stripes
The 17.5x15 km Banke complex is bounded by a ring-fault but one third of the enclosed area is occupied by basement rocks. The northern part of the complex is mountainous and comprises volcanic rocks, the central area of biotite granite gives low relief, while the south is a plain underlain by basement rocks. The volcanic rocks comprise a pre-caldera sequence, preserved along the southern margin of the granite, and an intra-caldera series occupying most of the northern part of the complex. The pre-caldera volcanics include laminated and lithic tuffs, welded ignimbrites and an area of basalt. The intra-caldera series includes peralkaline ignimbrites with strongly developed eutaxitic and spherulitic structures that are rich in crystals of alkali feldspar and quartz with poikilitic or spongy masses of arfvedsonite. Other ignimbrites contain ferrohedenbergite, zircon, allanite and fluorite. A discontinuous ring-dyke is composed of a granite porphyry which is variable in texture, degree of alteration and, to some extent, in mineralogy. Apart from alkali feldspar and quartz, combinations of ferrohedenbergite, fayalite, sodic amphibole and biotite occur with, in one area, veins of aegirine; accessory zircon, fluorite and apatite are usual. In one part of the ring-dyke the granite is mineralized with the development of pyrite, galena, sphalerite, chalcopyrite and minor molybdenite and cubanite. The mineralization is described by Olatunji and Ekwere (1986). The biotite granite is poorly exposed but occupies a quarter of the complex. It is variably coarse- or medium-grained and of perthite, discrete grains of albite, quartz, biotite, abundant topaz and accessory fluorite. Close to the northwestern and southwestern margins the granite has been heavily greisenised, individual veins being up to 10 cm wide but with zones of veins as much as 100 m across. The alteration may extend into adjacent country rocks. The veins consist of quartz, mica and topaz with accessory zircon and cassiterite and occasional fluorite, while wolframite occurs only in greisens cutting the basement. Cassiterite is apparently more abundant in veins cutting basement rocks than in those within the biotite granite. The fullest general account of the geology is that of Jacobson and MacLeod (1977) who give two analyses of rhyolitic rocks. Average compositions of biotite granite and greisen, including trace element data, are in Ekwere (1985). Bennett (Bennett et al., 1984) has made minor modifications to the mapping of Jacobson and MacLeod and gives analyses of ferrohedenbergite. Details of a gravity survey are presented by Ajakaiye (1976).
AJAKAIYE, D.E. 1976. A gravity survey over the Nigerian Younger Granite province. In C.A. Kogbe (ed) Geology of Nigeria. 207-24. Elizabethan Publishing, Lagos. BENNETT, J.N., TURNER, D.C., IKE, E.C. and BOWDEN, P. 1984. The geology of some northern Nigerian anorogenic ring complexes. Overseas Geology and Mineral Resources, 61: 1-65.EKWERE, S.J. 1985. Li, F and Rb contents and Ba/Rb and Rb/Sr ratios as indicators of postmagmatic alteration and mineralization in the granitic rocks of the Banke and Ririwai Younger Granite complexes, northern Nigeria. Mineralium Deposita, 20: 89-93.JACOBSON, R.R.E. and MACLEOD, W.N. 1977. Geology of the Liruei, Banke and adjacent younger granite ring-complexes. Bulletin, Geological Survey of Nigeria, 33: 1-117.OLATUNJI, J.A. and EKWERE, S.J. 1986. Alteration and base metal sulfide mineralization in the porphyries of the Banke Complex, northern Nigeria. Economic Geology, 81: 984-89.RAHAMAN, M.A., VAN BREEMEN, O., BOWDEN, P. and BENNETT, J.N. 1984. Age migrations of anorogenic ring complexes in northern Nigeria. Journal of Geology, 92: 173-84.
