Geochemistry and mineralization of Buru and Kuge volcanic carbonatite centres, Western Kenya

Western Kenya hosts a number of Tertiary and Quaternary alkaline volcanic carbonatite centres, such as Rangwa, the North and South Ruri centres, Kuge, Homa Mountain and Legetet which are located along an old Precambrian major shear zone lying within the Nyanza rift, off the main Kenyan (Gregory) rift. The centres consist of agglomerates and breccias with mixed clasts of silicate rocks and carbonatites, interbedded with carbonatitic and nephelinitic tuffs. The volcanic assemblage is transected by high level sheets and dykes of calcite carbonatite and ferrocarbonatite which were probably later feeders for the volcanic eruptions. Carbon, oxygen, and sulphur isotopic compositions were determined for calcite, siderite and barite from the Buru and Kuge carbonatite centres. Wide ranges in the isotopic compositions of the minerals were observed with values for delta13C and delta18O for the Buru calcites ranging from +1.27 to -3.23‰ (PDB) and +11.25 to +26.21‰ (SMOW). The delta13 C and delta18O for the Kuge calcites are -3.11 to -8.44‰ (PDB) and +18.09 to +25.73‰ (SMOW). The Buru siderites plot in a narrow and restricted range at -3.07 to -4.39‰ (PDB) and +12.61 to +16.10‰ (SMOW). Data on the sulphur isotopic composition from the Buru hill carbonatite show a fairly widespread variation in delta34S ranging from +4.50 to +12.40‰ (CDT), whereas Kuge hill displays a slightly more homogenous isotopic composition with values ranging from +1.10 to +5.10‰ (CDT). The carbon and oxygen isotopic compositions from the Buru and Kuge carbonatite centres do not retain the primary isotopic signatures expected for magmatic primary carbonatites. Most of the variations in isotopic composition have been attributed to secondary processes involving low temperature (60° to 144°C) hydrothemal alteration and isotopic exchange between the carbonatites and fluids (meteoric water). Higher delta18O values (+21.91 to +26.21‰) with a significant increase in delta13Cvalues (-1.48 to +1.27‰) shown by the most oxidized samples from the Buru carbonatite may indicate the involvement of supergene exchange with atmospheric CO2 at relatively lower temperatures (