Juquia is an asymmetric, zoned intrusion of 14 km2 intruded into Precambrian gneisses of the Acungui Group, which are fenitized in places. The major rock types are pyroxenites, containing a titanian augite, peridotites and alkali gabbros with a circular zoned carbonatite on the western side of the complex; carbonatite dykes are widespread. Nepheline syenite is found in two areas, one of which lies to the northwest of the main complex; ijolite-melteigite is confined to several small masses, mainly rimming the carbonatite. The outer part of the carbonatite is an apatite beforsite which envelops a core of banded ankerite-dolomite carbonatite. Among accessory minerals recognized in the carbonatite are apatite, magnetite, ilmenite, baryte, pyrochlore, ancylite, norsethite, pyrite and galena. Nepheline syenites comprise K-feldspar, nepheline, aegirine-augite and biotite and are considered to represent feldspathized ijolites (Born, 1971, p. 223). The ijolites consist of nepheline, aegirine-augite and occasional biotite. Apart from the carbonatite dykes, lamprophyres and tinguaites also occur within and around the complex.