TY - JOUR
T1 - Micro-characterisation of cassiterite by geology, texture and zonation
T2 - A case study of the Karagwe Ankole Belt, Rwanda
AU - Nambaje, Claude
AU - Eggins, Stephen M.
AU - Yaxley, Gregory M.
AU - Sajeev, K.
N1 - Publisher Copyright:
© 2020 Elsevier B.V.
PY - 2020/9
Y1 - 2020/9
N2 - Field geology, petrography and texturally controlled compositional studies were carried out on cassiterite from the Karagwe Ankole Belt (KAB) in Rwanda, to understand the tin (Sn) ore formation and cassiterite chemistry. It is identified that muscovite-rich veins are potentially Sn-bearing, indicating that acid neutralising muscovitisation reactions were part of mechanisms for the Sn precipitation as a result of fluid-rock interaction. Pleochroism and light to dark brown colour zonation of cassiterite relates to compositional variation of elements substituting for Sn. Dark brown zones are enriched in Nb, Ta, Ti and Fe relative to the lighter zones which approach pure (greater than 99%) SnO2. X-ray elemental maps show that elevated Nb and Ta concentrations are more correlated with darker brown cassiterite than are Ti and Fe. Intra-grain compositional analysis shows that the internal textures of cassiterite are also defined by trace element distribution with an enrichment of Ta, Nb, Fe, V, Al and U in darker cathodoluminescence (CL) bands relative to lighter bands. One or more of these elements may be quenching the CL, whereas Ti and W may activate the CL. These elements are incorporated within cassiterite via the substitution: 2-nNb,Ta,V5++[1-nFe,Mn,Ca,Mg2++n(Al,Sc,Fe)3+]↔(3-n)Sn4+where0≤n≤1. Ti, along with other tetravalent elements (Zr, Hf, U, W), is incorporated in cassiterite by direct substitution, Ti4+ ↔ Sn4+. The presence of small randomly disseminated rutile inclusions in cassiterite is attributed to the isostructural nature of these two minerals. Enrichment of Ta and Nb in the cassiterite formed in high temperature environments reveals the potential of cassiterite as an additional resource for these critical metals.
AB - Field geology, petrography and texturally controlled compositional studies were carried out on cassiterite from the Karagwe Ankole Belt (KAB) in Rwanda, to understand the tin (Sn) ore formation and cassiterite chemistry. It is identified that muscovite-rich veins are potentially Sn-bearing, indicating that acid neutralising muscovitisation reactions were part of mechanisms for the Sn precipitation as a result of fluid-rock interaction. Pleochroism and light to dark brown colour zonation of cassiterite relates to compositional variation of elements substituting for Sn. Dark brown zones are enriched in Nb, Ta, Ti and Fe relative to the lighter zones which approach pure (greater than 99%) SnO2. X-ray elemental maps show that elevated Nb and Ta concentrations are more correlated with darker brown cassiterite than are Ti and Fe. Intra-grain compositional analysis shows that the internal textures of cassiterite are also defined by trace element distribution with an enrichment of Ta, Nb, Fe, V, Al and U in darker cathodoluminescence (CL) bands relative to lighter bands. One or more of these elements may be quenching the CL, whereas Ti and W may activate the CL. These elements are incorporated within cassiterite via the substitution: 2-nNb,Ta,V5++[1-nFe,Mn,Ca,Mg2++n(Al,Sc,Fe)3+]↔(3-n)Sn4+where0≤n≤1. Ti, along with other tetravalent elements (Zr, Hf, U, W), is incorporated in cassiterite by direct substitution, Ti4+ ↔ Sn4+. The presence of small randomly disseminated rutile inclusions in cassiterite is attributed to the isostructural nature of these two minerals. Enrichment of Ta and Nb in the cassiterite formed in high temperature environments reveals the potential of cassiterite as an additional resource for these critical metals.
KW - Cassiterite chemistry
KW - Colour zonations
KW - Internal texture
KW - Karagwe Ankole Belt
KW - Micro-characterisation
KW - Niobium and Tantalum
UR - http://www.scopus.com/inward/record.url?scp=85085974675&partnerID=8YFLogxK
U2 - 10.1016/j.oregeorev.2020.103609
DO - 10.1016/j.oregeorev.2020.103609
M3 - Article
SN - 0169-1368
VL - 124
JO - Ore Geology Reviews
JF - Ore Geology Reviews
M1 - 103609
ER -