Bobi-Seguela

North of Seguela around Bobi and Toubabouko a number of sheared dikes cut Precambrian granitic rocks and were described by Bardet (1974) as 'meta-kimberlites'. Although much altered the geochemistry and remnant mineralogy indicate that they are lamproitic. Alluvial deposits containing diamonds are located in the vicinity of the dykes with a few patches more widely scattered. The dyke at Toubabouko has been traced for about 3.5 km. The rock is nodular with abundant talcose pseudomorphs after olivine, much phlogopite and an Fe-Ti phase. Some facies contain quartz and chlorite. Mitchell (1985) refers to "Fresh leucite phlogopite lamproite from the Kohue River area", and as the Kohue River flows immediately to the east of the Toubabouko dyke he is presumably referring to this locality; he gives analyses of the phlogopite of this rock. The Bobi occurrence consists of at least three dykes which trend in different directions and have been named the 'Prince', 'Intermediate' and 'Princess' dykes - going from north to south (Bardet, 1974). The rock of the Prince dyke is partly micaceous peridotite and partly lamproite with leucite, and that of the Intermediate dyke a leucite lamproite also but with some peridotite. The Princess dyke passes from a micaceous peridotite to leucite lamproite at the northern and southern ends which Bardet (1974) called fitzroyite. The tenor of diamond in the peridotite fractions is locally high at more than 10 carats per cubic metre, but the lamproitic parts are barren of diamond. A sample of Bobi dyke investigated by Mitchell (1985) was completely altered to a fine-grained mixture of carbonate, chlorite and anatase but contained fresh, corroded phlogopite phenocrysts analyses of which are given.