A recent renewal of interest and research into Nb, Ta, Sn, W, Li and Au deposits of the Central African Mesoproterozoic Karagwe-Ankole belt (KAB) has led to new insights regarding its metallogenesis. This study provides an overview of the geodynamic framework of the KAB, including its magmatism and mineralization. During compressional deformation, barren quartz veins are intensely folded. A second quartz vein generation developed inside the fold hinges. Reverse faults occur in the hinge zone of such folds. After compression, but still during tectonism, Neoproterozoic granite intrusions (G4) are emplaced. Hydrothermal W-rich quartz veins with a mixed magmatic and metamorphic origin formed after folding and cleavage development. Highly fractionated and often metasomatized pegmatites are enriched in Ta-Nb and Sn. Extensional tectonism is evidenced by boudinaged pegmatites, which post-date vein-type W mineralization. Sigmoidal quartz veins and shear zones are an expression of late tectonic shearing. Gold mineralization at Byumba (Rwanda) could be related to this shear phase, but a hydrothermal magmatic source for this deposit cannot be excluded. Sn mineralization is associated with greisenization of pegmatites and with quartz veins. These Sn deposits originate from magmatic fluids, exolved from pegmatitic melt and postdate folding and faulting.
Kambove West is a sediment-hosted Cu-Co deposit in the Neoproterozoic Mines Subgroup, which is mainly composed of dolomitic shales and dolostones. Key factors and processes responsible for high-grade mineralization were identified by studying multiple cores. The stratigraphic position of the mineralized zones adjacent to the Roches Siliceuses Cellulaires and brecciated zones are the most important factor controlling the formation of a high-grade mineralization as they most likely acted as conduits for the mineralizing fluids. The presence of organic matter, stromatolite fragments and anhydrite pseudomorphs promoted mineralization. The organic matter caused a reducing environment required for the precipitation of the sulfides. Sulfate-reducing microorganisms, which were main components of Precambrian stromatolite communities, could also have caused the necessary reduction reactions. In addition to sulfate from the pores of the sediment, anhydrite formed a sulfate source.
A paragenesis with six stages was established based on microscopic observations and cold cathodoluminescence petrography. Two major hypogene mineralization phases, which consist of pyrite, chalcopyrite, bornite, chalcocite and carrollite, formed during diagenesis and low-grade metamorphism, and were succeeded by supergene remobilization, of which chalcocite forms the dominant Cu-phase in the cementation zone. The distinction between hypo- and supergene chalcocite is based on the identification of multiple microtextures, chalcocite polymorphism, and the association with iron and other (hydro)oxides or late generations of dolomite. However, none of the features are conclusive. The combination of them is highly suggestive for a supergene remobilization.
This study presents Raman spectroscopic analyses of melt inclusions in tourmaline from tourmaline-quartz-(muscovite) assemblages of common pegmatites of the Gatumba-Gitarama area (Rwanda).The melt inclusions show a main mineralogy composed of muscovite, a-quartz, moga ´nite, dickite, and minor feldspars which demonstrate, in combination with the observation of dawsonite, nahcolite, jeremejevite, and childrenite daughter minerals, a CO 2 -, H 2 O-, B 2 O 3 -, and P 3 O 4 -enriched peraluminous boro-aluminosilicate composition for the trapped melt.The variable amount of acidic interstitial fluids inside the melt inclusions resulted mainly from heterogeneous trapping of omnipresent, exsolved aqueous fluids during melt inclusion entrapment.Aliquots of this exsolved alkali-rich aqueous fluid phase are preserved in the numerous coexisting fluid inclusions in tourmaline.The observed mineralogy and composition of the melt inclusions deviates strongly from a bulk single-phase melt crystallization model for pegmatite formation.Based on reported experimental, theoretical, and natural constraints, an alternative hypothesis can offer an explanation for the formation of the anomalous but omnipresent residual melts trapped in tourmaline: i.e., the immiscibility of a hydrous fluid and a boro-aluminosilicate melt from the residual bulk aluminosilicate melt.The chemically anomalous composition of the immiscible boro-aluminosilicate melt trapped inside the melt inclusions can explain the mineralogical transition from a granitic mineral mode towards a schorl-quartz-(muscovite) assemblage within the common mineralogical zonation of the pegmatite dike.
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