REE MINERALIZATION AT GAKARA, BURUNDI - EVIDENCE FOR ANOMALOUS UPPER-MANTLE IN THE WESTERN RIFT-VALLEY

The 587 Ma-old hydrothermal bastnaesite mineralization of Gakara in th e western branch of the East African Rift in Burundi has geological an d chemical features typical of carbonatite-related REE ore deposits, b ut Nd, Ce, and Sr isotope data on the bastnaesite indicate a time-inte grated LREE-enriched source with epsilon(Nd)(T) - 11.9, epsilon(Ce)(T) 1.6, and Sr-87/Sr-86 greater than or equal to 0.7075. The mid-Protero zoic Nd model age of this source coincides with the formation of the i ntracratonic Kibaran basin of central Africa and may indicate a mid-Pr oterozoic rift-related event of small-scale, but locally very effectiv e, mantle metasomatism in this region. The formation of REE ore deposi ts in carbonatite environments is possibly linked to enriched mantle d omains.