Tectonic evolution and tin mineralisation of the Karagwe–Ankole Belt, Rwanda

Abstract

Studies of the tectonic evolution and associated ore genesis through time are fundamental to understanding Earth’s crustal growth, evolution, and related enrichment in metal deposits. The present PhD research work has focused on the tin (Sn) ore genesis and tectonic evolution of the Karagwe–Ankole Belt (KAB), with a case study area of Rwanda, east-central Africa. This work addresses the controversial geodynamic evolution of the KAB and puts mineralisation in a tectonic context which has been debatable. This thesis work has discovered Archean crust in the Rusizian terrane of the Western Domain of KAB, SW Rwanda, where the basement was previously thought to be Paleoproterozoic. The discovery of the reported Neoarchean basement provides new insight into the tectonic setting of the KAB by defining the reworked north-western margin of the Tanzania Craton. This thesis also presents evidence for high-pressure granulite facies metamorphism in the Western Domain of the KAB, providing new insight into the tectonic evolution of the orogenic belt during the amalgamation of Rodinia. The present thesis provides constraints from petrology, geochemistry and geochronology on the granitic rocks from Rwanda, further indicating a collisional or convergent setting for the Mesoproterozoic evolution of the KAB. In the present work, detailed ore geology and micro-textural characterisation of cassiterite, representing major mineralisation in the KAB, have been carried out to understand the mechanism of tin ore formation and cassiterite chemistry. With simple modifications, the sensitivity of the SIMS U-Pb technique was improved, and a meaningful precision was achieved in the dating of hydrothermal cassiterite with sub-ppm U contents. This thesis reports the first SHRIMP direct U-Pb dating of cassiterite mineralisation in the KAB, Rwanda, and through the zircon and monazite dating of igneous rocks that are potentially linked to the mineralisation, places the several styles and episodes of mineralisation in a tectonic context. It also shows the value of monazite in dating felsic/fractionated igneous rocks in which the zircon has suffered major radiation damage. The links demonstrated between mineralisation, magmatism, geological setting and tectonism not only help to clarify the mineralisation process, but also will assist in targeting exploration for further tin deposits in both Rwanda and neighbouring countries. Furthermore, this thesis newly addresses xanthan gum-based investigations into the surface chemistry of cassiterite and beneficiation of cassiterite tailings, contributing to a new development for the recovery of a good grade of the cassiterite lost in gravity tailings from the world class tin deposits of the KAB, Rwanda