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

Nambaje, C and Eggins, SM and Yaxley, GM and Sajeev, K (2020) Micro-characterisation of cassiterite by geology, texture and zonation: A case study of the Karagwe Ankole Belt, Rwanda. In: Ore Geology Reviews .

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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. © 2020 Elsevier B.V.

Item Type:	Journal Article
Publication:	Ore Geology Reviews
Publisher:	Elsevier B.V.
Department/Centre:	Division of Mechanical Sciences > Centre for Earth Sciences
Date Deposited:	05 Nov 2021 05:50
Last Modified:	05 Nov 2021 05:50
URI:	http://eprints.iisc.ac.in/id/eprint/65768

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