East Cameroon
Introduction
The southern Cameroon basement is essentially made up of rocks belonging to the Congo Craton and the Pan-African metamorphic belt (Lasserre and Soba, 1976). This part of the Pan-African metamorphic belt has been subjected to various weathering processes that have controlled the distribution of major and trace elements (Temgoua et al., 2002, Bitom et al., 2003).
The weathering of serpentinites (in the South-East part of the basement) has led to secondary mineralization dominated by chlorites, smectites, talc, goethite, maghemite, hematite, kaolinite and gibbsite (Yongué-Fouateu et al., 2006). This weathering has also liberated some chemical elements that show various types of behaviour in the weathering profile.
For example, Yongué-Fouateu et al. (2006) showed that highest concentrations of nickel and cobalt are located in the saprolite zone. In contrast, weathering mobilises uranium through dissolution of primary minerals from the coarse saprolite whereas thorium precipitates in situ (Moreira-Nordemann and Sieffermann, 1979). Muller and Calas (1989) showed that U and Th concentrations increase slightly from the bottom to the top of a profile on gneiss at the forest-savanna boundary in Cameroon.
The REE studies have proved their redistribution along the weathering profile instead of being carried away in solutions (Middelburg et al., 1988, McLennan, 1989). REE were leached from the top and accumulated at the bottom of the profile. They were adsorbed at the interfaces of the new formed clay minerals, Fe-oxides and residual minerals (Decarreau et al., 1979, Laufer et al., 1984). Under hot-humid climatic conditions, enrichment of light REE upon heavy REE is observed (Balashov et al., 1964).
Very few studies have documented the behaviour of REE, Th and U in tropical weathering profiles (Muller and Calas, 1989, Braun et al., 1990, Marker and de Oliveira, 1990, Boulangé and Colin, 1994, Braun et al., 1998). In South Cameroon particularly, the study of these elements in soils remain limited to investigations of lateritic profiles on syenite in the rainforest environment (Braun et al., 1990, Braun et al., 1993) and of soil sequence hosted in gneiss at the forest-savanna boundary zone (Braun et al., 1998). The geochemistry of PGE in ultramafic rocks is well understood as compared to that of PGE in weathered materials from these ultramafic rocks (Bowles et al., 1994, Salpéteur et al., 1995, Tashko et al., 1996, Traoré et al., 2006). No previous investigations have been carried out on the behaviour of trace, rare earth and platinum-group elements in laterites related to ultramafic rocks in the Central African rainforest. The aim of this study is to document and explain the mobilization and redistribution of major and trace elements in relation with the different stages of serpentinite weathering along lateritic profiles of two separate bodies of the Lomié ultramafic complex in the South-East of Cameroon.
Section snippets
Geographical and geological setting
The Lomié ultramafic complex is located in South-East Cameroon (Fig. 1), with a rainfall of about 1650 mm per year and an average temperature of 23.5 °C, typical of dense humid forests (Letouzey, 1985). Such climatic conditions are suitable for supergene weathering leading to the formation of red ferrallitic soils (Yongué-Fouateu et al., 2006). The ultramafic complex is part of the South Cameroon plateau, with a moderate hilly geomorphology (750 m) and large swampy valleys.
The Lomié ultramafic
Sampling and analytical procedures
Two rock samples were collected, one (NK6) in the Kongo-Nkamouna ultramafic body (Fig. 2) and the second (MA6) in the Mang North site (Fig. 3).
Fourteen samples (eight from Kongo-Nkamouna and six from Mang North) were collected from two pits. In general, one sample per horizon was used for mineralogical and chemical analyses. However, in the nodular horizons, separate samples of matrix and nodules were collected, and two samples were collected from the coarse saprolite of the Nkamouna site
Petrology of serpentinites
Serpentinites are greenish and massive rocks, containing rare inclusions of ore minerals. Under optical microscope, the serpentinites are seen to be made up of close to 90% antigorite as tabular crystals and 10% of opaque minerals (magnetite, olivine and chromite) (Table 1), with a “mesh” structure. The opaque minerals form plates of few millimetres in length.
