干旱区生态环境知识资源中心

Some terrestrial areas have favourable environmental features that allow the preservation and scientific study of processes which can be used as geomarkers. The crater-like Richat structure (Sahara desert, Mauritania) is a recently proposed protected natural area, which can be considered as one of these sites. Volcanic, hydrothermal and arid environments merge together, making easy their investigation in the field and their later characterization by the use of a set of geochemical and mineralogical techniques (XRD, transmitted and reflected light microscopy, SEM-EDX, laser-Raman spectroscopy, XRF, ICP-MS and stable isotope spectroscopy).

The study presented here focuses on the spectacular silica-rich megrabreccias which crop out, as a large and thick lenticular body, at the centre of the Richat structure. Based on paragenetic relationships, six types of silica-rich megabreccias were identified. The link between the geochemical and mineralogical characteristics was established, from macro to micro scale, showing the existence of: a) variable distributions of the major, minor and trace elements; b) polymorphic phases; c) micrometric cataclastic features and d) different textural intergrowths and degrees of crystallinity of some minerals. The oxygen and carbon isotopic signatures of the carbonates, within the silica-rich megabreccias, point out a meteoric (low-temperature hydrothermal) origin for the source waters.