An assessment of Sn-W-Nb-Ta oxide minerals from the Bugesera district of Rwanda and geological context
Date
2018
Authors
Buurman, Innes
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Abstract
The Sn‐W‐Nb‐Ta oxide minerals of Rwanda are rarely found in the same area and predominantly occur
in different deposits. Gatumba is known for its pegmatite related Nb‐Ta, and Sn mineralisation,
Nyakabingo for its hydrothermal vein type W mineralisation, and Rutongo for its hydrothermal vein
type Sn mineralisation. Very little geological research has been performed on the Bugesera district in
Rwanda. Therefore, the aim of this study is to enhance the knowledge base on the Bugesera district
by studying four mining areas (Nyagasagara, Gakurazo, Kageyo and Kageyo Extension) on the Hard
Metal Rwanda (HMR) license, their locations relative to regional structures and G4 granites, and the
mineral grade at each of the deposits. The mineralogy and mineral chemistry were only studied on
the concentrate material from Nyagasagara, for comparison with literature and to determine whether
the mineralisation was primary or secondary. The HMR license is located in between the Bugesera
granite and the Bugesera syncline fold. Nyagasagara and Kageyo occur adjacent (< 4km) to known
large scale structures, granites and/or pegmatites, Gakurazo is located in between Nyagasagara and
Kageyo, and the Kageyo Extension is located on the rim of the Bugesera granite. No outcrops of granite
and/or pegmatite were observed at any of the sites. The highest mineral grade was recorded at
Nyagasagara Tunnel 1 (5.56% cassiterite) with the lowest grade recorded at Kageyo and Kageyo
Extension (0.01% coltan). Around 60% of the cassiterite grains from Nyagasagara, reports to the
coarser size fraction of +4 mm and +2 mm, with the shape of the grains ranging from angular to very
angular. Cassiterite is the only oxide mineral of interest present at Nyagasagara, as confirmed by the
whole rock chemistry and optical microscopy on the concentrate samples from Nyagasagara. The
mineral chemistry of the cassiterite plots in the rare element granite and pegmatite field on the binary
Nb + Ta and Fe + Mn diagram. Most of the cassiterite contains more Nb compared to Ta (Nb>Ta) and
minor Fe2O3. The cassiterite also contained numerous coltan inclusions and exsolution products. This
study concludes that the HMR license area is situated in a suitable regional geological setting for Sn‐
W‐Ta‐Nb oxide minerals to be found. However, the Nyagasagara deposit is an eluvial deposit
consisting of only cassiterite minerals, which have a magmatic origin, believed to be related to rare
element granites.
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Citation
Buurman, Innes, (2018) An assessment of Sn-W-Nb-Ta oxide minerals from the Bugesera district of Rwanda and geological context, University of the Witwatersrand, Johannesburg, https://hdl.handle.net/10539/25877.