An approximately circular, 4 km diameter intrusion, Hossere Tchegui forms a hill which is composed predominantly of peralkaline syenites, that are cut by a small intrusion and dykes of trachyte. Gabbro forms an arcuate intrusion in the northwest. The Precambrian schists into which the intrusion is emplaced are thermally metamorphosed. The syenites comprise an outer ring which is cut by later syenites, the former having been mapped as 'white' and the latter as 'brown' syenites (Lasserre, 1978). The syenites contain quartz, alkali feldspar, aegirine, which is zoned with deeper green cores, riebeckite, biotite, fluorite, calcite and apatite.