Early Neoproterozoic rare metal (Sn, Ta, W) and gold metallogeny of the Central Africa Region: a review

Abstract

The four metals of economic significance in the Central Africa or Great Lakes region, i.e. gold, tin, tantalum and tungsten, are part of one composite metallogenic system that operated about 980¡20 Ma. The main driving agent was peraluminous ilmenite-series granite magmatism, synchronous with intracratonic compression and associated with the final amalgamation of the supercontinent Rodinia. The granitic melts were emplaced at intrusive levels of §2 kbar ( §8 km); the intrusions display a variable and often advanced degree of fractionation, including abundant Sn-Ta-Li-Be-Rb-Cs pegmatites, and are associated with hydrothermal systems enriched in tin, tungsten and/or gold. Based on cumulative past production and present metal prices, gold in hydrothermal quartz veins is the major commodity, followed by tin either in rare metal pegmatites or in sheeted, hydrothermal quartz veins. Many deposits in the province occur in siliciclastic metasedimentary, or metabasaltic roof rocks above parental granites; mainly in its western part, the zone of mineralisation retracts into the granite roof. Typically in the first case, antiformal sites acted as fluid escape zones, with carbonaceous or metabasaltic rocks as chemical traps for tungsten and gold. Examples of pegmatitic and magmatic-hydrothermal deposits are presented in some detail in order to illustrate characteristics and genetic controls, and to support the metallogenic hypothesis here advanced. Impeding strategic exploration, published elements of understanding the evolution and mineralisation of the Kibara belt are contradictory and essential links are missing, foremost an understanding of the 1 Ga flare up of fertile granites. Towards solving this conundrum we suggest that the key is delamination of the mantle lithosphere and dense mafic lower crust, residual after extraction of voluminous 1?38 Ga granitic melts. During pan-Rodinian orogenic events, the Tanganyika spur of the Tanzania craton acted as an indenter whose impact caused foundering of the early Kibaran lithosphere. Consequent influx of asthenospheric heat triggered large-scale crustal melting that resulted in the tin granites. The stress state was largely compressive but possibly punctuated by short or local extensional events. The correlation of geological evolution and mineralisation substantiates the formal recognition of a Kibara Metallogenic Domain, which is composed of two units: The Mesoproterozoic (1?4 Ga) Kabanga-Musongati nickel (¡copper, cobalt, platinum) province; and the early Neoproterozoic (1 Ga) Kibara rare metal and gold province that is the main subject of this paper. The present understanding of the operating metallogenic systems remains limited. Regarding the application of modern concepts and technologies, this province is drastically underexplored.

Tracing fluid saturation during pegmatite differentiation by studying the fluid inclusion evolution and multiphase cassiterite mineralisation of the Gatumba pegmatite dyke system (NW Rwanda)

“…1A; Tack et al, 2010). The Karagwe-Ankole Belt hosts, together with the associated Kibara Belt, the Early-Neoproterozoic Kibara Metallogenic Province (Hulsbosch, 2019;Pohl et al, 2013). This granite-related ore province consists of the Early-Neoproterozoic "Kibara tin granites" (i.e.…”

“…Currently, the GDS is artisanally mined and open pit reserves of 26 MT at a grade of 153 ppm Sn, ppm Ta and 82 ppm Nb have been reported (Pohl et al, 2013).…”

Géochimie des muscovites comme indicateur du fractionnement des pegmatites de la région de Kabarore-Mparamirundi (nord-ouest du Burundi, Afrique centrale) [Geochemical signature of muscovites as pathfinder for fractionation of pegmatites in the Kabarore-Mparamirundi area (northwestern Burundi, Central Africa)]

“…Il s'agit d'un "granite à étain" et est la source parentale des pegmatites à Nb, Ta, Sn, Li, Be et des veines de quartz hydrothermales riches en Sn et W observées à plusieurs endroits au Rwanda, au Burundi, en Ouganda et en R.D. Congo (Varlamoff, 1972;Pohl & Günther, 1991;Pohl, 1994;Romer & Lehmann, 1995;Dewaele et al, 2010;De Clercq, 2012;Goldmann et al, 2013;Pohl et al, 2013;Hulsbosch et al, 2014Melcher et al, 2015;Hulsbosch, 2019). Les pegmatites du Burundi ont été datées à 969 ± 17 Ma (roche totale Rb-Sr; Lehmann et al, 1994) et 969 ± 8 Ma (muscovite Rb-Sr; Brinckmann & Lehmann, 1983).…”

Geochemistry of feldspar and muscovite from pegmatite of the Gatumba area, Karagwe Ankole Belt: implications for Nb–Ta–Sn mineralisation and associated alterations

“…The metals are dominantly hosted in pegmatites or quartz veins. According to [4], the pegmatites of the KAB can be found mineralised with Nb-Ta minerals, and cassiterite with accessory minerals such as amblygonite, spodumene, beryl, apatite and tourmaline while mineralised quartz veins mainly contain cassiterite or wolframite. These primary mineralisations observed in quartz veins, greisens and pegmatites are also found in placer deposits as secondary mineralisations.…”