The Mata Preto complex is intruded into the Precambrian metasediments of the Acungui Group and Tres Corregos granites but shows little topographical expression. The intrusion has been investigated in detail only in a small area in the southwest and a rather larger area in the northwest of the complex. The earliest rocks are ijolites and nepheline syenites which were followed by the intrusion of tuffs and breccias with a matrix of carbonate and silica and containing xenoliths of alkaline rocks, fenite and granite. Carbonatites were then emplaced, followed by various types of phonolite. The carbonatite of the southwest of the complex consists of an earlier feldspathic carbonatite breccia and a ferruginous carbonatite with local enrichment in REE, Th and P. A much more extensive area of carbonatite in the northwest of the complex consists essentially of sovites containing abundant fluorite and pyrite.