The Muri alkaline complex is centred on the Muri Mountains, which run along the Guyana-Brazil border, and is emplaced in Precambrian granodiorite and gneiss which are locally fenitized. At the northeastern end of the complex rises the conical hill of Twareitau, while in the southwest the complex broadens, part lying in Brazil and known as Mutum (see Brazil No. 5), and part in Guyana, the latter developing a series of concentric ridges (C.N. Barron, personal communication, 1985). It is not clear whether the northeastern and southwestern parts are continuous. The area of the complex is at least 40 km2, and may be considerably more. The principal rock types are nepheline syenites of a range of textural types and consisting of microcline microperthite, up to 30% nepheline, sometimes altered or replaced by cancrinite and sodalite, aegirine-augite, amphibole, biotite and accessories including abundant zircon. Ijolite, and particularly urtite, are also present. Enclaves and xenoliths of phonolitic rocks are widespread in the nepheline syenites.
Twareitau Mountain has an area of some 5 km2 and is covered by a laterized breccia or conglomerate which in places can be seen to rest on a cavernous, potholed phosphate-rich rock, 12 analyses of which averaged 19% P2O5 (Barron, 1981, p. 4). X-ray diffraction indicated strengite and a member of the goyazite- florencite series as major phases (op. cit.). It is considered very probable that carbonatite underlies Twareitau, although drilling has not been to a sufficient depth to reach unweathered rock.