La mineralisation d'or dans le nord-ouest du Burundi est liee a une zone de cisaillement post-kibarienne du Proterozoique superieur, d'orientation NNOSSE, de soixante kilometres de long sur dix kilometres de large, qui s'etend de Ntendezi/Rwanda au NNO jusqu'a Ndora/Burundi au SSE. Il y a deux types differenes de mineralisations auriferes, tous les deux lies aux zones de cisaillement et fracture dans la meme region.

Type 1: L'or apparait dans des filons hydrothermaux de quartz-tourmaline- muscovite-rutile-sulfures-oxydes de fen Il est dissous dans les sulfures et, dans des zones d'alteration supergene, se presente egalement sous la forme d'or natif associe aux oxydes de fen La zone orientale de la region aurifere, a Rubembe et Ndora, se superpose avec le secteur minier oriental des pegmatites a niobium-tantale et des filons Sn-W lies aux granites a etain post-kibariens. Les deux systemes ont des ages radiometriques comparables: les pegmatites ont ete datees a 965 +/- 5 m.a. (U/Pb sur colombite), les filons d'or-quartz a 908 +/- 21 m.a. (Rb/Sr sur muscovite; systeme partiellement perturbe). En outre, il existe des concordances en ce qui concerne l'inventaire des inclusions fluides et des conditions physico-chimiques semblables pour la mise en place des mineralisations hydrothermales auriferes et staniferes. Ces resultats indiquent un lien genetique etroit entre la mineralisation mesothermale d'or et les systemes hydrothermaux lies au magmatisme granitique G4 a etain.

Type 2: L'or est egalement present dans des breches hydrothermales ferrugineuses panafricaines. Les parties auriferes de ces breches vent limitees a la meme zone de mineralisation d'or susmentionnee. Les breches ont ete datees a 535+/-2 m.a. (U/Pb sur monazite de la matrice de la breche) et representent un milieu epithermal et de haut degre d'oxydation. La presence de fragments de quartz de l'ancien type mesothermal dans ces breches et l'inventaire chimique suggerent que les breches auriferes representent une remobilisation de l'ancien stock metal de la mineralisation de type 1. On obtient ainsi un modele a deux etapes pour les mineralisations d'or au NW de Burundi: Mineralisation aurifere mesothermale post-kibarienne liees au partie marginale des systemes hydrothermaux associee avec des granites a etain et remobilisation panafricaine au cours d'un systeme epithermal lie a une tectonique cassante de redressement et de rift.

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.