Caractérisation pétrologique, hydrogéochimique et reconstitution paleoenvironnementale des formations travertineuses de Ngol et de Bongongo (Ouest-Cameroun)

Abstract

The study area Bongongo is located in the Ekondo-Titi Subdivision, Ndian Division in the Southwest Region, while Ngol is located in the Manjo Subdivision, Moungo Division in the Littoral Region of Cameroon. The aim of this work is to determine the origin, typology, conditions of emplacement and to reconstruct the palaeoenvironmental signatures of the Ngol and Bongongo peri volcanic travertine deposits. The travertines of Bongongo outcrop in a swampy undergrowth with three main observable facies consisting of: an unconsolidated facies consisting essentially of carbonate sludge, consolidated facies in the form of a dome, and solid consolidated facies. Those of Ngol are located in a mountainous landscape and precipitate mainly in a fluvial environment with four main observable facies comprising of: slope, fall, waterfall and dam. Microscopic observations revealed associations of micritic calcite, microsparite and the existence of microbial and fungal structures. The travertines of both sites are mainly calcareous, consisting of more than 90% calcite and 5% quartz, with CaO contents greater than 50% in all samples. Stable isotope data for carbon 13 (1.4 ‰ and 0.5 ‰ PDB) and oxygen 18 (22.4 ‰ and 24.5 ‰ SMOW) for Ngol and Bongongo travertines are in agreement with the current global deposit environments of travertines. The geochronological data show that the travertine formations are plio quaternary. Thus, the older Ngol travertines have an age of 3524.4634 ± 10.79 ka b2k whereas those of Bongongo are 50.36 ± 9.40 ka b2k. The physical and hydrogeochemical data of Ngol waters show high source values for some cations and anions such as: Ca2+ (484.6 mg/l and 130 mg/l), NH4 + (336.4 mg/l and 198.9 mg/l), Na + (119.1 mg/l and 239.5 mg/l), HCO3- (3.782 mg/l) and SO4- (93.66 mg/l). The conductivity at the sources is 2950 μS/cm and 1640 μS/cm. The pH varies from 6.64 to 9.43. The results of sedimentological and hydrogeochemical analyzes suggest that the precipitation of travertines would be mainly controlled by morphological, physical, neotectonic, climatic and chemical factors. The δ13C values of the Bongongo and Ngol travertins are in the intermediate range of the thermogenic and meteogenic forming travertins (-2/ + 3 ‰ PDB). However, the geothermal environment and high temperature water sources (31°C and 49°C) define them as thermogenic. Also, the δ13C (CO2) values recalculated from two different equations admit that the CO2 present in the travertines is of magmatic origin, which is consistent with the tectonic and geothermal framework of the Bongongo and Ngol travertines, which are located on an active volcanic zone. The distribution of major, trace and Rare Earth elements, as well as the high Mg/Sr and Mn/Sr ratios suggest an open system conditions during the early diagenesis stage, at the time of carbonate precipitation. More open conditions and diagenesis of early-stage interstitial fluids led to different degrees of alteration of carbonate minerals, including a decrease in Sr content and an increase in Mn content. The hydrogeochemical data testify to the strong mineralization of Ngol waters. The positive correlations between the Ca2+, NH4+ , Na+ , K+ and Mg2 + ions attribute a double origin to the ions present in the water that would come from both the water-rock interaction and the anthropogenic action by the use of the fertilizers.