Geochemistry of feldspar and muscovite from pegmatite of the Gatumba area, Karagwe Ankole Belt: implications for Nb–Ta–Sn mineralisation and associated alterations

Abstract

The studied pegmatites suggest a sodium-dominant pegmatitic melt where micas and albites are dominant over potassium minerals. The mapped pegmatites from the Gatumba area are of interest in Karagwe Ankole Belt for the study of rare-element pegmatites. Previous works on the Gatumba pegmatites included geochemistry and geological mapping; and further studies are required for focused exploration and mine planning. For this reason, field investigation, petrographic and mineral chemistry studies on pegmatites were carried out. Thin sections of the rock samples collected were studied under a transmitted light microscope. Electron Microprobe Analysis and Energy Dispersive X-Ray Spectroscopy were used for mineral chemistry and identification; Back Scattered Electron analysis was used for mineral texture. Major minerals identified in the pegmatite bodies are quartz, plagioclase and micas. Albites (An0.1–0.8; Ab98.7–99.6 and Or0.3–0.5) are found as the dominant-plagioclase minerals in the pegmatites from the Gatumba area. Albites are precursors to kaolinites, and the albitisation and kaolinisation processes altered the primary emplaced pegmatites during the mineralisation of the Nb–Ta, Sn rare metals, which precipitated into columbite-tantalite and cassiterite. Muscovites are the most common mineralogical indicators of strongly peraluminous composition in plutonic rocks. Crystallisation, muscovitisation and kaolinisation accompanied the precipitation of prompted cassiterite and columbite-tantalite in the pegmatites of the study area.