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

Acke, Jolan; Dewaele, Stijn; Barros, Renata; Burlet, Christian; Nachtergaele, Simon; Uwiringiyimana, Justin; Fußwinkel, Tobias; Borst, Anouk

doi:10.5382/econgeo.5173

Article (Scientific journals)

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

Acke, Jolan; Dewaele, Stijn; Barros, Renata et al.

2025 • In Economic Geology and the Bulletin of the Society of Economic Geologists, 120 (5), p. 1207 - 1234

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https://hdl.handle.net/2268/336961

DOI
10.5382/econgeo.5173

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Keywords :

Cathodoluminescence microscopy; Central Africa; Deformation process; Drill core; Elemental geochemistry; Granitic plutons; Host rocks; Hydrothermal alterations; Mineral evolutions; Petrographic microscopy; Geophysics; Geology; Geochemistry and Petrology; Economic Geology

Abstract :

[en] 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.

Disciplines :

Earth sciences & physical geography

Author, co-author :

Acke, Jolan; Department of Earth and Environmental Sciences, KU Leuven, Leuven, Belgium ; Mineralogy and Petrology Research Group, Ghent University, Gent, Belgium

Dewaele, Stijn; Mineralogy and Petrology Research Group, Ghent University, Gent, Belgium

Barros, Renata; Geological Survey of Belgium, Royal Belgian Institute of Natural Sciences, Brussels, Belgium ; EuroGeoSurveys, Brussels, Belgium

Burlet, Christian; Geological Survey of Belgium, Royal Belgian Institute of Natural Sciences, Brussels, Belgium

Nachtergaele, Simon ; Université de Liège - ULiège > Urban and Environmental Engineering

Uwiringiyimana, Justin; Trinity Metals Group, Rwamagana, Rwanda

Fußwinkel, Tobias; Institute of Applied Mineralogy and Economic Geology, RWTH Aachen University, Aachen, Germany

Borst, Anouk; Department of Earth and Environmental Sciences, KU Leuven, Leuven, Belgium ; Geodynamics and Mineral Resources, Royal Museum for Central Africa, Tervuren, Belgium

Language :

English

Title :

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

Publication date :

2025

Journal title :

Economic Geology and the Bulletin of the Society of Economic Geologists

ISSN :

0361-0128

eISSN :

1554-0774

Publisher :

Society of Economic Geologists, Inc

Volume :

120

Issue :

Pages :

1207 - 1234

Peer reviewed :

Peer Reviewed verified by ORBi

Additional URL :

https://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/120/5/1207/7348286/5173_acke_et_al.pdf

Funding text :

We thank Lionel Sematuro, Missionnaire Mbanza, Alastair Goodship, Daniel Kwizera, Sam Ryumugabe, and Neza Damascene of Trinity Metals for facilitating the field work at Musha-Ntunga and providing core sections and geochemical data for this study. Jean-Claude Ngaruye and colleagues of the Rwanda Mines, Petroleum and Gas Board are acknowledged for authorizing field work and export of samples, with assistance of Daniel Baudet and Gerard Nimpagaritse of the Royal Museum for Central Africa. We thank Herman Nijs (KU Leuven) for the high-quality thin sections, Dominique Jacques for sharing valuable insights on deformational microstructures, and William Keyser for providing an external spodumene sample standard. LIBS analyses at the Geological Survey of Belgium were funded by the LIBS-SCReeN project (BRAIN 2.0). The PhD research of Jolan Acke is funded by grant 11PDY24N of the Research Foundation Flanders (FWO). Anouk Borst acknowledges funding by the Belspo FED-tWIN program (Prf-2019-051-GEMMA). We further thank John Dilles and an anonymous reviewer for detailed comments on the manuscript and Tom Benson and David Cooke for the editorial handling.

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