Chiriguelo is an approximately circular complex about 6 km in diameter. Precambrian schists and quartzites, overlain by Palaeozoic sedimentary rocks, are domed around the complex, and on the southern and eastern margins these rocks are overlain unconformably by basalts of the Alto Parana Formation. The complex contains a central sovitic core which is intersected by rodbergite and late carbonatite stringers. There is an annular intrusion of apatite-melanite syenite the whole being surrounded, and partly capped, by potassic fenites. Fenitization extends unevenly into schists and gneisses in which concentrations of thorite and REE-bearing minerals occur.