The recognition of meteoric-hydrothermal and supergene processes in volcanic carbonatites, Nyanza Rift, western Kenya, using carbon and oxygen isotopes

Carbon and O isotopic compositions were determined for calcite and siderite from the Buru and Kuge carbonatite centres. Wide ranges in the isotopic compositions of these minerals were observed with values of δ13C and δ18O for the Buru calcites ranging from +1.3 to −3.2% (PDB) and + 11.3 to +26.2% (SMOW), respectively. The δ13C and δ18O ranges for the Kuge calcites are −3.1 to −8.4% (PDB) and +18.1 to + 25.7% (SMOW). The Buru siderites plot in a narrow and restricted range at −3.1 to −4.4% (PDB) and +12.6 to + 16.1% (SMOW). Calcite and siderite from the Buru and Kuge volcanic carbonatite centres do not retain their isotope signatures expected for primary magmatic carbonatites. Most of the variations in CO isotope composition can be attributed to secondary processes involving low-temperature hydrothermal alteration and isotope exchange between carbonatites and meteoric water. Higher δ18O values (+21.9 to +26.2%) with a significant increase in δ13C values (−1.5 to + 1.3%) shown by the most oxidised samples from the Buru carbonatite may indicate the involvement of supergene exchange with atmospheric CO2 at surface temperatures.