Variations in Tourmaline Morphology and Geochemistry from Quartz Veins, Pegmatites, and Metasomatized Host Rocks: Insights from the Karagwe-Ankole Belt (Rwanda)

Abstract Tourmaline is an excellent petrogenetic indicator, due to its large chemical variability and its broad P-T stability range, and has been used as a proxy to track various geological processes. Here tourmaline is used to study the crystallization conditions in quartz veins, pegmatites, and the surrounding metasediments from the Karagwe-Ankole Belt in central Africa. The quartz veins and pegmatites host numerous Sn-Ta-W-Nb deposits that have been genetically linked to a specific generation of S-type leucogranite intrusions. We study the textural and compositional variations between tourmaline from quartz veins, pegmatites, and metasediments through detailed petrography and major and trace element compositional analyses. Tourmalines hosted in pegmatites and quartz veins share several textural and morphological characteristics, whereas some textures appear to be unique to pegmatites, indicating differences in crystallization conditions. Inwards flaring tourmaline prisms occur in both the quartz veins and the pegmatites, but quartz-tourmaline intergrowths are unique to the pegmatites. This is likely due to different viscosities of the aqueous fluid precipitating the quartz veins and the melts crystallizing the pegmatites. Two different tourmaline morphologies also occur in the altered host rock, which is mainly related to the mineralogy of the host rock. The geochemical composition indicates that all tourmaline types studied here, regardless of crystallization environment, have a magmatic signature. A broad compositional trend suggests that this fluid was mixed at various degrees with a host rock component. Compositional trends in, e.g., Al, Fe, Mg, Ti, and Na, from quartz veins through the host rocks are linked to fluid–rock interactions, such as muscovitization and tourmalinization, between the aqueous fluid and the surrounding metasediments.