Sedimentological and geochemical study of the Bongongo and Ngol travertines located at the Cameroon Volcanic Line
Résumé
A sedimentological and geochemical study was carried out on the travertines from Bongongo and Ngol locations along the Cameroon Volcanic Line. This work seeks to ascertain: (1) the origin and typology of the studied travertine deposits, (2) the palaeo-environmental depositional model and (3) the typical characteristic of the hydrothermal system. The distribution and typology of travertine deposits were performed from natural outcrops. Oxygen and carbon stable isotope methods have been employed in the detail study of representative travertine samples from Bongongo and Ngol. The different travertine deposits along Ngol stream valley occur around waterfall/cascade, slopes and pools. At Bongongo the travertine precipitation can be linked to temporal fluctuation of carbonate mineral saturation level in rising thermal fluids and the depth of the boiling zone. The hydrothermal system of Ngol and Bongongo has been clarified using results of geochemical analyses of stable isotope, which can be applied regionally. The studied travertines are characterized by δ 18 O values from −8.4 to −6.4‰ V-PDB for Ngol and −5.8 to −5.9‰ V-PDB for Bongongo and δ 13 C values from 0.4 to 0.5‰ V-PDB for Ngol and from 1.1 to 2‰ V-PDB for Bongongo, in accordance with global travertine deposit environments. The values of stable carbon isotopes analyses of Bongongo and Ngol travertines are very far from the isotopic composition of magmatic carbon. Based on the geothermal environment, the high temperature parent solution (31 °C and 49 °C) and stable isotope study, it can be considered that the studied travertine from Bongongo and Ngol are charasteristic of the thermogenic type.