Micro-characterisation of cassiterite by geology, texture and zonation: A case study of the Karagwe Ankole Belt, Rwanda

Claude Nambaje, Stephen M. Eggins, Gregory M. Yaxley, K. Sajeev*

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    34 Citations (Scopus)

    Abstract

    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.

    Original languageEnglish
    Article number103609
    JournalOre Geology Reviews
    Volume124
    DOIs
    Publication statusPublished - Sept 2020

    Fingerprint

    Dive into the research topics of 'Micro-characterisation of cassiterite by geology, texture and zonation: A case study of the Karagwe Ankole Belt, Rwanda'. Together they form a unique fingerprint.

    Cite this