Petrographic and mechanical study of aggregates from Central Chad: a case for natural selection of hard rocks

This study focusses on the scarce resources of high-quality silicated rock aggregates in Central Chad, a wide area almost devoid of hard rock outcrops. We provide an original study of the petrographic and mechanical characteristics of Alfallé massif together with a review of quarries in activity or potentially exploitable. The resistance to degradation of the prepared aggregates were evaluated using the standard test of Los Angeles and micro-Deval. The Alfallé massif is made up of three lithologies: alkali feldspar coarse granite, biotite-rich fine-grained quartz monzonite and biotite-rich fine-grained gneiss. The mechanical test results obtained indicate that the materials studied have excellent to satisfactory properties, and that the Alfallé massif provides the best characteristics among six different quarrying areas in Central Chad. Moreover, the three quarries that were once isolated islands in the mega-Chad paleolake yield on average higher resistance. We thus propose that these rare residual reliefs within the mega-Chad paleolake area reached optimal mechanical properties through a natural selection process, specific of the paleolake erosional conditions.