PROTEROZOIC GOLD MINERALIZATION IN NW BURUNDI

In Burundi two types of primary gold mineralization can be distinguished: sulfide vein assemblages with gold related to the post-Kibaran G4-granite magmatism (900-1000 Ma) and Pan-African ferruginous breccia zones with gold dated around 640 Ma. The post-Kibaran sulfide assemblages have a characteristic bismuth component and are accompanied by the early gangue assemblage of quartz-tourmaline-rutile-magnetite. The elevated Sn and W contents and the spatial and temporal relationships with highly fractionated granitic rocks point to a magmatic input into the Au-bearing hydrothermal system. The Pan-African gold-bearing ferruginous breccia zones have relatively high Ba contents and are characterized by an early sulfide stage and subsequent extensive hydrothermal leaching and transformation into hematite/limonite in an oxidizing environment. The hematite/limonite assemblage is host to native gold mineralization with lenticular gold aggregates up to several millimeters in size. The breccia-style gold mineralization is thought to represent redistributed gold from the post-Kibaran sulfide stage, which was remobilized during the Pan-African structural reactivation.