Major and trace elements behaviour in three weathering profiles developed on syenitic rocks in Ina pluton (syenitic set of Linté, Central Cameroon)

Abstract

The present work deals with the behaviour of major, trace and rare earth elements in three weathering profiles (P1, P2, P3) developed on syenites from Ina pluton in central Cameroon. The mineralogical composition of weathering materials was determined using X-ray diffraction analysis. Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) have been used to determine the geochemical composition of parent syenites and the overlying weathered materials. The parent rocks of the studied profiles are quartz alkali syenites and alkali syenite. They have metaluminous character, granular porphyroid texture and were emplaced in a subduction context. Macroscopically, the weathering profiles mainly show from bottom to top: (i) coarse saprolite; (ii) fine saprolite; (iii) nodular horizon and (iv) superficial loose clay horizon. The upslope profiles (P1, P2) are less developed than the slope profile (P3). The alteration (CIA, MIA and PIA) and lateritisation (IOL) indices values show that upslope weathering profiles are characterised by moderate weathering and weak lateritization while, slope profile is intensely weathered and weak to moderate lateritization. Kaolinite, quartz, gibbsite and alkali feldspars are the common minerals to all 3 profiles. Illite, ilmenite, hematite and magnetite are specific to profiles P1 and P2, while halloysite and goethite are only present in P3 weathering profile. Geochemical data and mass balances show that SiO₂, K₂O, Na₂O, CaO, MgO, P₂O₅, some LILE (Ba, Sr, Rb) and Zn are leached with varying intensities in the studied profiles. Inversely, Al₂O₃, Fe₂O₃, TiO₂, Cs, Pb, HFSE and some transition elements (Ni, Co, Cr, V, Cu) are enriched overall. REE contents increase in the weathering products of P1 and P2 profiles mainly in deep horizons while they decrease in those of P3 profile. The highest Y contents are recorded in the lower horizons of all the weathering profiles. The HFSE, REE and certain transition elements (Ni, Co, Cr, V and Cu) distribution are controlled by clays (kaolinite, halloysite, illite), iron and aluminium sesquioxides (hematite, magnetite, goethite and gibbsite), secondary phosphate minerals such as florencite and residual accessory minerals such as apatite. The negative anomalies observed in most weathering materials are related to the reducing conditions of the environment reflected by very low Eh (-32- 87.7 mv) and moderate pH (4.7–6.9) values. The positive anomalies registered at the top horizons of studied profiles are due to the fixation of Ce³⁺ cations by adsorption on the surface of iron and aluminium hydroxides or by the presence of Ce⁴⁺ in the residual accessory minerals such as zircon. It could also be explained by external influences such as solutions and/or solids that may have affected their genesis. The negative Eu anomalies are mainly obtained from weathering of Eu-bearing minerals. The studied weathering products are enriched in some chemical elements, mainly REE (up to 626.36) and Zr (up to 2650 ppm), which could be explored further.