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Element mobility and metasomatic halos around Li-rich pegmatites in the Musha-Ntunga area, Rwanda: implications for lithium exploration in tropical environments
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DOI：10.1007/s00126-026-01466-z.png)
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Abstract
En 中文
Rare-metal pegmatites are important sources of lithium and other critical metals essential for green-energy technologies. In the Musha–Ntunga area of Rwanda, spodumene-bearing lithium–cesium–tantalum (LCT) pegmatites intrude Mesoproterozoic metasedimentary rocks of the Karagwe–Ankole Belt. This study integrates petrography, mineral chemistry, and whole-rock geochemistry to investigate pegmatite crystallization, the magmatic–hydrothermal transition, and host-rock metasomatism related to pegmatite-derived fluid migration. Drill cores intersect pegmatite dykes up to 100 m thick, with reported lithium concentrations locally reaching 20700 ppm (4.5 wt% Li₂O). The pegmatites record a four-stage evolution: (I) primary magmatic crystallization of spodumene, montebrasite, feldspar, quartz, and muscovite; (II) magmatic–hydrothermal overprinting characterized by albitization, muscovitization, and lepidolitization; (III) deformation; and (IV) low-temperature hydrothermal alteration, including eucryptite, lithiophilite, and cookeite. Pegmatite-derived fluids produced 5–25 m-wide metasomatic halos in the metasedimentary host rocks, marked by tourmaline, micas, chlorite, and holmquistite. Geochemical data show that metasomatic micas in the host rocks are enriched in the fluid-mobile elements Li, Rb, and Cs, with Fe sourced from the host rock, whereas Ta and Nb display limited fluid mobility and are enriched only near pegmatite contacts. Pegmatites that lost fluids into permeable host rocks preserve stronger primary magmatic signatures and higher lithium contents, whereas restricted fluid migration into host rock promoted the development of internal metasomatic alteration assemblages. Tropical weathering caused extensive kaolinization of primary Li-bearing phases within the pegmatites. Crucially, the metasomatic halos retain elevated Li, Cs, and Rb contents throughout the weathering profile, making them robust exploration vectors in deeply weathered tropical environments.
Keywords:
Spodumene
LCT pegmatites
Musha-Ntunga
Magmatic-hydrothermal alteration
Host rock metasomatism
Element mobility
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