Mobilization and redistribution of major and trace elements in two weathering profiles developed on serpentinites in the Lomié ultramafic complex, South-East Cameroon

https://doi.org/10.1016/j.jafrearsci.2007.10.006Get rights and content

Abstract

The behaviour of major and trace elements have been studied along two serpentinite weathering profiles located in the Kongo-Nkamouna and Mang North sites of the Lomié ultramafic complex.

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. Normalized REE patterns according to the CI chondrites reveal that: (i) all REE are below chondrites abundances in the Mang North sample; (ii) the (La/Yb)N ratio value is higher in the Nkamouna sample (23.72) than in the Mang one (1.78), this confirms the slightly more weathered nature of the Nkamouna sample. Normalized PGE patterns according to the same CI chondrites reveal a negative Pt anomaly in the Mang sample. The Nkamouna sample is characterized by a flat normalized PGE pattern.

All element contents increase highly from the parent rock to the coarse saprolite.

In the weathering profiles, Fe2O3 contents decrease from the bottom to the top contrarily to Al2O3, SiO2 and TiO2. The contents of alkali and alkaline oxides are under detection limit.

Concerning trace elements, Cr, Ni, Co, Cu, Zn and Sc decrease considerably from the bottom to the top while Zr, Th, U, Be, Sb, Sn, W, Ta, Sr, Rb, Hf, Y, Li, Ga, Nb and Pb increase towards the clayey surface soil. Chromium, Ni and Co contents are high in the weathered materials in particular in the saprolite zone and in the nodules.

REE contents are high in the weathered materials, particularly in Nkamouna. Their concentrations decrease along both profiles. Light REE are more abundant than heavy REE. Normalized REE patterns according to the parent rock reveal positive Ce anomalies in all the weathered materials and negative Eu anomalies only at the bottom of the coarse saprolite (Nkamouna site). Positive Ce anomalies are higher in the nodular horizon of both profiles. An additional calculation method of lanthanide anomalies, using NASC data, confirms positive Ce anomalies ([Ce/Ce]NASC = 1.15 to 60.68) in several weathered materials except in nodules ([Ce/Ce]NASC = 0.76) of the upper nodular horizon (Nkamouna profile). The (La/Yb)N ratios values are lower in the Nkamouna profile than in Mang site.

PGE are more abundant in the weathered materials than in the parent rock. The highest contents are obtained in the coarse saprolite and in the nodules. The elements with high contents along both profiles are Pt (63–70 ppb), Ru (49–52 ppb) and Ir (41 ppb). Normalized PGE patterns show positive Pt anomalies and negative Ru anomalies.

The mass balance evaluation, using thorium as immobile element, reveals that:

  • major elements have been depleted along the weathering profile, except for Fe, Mn and Ti that have been enriched even only in the coarse saprolite;

  • all the trace elements have been depleted along both profiles, except for Cr, Co, Zn, Sc, Cu, Ba, Y, Ga, U and Nb that have been enriched in the coarse saprolite;

  • rare earth elements have been abundantly accumulated in the coarse saprolite, before their depletion towards the top of the profiles;

  • platinum-group elements have been abundantly accumulated in the coarse saprolite but have been depleted towards the clayey surface soil.

Moreover, from a pedogenetical point of view, this study shows that the weathering profiles are autochtonous, except in the upper part of the soils where some allochtonous materials are revealed by the presence of zircon grains.

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:

  • 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);

  • 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:

  • (1)

    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

References (52)

  • R.T. Lowson et al.

    238U/238U and 230 Th/234Th activity ratios in minerals phases of alteritic weathered zone

    Geochim. Cosmochim. Acta

    (1986)
  • A. Manceau et al.

    Crystal chemistry of trace elements in natural and synthetic goethite

    Geochim. Cosmochim. Acta

    (2000)
  • C. Manikyamba et al.

    Crustal growth processes as illustrated by the Neoarchaean intraoceanic magmatism from Gadwal greenstone belt, Eastern Dharwar craton, India

    Gondwana Res.

    (2007)
  • A. Marker et al.

    The formation of rare earth element scavenger minerals in weathering products derived from alkaline rocks in SE-Bahia, Brazil

    Chem. Geol.

    (1990)
  • W.F. McDonough et al.

    The composition of the Earth

    Chem. Geol.

    (1995)
  • J-J. Middelburg et al.

    Chemical processes affecting the mobility of major, minor and trace elements during weathering of granitic rocks

    Chem. Geol.

    (1988)
  • J.E. Mungall et al.

    Severe leaching of trachyte glass without devitrification, Terceira, Azores

    Geochim. Cosmochim. Acta

    (1994)
  • G. Ottonello et al.

    Petrogenesis of some Ligurian peridotites, II. Rare earth element chemistry

    Geochim. Cosmochim. Acta

    (1979)
  • E. Temgoua et al.

    Démantèlement des paysages cuirassés anciens en zones forestières tropicales d’Afrique centrale: formation d’accumulations ferrugineuses actuelles en bas de versant

    C. R. Géosci.

    (2002)
  • D. Traoré et al.

    Platinum and palladium mobility in supergene environment: the residual origin of the Pirogues River mineralization, New Caledonia

    J. Geochem. Explor.

    (2006)
  • S.A. Wood

    The role of humic substances in the transport and fixation of metals of economic interest (Au, Pt, Pd, U, V)

    Ore Geol. Rev.

    (1996)
  • R. Yongué-Fouateu et al.

    Nickel and cobalt distribution in the laterites of the Lomié region, South-East Cameroon

    J. Afr. Earth Sci.

    (2006)
  • Th. Augé

    Minéraux du groupe du platine en Nouvelle-Calédonie

    C. R. Acad. Sci. Paris

    (1993)
  • Y. Balashov et al.

    The effects of climate and facies environment on the fractionation of rare earth elements during sedimentation

    Geochem. Int.

    (1964)
  • J.F.W. Bowles

    The development of platinum-group minerals in laterites

    Econ. Geol.

    (1986)
  • J.F.W. Bowles

    The development of Platinum-Group Minerals (PGM) in laterites: mineral morphology

    Chron. Rech. Min.

    (1995)
  • Cited by (0)

    View full text