Magmatic, Magmatic-Hydrothermal, and Deformational Mineral Evolution of Spodumene Pegmatites from the Musha-Ntunga Area (Rwanda)

Pegmatites in the Mesoproterozoic Karagwe-Ankole belt of Central Africa are associated with large granitic complexes that were emplaced around 1 Ga. This study analyzes drill core samples of fresh albite-spodumene pegmatites from the Musha-Ntunga area (East Rwanda), spatially associated with the Lake Muhazi granitic pluton. We combine petrographic and cathodoluminescence microscopy with Raman spectroscopy and elemental geochemistry to study the paragenetic sequence, microtextural variations, and lithium distribution, from the magmatic and magmatic-hydrothermal stages to the hydrothermal stage and during deformation processes. Five textural types of spodumene are distinguished. Coarse-grained spodumene type 1 and symplectitic type 2 are interpreted to have formed during primary magmatic crystallization, whereas spodumene types 3 and 4 formed during magmatic-hydrothermal alteration. Deformation locally affected the pegmatite intrusions. Spodumene type 1 crystals deformed in a brittle and ductile manner, displaying sigma-clast-shaped porphyroclasts (“spodumene fish”) and boudinage textures. The large strained spodumene crystals were also partially recrystallized to fine-grained elongated crystals (type 5), which occur in bands along with mica, quartz, and apatite and define the main orientation of foliation. Montebrasite occurs both as a late primary magmatic phase with spodumene and as a secondary phase that recrystallized during magmatic-hydrothermal alteration and deformation. Eucryptite, lithiophilite, and cookeite occur as late-stage hydrothermal phases, replacing primary lithium assemblages. Associated phases muscovite, apatite, microcline, albite, quartz, and columbite-tantalite further demonstrate the transition from a magmatic to a (magmatic-)hydrothermal and deformational regime. Elevated lithium contents in tourmaline within the metasedimentary host rock indicate dispersion of lithium into the host rock during pegmatite emplacement, subsequent crystallization, and alteration. The results of this multimethod approach demonstrate that different generations of lithium-bearing minerals and associated textures not only record the full transition from a magmatic to hydrothermal regime but also document deformation-related processes that can impact the distribution of metals within pegmatites.