Ririwai (Liruei)

Ririwai is one of the most studied of the Nigerian ring-complexes with an extensive literature on the geology, petrology and mineralization. The complex of 17x16 km forms an isolated massif rising to over 500 m above the surrounding plains and with steep outer slopes against a ring-dyke of porphyritic fayalite granite. About half of the complex is composed of volcanic rocks, the rest of peralkaline and biotite granites. The volcanics are the earlier and were divided by Jacobson and MacLeod (1977) into an early extrusive group, including rhyolites, tuffs, breccias and basalts, and an early intrusive group which includes further rhyolites, tuffs and basalts together with vent agglomerates and explosion breccias; a late intrusive volcanic phase comprises quartz-pyroxene-fayalite porphyry, rhyolites and breccias. The early basalts are not voluminous and much altered but appear to be a series from alkali olivine basalt through hawaiite to mugearite. The rhyolitic rocks include a high proportion of ignimbrites with varying degrees of welding and of peralkalinity. The development of aegirine and arfvedsonite is assumed by most researchers to be a primary magmatic feature but Kinnaird et al. (1985) argue that it is a subsolidus mineralogy related to late fluid phases. Occasional areas of compact aphyric rocks may be lavas or examples of extreme welding and compaction. Three major volcanic vents have been preserved within the complex the largest of which, Dutsen Shetu, has a crude annular structure and concentrically disposed vent agglomerates and other rocks around a central fayalite porphyry plug. A general succession of 19 units in the northwestern part of the complex is described in some detail by Jacobson and MacLeod (1977) with similar accounts for other areas of extrusive volcanics within the complex, while the intrusive volcanic rocks, which are centred on three complexes, are similarly described in detail. The volcanic rocks are probably preserved because of development of a caldera, a plutonic phase being initiated by emplacement of a granite porphyry ring-dyke along the outer margin of which is a major ring-fault. The ring-dyke has been truncated by later intrusions of granite and in the southwest it is narrow and discontinuous. The rock consists of quartz, alkali feldspar, fayalite, ferrohedenbergite - aegirine-hedenbergite, ferrorichterite, aenigmatite, Fe-Ti oxides and accessories including allanite and fluorite. Three areas of peralkaline granite are located in the southeast of the complex. A small intrusion cutting the ring-dyke comprises phenocrysts of perthite and quartz in a groundmass of alkali feldspar and quartz, often graphically intergrown, and prisms of arfvedsonite. The largest late granite contains aegirine and arfvedsonite and accessories including astrophyllite, aenigmatite and fluorite. In the third granite, known as the Kaffo Valley albite-riebeckite granite, aegirine is subordinate to, and generally enclosed by, lithian arfvedsonite with accessories including cryolite, thomsenolite, astrophyllite, topaz and uraniferous pyrochlore. A large biotite granite covering some 30 km2 occupies the central area of the complex and is itself cut by small bodies of biotite granite and microgranite. Accessories of these granites include fluorite, columbite, xenotime and thorite, and the roof area of the granite has been extensively altered by late stage magmatic and post-magmatic fluid processes (Kinnaird et al., 1985; Kinnaird, 1985). Subsolidus episodes of albitization that accompany columbite mineralization of the biotite granites are described by Martin and Bowden (1981) and detailed descriptions, with numerous analyses, of thorite and coffinite, both of which are exceptionally rich in Zr, are given by Pointer et al. (1988) and of zircon in the same rocks by Pointer et al. (1989). The fullest account of the geology and some analyses of the main rock types and of mica, aenigmatite and aegirine will be found in Jacobson and MacLeod (1977). Analyses of amphiboles will be found in Borley (1963), other mafic minerals in Borley (1976a) and aenigmatite in Borley (1976b). Fadipe (1987) gives a partial analysis of pyrochlore. The fullest chemical and mineralogical data are in Kinnaird et al. (1985), including REE and other trace elements for rocks and analyses of columbite. The geochemistry of the granites, with particular reference to their mineralization and ore-bearing potential, has been studied by Ekwere and Olade (1984), while Ekwere (1985) considers the same topics in the light of Li, F, Rb, Ba and Sr data. Analyses for U and Th in numerous granites and some volcanic rocks are presented by Bowden et al. (1981) and REE data by Bowden et al. (1979); Rb-Sr isotopic data are to be found in van Breemen et al. (1975). Pb, Sr and Nd isotope analyses are given for three granites by Dickin et al. (1991). The chemistry of the Kaffo Valley granite has been investigated by Orajaka (1986). A gravity survey over Ririwai is described by Ajakaiye (1968).