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Paper Number: 10
Chemistry of
cassiterite, ferberite and columbite-tantalite samples from Rwinkwavu,
Bugarura-Kuluti and Musha-Ntunga mineral districts of the South-Eastern
Rwanda.
Ngaruye,
J.C.1 and Gauert, C.2
2Department
of Geology, University of the Free State, Bloemfontein, South
Africa
The mines of Eastern Rwanda are historically known to host
cassiterite, ferberite and niobo-tantalite (“coltan”) mineralization.
The geology of that area is dominated by meta-sedimentary rocks of
Mesoproterozoic age. The latter were deformed during the Kibaran Orogeny
(1.4 to 1 Ga) and intruded by two generations of granite: G1-3 granites
of ca. 1380+/-10 Ma and G4- granites of ca. 986+/-8 Ma (Tack et al.,
2010). The mineralization exploited here, are related to the G4 granite
which functioned as the heat source for the mineralizing fluids. The
SEM-EDX/WDX investigations on cassiterite samples revealed that the
dominant substitutions were of the Sn4+ ↔︎ (Ta,
Nb)4+, 3 Sn4+ ↔︎ 2(Ta, Nb)5+ +(Fe, Mn)
2+ and/or Fe3+ + OH- ↔︎ Sn4+
+ O2- types (Moller et al., 1988). Laboratory analysis of
coltan mineralization from increasingly distal veins showed evolution
trends from ferro-columbite to mangano-tantalite compositions indicating
the increase of Ta and Mn with advanced differentiation of pegmatites.
The fluid inclusion studies showed that the mineralizing fluids were
composed by more than one type of cations, salinities below 17.5 wt. %
NaCl equiv. and Th between 103 and 360°C (Ngaruye, 2011).
They confirmed that the coltan precipitation closer to the granites was
achieved in pneumatolytic conditions whereas the cassiterite and
ferberite mineralization precipitated in hydrothermal environments
possibly derived from a mixture between magmatic fluids and meteoric to
connate waters. A conceptual metallogenetic model was developed and
includes a multi-stage hydrothermal circulation induced by progressive
granitic magmatism and precipitation of Sn, W, Nb-Ta minerals in
pegmatitic veins cross-cutting rocks strata acting as chemical
traps.
Reference
Möller, P., Dulski, P., Szacki, W, Malow, G. and Riedel, E. (1988).
Substitution of tin in cassiterite by tantalum, niobium, tungsten, iron
and manganese. Geochimica et Cosmochimica Acta,
52, 1497-1503.
Ngaruye, J.C. (2011), Petrographic and geochemical investigation
of Sn - W - Nb - Ta - pegmatites and mineralized quartz veins in
southeastern Rwanda, Magister Thesis, University of the Free State,
Bloemfontein, South Africa, p182.
Tack, L., Wingate, M.T.D., De Waele, B., Meert, J., Belousova,
E., Griffin, B., Tahon, A. and Fernandez-Alonso, M. (2010). The 1375 Ma
“Kibaran event” in Central Africa: Prominent emplacement of bimodal
magmatism under extensional regime. Precambrian research,
180, 63-84.