La géologie et la minéralisation primaire de l'or de la Chaîne Kibarienne, nord-ouest du Burundi, Afrique Orientale

Gold mineralization in NW Burundi is located in a NNWSSE aligned zone of 60 by 10Ekm which extends from Ntendezi/Rwanda in the north to Ndora/Burundi in the south. This area is underlain by a 1300 Ma-old low- to medium-grade clastic sedimentary sequence with minor volcanic intercalations, and by granitic intrusions of Kibaran (1265--1210 Ma) and post-Kibaran age (970 Ma). There are two different types of gold mineralization, both related to shear and fracture zones within the same area. Type 1: Mesothermal quartz-tourmaline-muscovite-(rutile) veins with a Bierich sulfide and a subsequent hematite stage. Au occurs as refractory microgold in pyrite and arsenopyrite, and as visible gold from supergene enrichment/ weathering. The Au-bearing hydrothermal veins spatially overlap with granite-related rare metal pegmatite/ hydrothermal systems in the SE part of the study area near Ruhembe and Ndora. Radiometric ages for Ta pegmatites of 965 +/-5 Ma (U-Pb on columbite) and of 908 +/-21 Ma (Rb-Sr errorchrone on muscovite) for Au-bearing quartz veins suggest roughly synchronous formation. Overlapping chemical features (Sn, B, Bi) and fluid inclusion data indicate very similar physicochemical conditions of formation (400°C, 2 kbar, with late-stage isochoric evolution to 200°C and 1 kbar) for both tin and gold mineralization, and point to a genetic relationship with the post-Kibaran G4 granite magmatism. Type 2: Epithermal hematite/limonite breccia zones with visible gold and a characteristic Ba component. Au occurs in mm-large aggregates dominantly associated with pseudomorphically replaced pyrite cubes. The breccia bodies contain hydrothermal quartz clasts which are of mesothermal origin and which have fluid inclusion characteristics of the type 1 gold mineralization. The siliceous breccia matrix is of low-temperature (140-190°C) and low-pressure origin (ca 200 bar). Monazite from this matrix has a Panafrican U-Pb age of 535 +/-2 Ma, pointing to a period of rapid uplift, rifting and high heat flow for this time. These data suggest a two-stage model of formation for the gold mineralization of NW Burundi: Initial post-Kibaran mesothermal gold mineralization related to the peripheral parts of hydrothermal systems in association with the G4 rare metal granite magmatism, and later gold remobilization by epithermal fluid circulation during Panafrican uplift and rifting. Both mineralization styles are upgraded by present-day supergene enrichment processes.