Mineralogical and geochemical signatures of clays associated with rhyodacites in the Nefza area (northern Tunisia)

International audienceThe geology of northern Tunisia is marked by magmatic extrusion that occurred during the Middle Miocene (Langhian-Lower Tortonian), which led to the outcrops of rhyodacites in the Nefza-Tabarka region. This event is contemporaneous with the Alpine compressional phase, which is well-characterised in the western Mediterranean area, where intense fracturing and hydrothermalism occurred with evidence of metallogenic consequences. In this paper, a detailed study is presented on the acid volcanic rocks that outcrop at the core of the Oued Belif structure in the Nefza area of northern Tunisia. The results indicate that these series have undergone various transformations subsequent to their extrusion. These alterations include ferrugination, silicification, argilitisation and devitrification of volcanic glass. Petrographic observations demonstrated that the primary minerals, particularly feldspars, biotite and mesostasis glass, were affected by hydrothermal and meteoric weathering. The mineralogical study of the neogenic products revealed a nearly monomineral smectitic phase with relatively low levels of added illite and/or kaolinite. These neoformed smectites were classified as ferroan beidellites-nontronite based on thermal and crystallochemical analyses. Chemical analysis of the major elements, trace elements and rare Earth elements (REEs) show the presence of Al, Fe and K and an enrichment of REE in the clay fraction with a greater fractionation of light rare Earth elements (LREEs) compared with that of heavy rare Earth elements (HREEs). The abundance of these elements is attributed to their mobility during chemical weathering of acidic lavas and their adsorption by clay mineral