Under the electron microscope, the details of the metamorphic texture are visible (Fig. 4), with bright grains of Cr-rich magnetite
Nkamouna weathering profile
The profile is located on the eastern flank of the Edje river interfluve (Fig. 2). It is 18.70 m thick above the water table and presents from the base to the top, according to Tardy (1993), a coarse saprolite, a fine saprolite, a lower nodular horizon, an upper nodular horizon and a loose clayey horizon (Fig. 7). The Nkamouna weathering profile is dominated by Fe-oxides (magnetite, goethite and hematite) with high Fe2O3 contents (up to 40%) corresponding to the decrease of SiO2 and Al2O3
Distribution of trace elements
Two groups of trace elements can be distinguished on the basis of their behaviour along the lateritic profile:
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chromium, Ni, Co, Zn, Cu and Sc are more concentrated at the bottom of the coarse saprolite and strongly decrease towards the clayey surface soil (Table 3);
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yttrium, Ga, Pb, Th and U progressively increase throughout the weathering profile with a highest value in nodular horizons (Table 3). Zirconium, Sb, Nb, Hf, Cd, Mo, Sn, W, Cs, Rb, Sr and Ta show a similar behaviour to that of the
REE distribution and fractionation during weathering processes
Rare earth elements are globally less concentrated in Mang North than in Nkamouna. They increase drastically from the parent rock to the upper horizons (Table 4). In the Nkamouna site, REE are relatively enriched in the nodules (lower nodular horizon) whereas a similar phenomenon is instead shown at the bottom of nodular horizon (Table 4) of the Mang site. Light REE are more abundant than heavy REE in all weathered materials, and reveal maximum concentrations at the bottom of nodular horizons (
PGE distribution
In the weathering profiles, element contents increase greatly and vary between 3 and 70 ppb (Table 5). The elevated contents in Pt, Rh, Ir and Ru are obtained in the coarse saprolite particularly in Mang site. The highest PGE values are related to the coarse saprolite where Pt reaches 70 ppb (Mang North) and 63.8 ppb (Nkamouna). In the fine saprolite, Pt drops to 47 ppb (Mang North) and 31.2 ppb (Nkamouna) but increases afresh in the matrix of nodular horizons with an average value (35 ppb). The
Mass balance evaluation
It is concerned with the evaluation of element distribution during supergene weathering. Several methods have been applied on supergene weathering. In extreme weathering conditions, the mobility of refractory elements such as Al, Ti, Zr and even Th was demonstrated (Mungall and Martin, 1994, Nahon and Merino, 1997, Cornu et al., 1999, Kurtz et al., 2000). Braun et al. (2005) showed that the mobility of refractory elements can also depend on the presence of organic ligands in the soil solution.
Petrology of serpentinites
The serpentinites are characterized by high SiO2 and MgO contents, very low trace, rare earth and platinum-group element contents. These characters confirm the alteration of ultramafic rocks, origin of the Lomié serpentinites (Seme Mouangué, 1998). Lanthanides and PGE contents are higher in Nkamouna than in Mang North. The higher contents in these elements could be due to a new mobilization during the first stages of the weathering processes in Nkamouna. The Mang sample seems less affected by
Conclusions
The results of this study lead us to the following conclusions:
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The serpentinites are characterized by high SiO2 and MgO contents, very low trace, rare earth and platinum-group element contents. Lanthanide and PGE contents are higher in the Nkamouna sample than in Mang North, less affected by weathering processes. Normalized REE patterns according to the CI chondrites reveal that some light REE (La, Ce, Pr, Nd) enrichment process have occurred in the Nkamouna sample; all REE are below chondrites
Acknowledgements
This work was supported by Grants from Geology Department of University of Toronto (Canada) for stages of samples preparation and analysis. The authors are grateful to James E. Mungall (University of Toronto, Canada) and Germain M. Mboudou (University of Buea, Cameroon) for their helpful comments on the original manuscript. The editor (Pr. S. Muhongo), Dr. J.J. Braun and other two anonymous reviewers are gratefully thanked for their constructive comments. This work is a contribution to IGCP 479
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