Bukusu

The central area of Bukusu, covering about 50 km2, consists principally of intrusive carbonatites and silicate rocks but is poorly exposed. This area is surrounded by a prominent ring of hills of well exposed metasomatic rocks that extend over some 125 km2. There is no outcrop in the central area what little is known of the geology being based on data from a few boreholes and the mineralogy and chemistry of residual soils. The greater part of this area appears to be underlain by heterogeneous melteigite and ijolite with subordinate nepheline syenite. Pyroxenite, mica pyroxenite and hornblendite seem to form an outer ring. Both the ijolite series and the ultramafic rocks are cut by arcuate masses of carbonatite that only crop out at two localities. However, the distribution of the carbonatites is manifest from the discontinuous ridges it produces with overlying phosphate rocks and magnetite-rich float. The carbonatite comprises interbanded cone sheets of ankeritic sovite and rauhaugite with intervening screens of phoscorite (magnetite-olivine-apatite rock). At the centre of the complex is an agglomerate, which is highly potassic (Baldock, 1971), and which is intruded by small bodies of sovite, feldspathic sovite and tuffisite. There are also a number of centres at which, according to Baldock (1969), silica undersaturated syenites have been transformed into magnetite-rich silicate-carbonate rocks with which late-stage mineralization is associated. The metasomatized rocks around the complex are divided by Baldock (1973) into three zones. In the outer zone the granodioritic gneiss country rock is 'bleached' with an increase in the amount and grain size of the K-feldspar and quartz; the feldspar becomes clouded. In the second zone all mafic minerals, notably epidote and biotite, disappear and the rock consists almost solely of coarse K-feldspar and quartz. This zone passes gradationally into an inner zone of syenite which is free of quartz and dominantly composed of K-feldspar and limonite with minor albite and rare grains of a reddish biotite. Within the metasomatic zone on the eastern side of the complex, and extending round to the north, are concentric shear zones along which dyke-like bodies of trachytic rocks occur. One group, up to 10 m thick and called pseudotrachytes by Baldock (1973), consists of disorientated fragments of orthoclase in a feldspathic groundmass with limonite and pyrite. These rocks are thought to have been generated in situ by brecciation of the syenitic fenite. A second group of 'potassic trachytes', up to 5 m thick, consists of laths of orthoclase or microperthite with a trachytic texture; again limonite is abundant. Some recrystallization of the adjacent country rocks is evident and they are thought to be intrusive, although gradation from syenitic fenite through pseudotrachyte to potassic trachyte has been observed. The syenitic fenites contain up to 11% K2O and the trachyte dykes as much as 14% K2O (Baldock, 1973). Rock analyses and a discussion of the chemical changes during fenitization will be found in Baldock (1973). A description of calzirtite and the mineralogy of residual soils is given in Baldock (1968), and a geochemical survey of the soils in Baldock (1969). McCormick and Le Bas (1996) made a study of zoned phlogopites in carbonatite. Sm, Nd and Sr isotope data are available in Bell and Blenkinsop (1989).