Late orogenic gold mineralization in the western domain of the Karagwe-Ankole Belt (Central Africa): Auriferous quartz veins from the Byumbadeposit (Rwanda)
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External Organisational units
- Vrije Universiteit Brussel
- Department of Materials Science and Engineering, Ghent University
- Katholieke Universiteit Leuven
Gold mineralization in the KAB (Karagwe-Ankole Belt) of Central Africa dominantly occurs as quartz lode and ferruginous breccia systems. Primary gold mineralization in the KAB is possibly linked to a deformational event in the early-Neoproterozoic which is accompanied by shear movements and post compressional magmatism. The Byumba deposit is chosen due to its position in the KAB Mesoproterozoic metasedimentary stratigraphy (Akanyaru Supergroup), interesting structural trend (NW trending shear zone) and its closeness to magmatic intrusions. Explorative drillings from the Byumba deposit in Rwanda were logged and sampled for further petrographic investigation, in situ Rb-Sr geochronology (Laser Ablation Inductively-Coupled-Plasma Triple Quadrupole Mass Spectrometry) and Micro X-ray fluorescence (µXRF) major and trace element mapping. Distinct phases of folding and shear deformation are present at Byumba and three main phases of quartz veining are identified (pre-, syn-, post-folding). The first quartz vein generation is layer-parallel and folded along the bedding, indicative of a pre-folding origin (V1 generation). A second generation of quartz veins (V2a) is concentrated in fold hinges and related to the primary folding of the layers. Cleavage parallel and often boudinaged V2b quartz veins are emplaced during late-stage folding. Post-folding (V3) massive or sigmoidal quartz veins, crosscut the folds and cleavage. Element mapping of the gold by using µXRF shows that Chlorite-rich quartz veins (V3) host the primary gold mineralization in the form of sub-micron gold and of small disseminated blebs. Cores additionally recorded a supergene enrichment phase of gold at reduction zone boundaries. In situ Rb-Sr geochronology on white mica of the crosscutting post-folding quartz veins (V3) point towards an Early-Neoproterozoic age of ∼900 Ma. The gold mineralization at Byumba is interpreted to be related to the regional early-Neoproterozoic compressional deformation event based on 1) the post-folding timing of the auriferous veins, 2) the observation of shear-related textures both in the host-rock and the veins, and 3) the location of the Byumba deposit within a regional shear structure. This association with compressional shearing points towards an orogenic origin for gold mineralization. However, a magmatic-hydrothermal influence from contemporaneous granitic magmatism cannot be excluded.