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Structurally-controlled hydrothermal alteration in the syntectonic Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (Burundi): Implications for REE and HFSE mobilities

Abstract : The Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (URAPC), Burundi, is located along the western branch of the East African Rift. It comprises oversaturated and undersaturated syenites and a shallow level carbonatite body (the Matongo carbonatite) that does not outcrop but has been sampled by drill-cores. The elliptic map contour of the URAPC points to a syntectonic emplacement. Large shear zones that were active during magmatic emplacement have accommodated a regional NE-SW shortening. Mineralization features of late-magmatic to hydrothermal origin are associated with the carbonatite, which, by itself, contains a dense network of calcitic veins. HFSE mineralization occurring as zircon and ilmenite megacrysts can be found in an area of intense and extensive K-fenitization, which lead to the transformation of the surrounding syenite into a dominant K-feldspar + biotite mineral assemblage (Inamvumvu area). Carbonatitic dykes (overprinted by a hydrothermal alteration) are present a few kilometers north of the Matongo carbonatite, within highly deformed zones in the syenite. These dykes occur along with Na-fenites (resulting from the transformation of the feldspathoidal syenite into an albite-dominant paragenesis) and are enriched in REE-minerals (monazite and ancylite-(Ce)). Many magmatic (pegmatoid) dykes and hydrothermal (quartz + hematite) veins also occur in shear zones in the URAPC. Most of them can be interpreted as tension gashes. The chondrite-normalized REE patterns of some carbonatite whole rock samples are highly disturbed, in relation to post-magmatic hydrothermal alteration. The HFSE and REE distribution in the minerals from the hydrothermal veins/dykes (calcitic veins within the carbonatite, carbonatite dykes overprinted by a hydrothermal alteration in deformed zones, and zircon and ilmenite megacrysts) attests for a complex behaviour of REE during alteration. Oxygen and carbon isotope compositions of the Matongo carbonatite and the carbonatitic dykes have a magmatic signature, with 7.2 < δ18O (vs. SMOW) < 8.5‰ and -4.7 < δ13C (vs. PDB) < -5.4‰ in agreement with the Sr isotopic composition. The oxygen isotope composition of zircon and ilmenite megacrysts (δ18OZr = 4 to 4.7‰, δ18OIlm = -4.3 to -1.5‰ respectively) also point to a magmato-hydrothermal origin of the forming fluids. Some samples of the Matongo carbonatite and the carbonatitic dykes, with high δ18O values (δ18O = 8.6 to 21.8‰), show evidence of a medium- to low-temperature hydrothermal alteration event by an aqueous fluid. Calcitic veins in the carbonatite record another alteration event, outlined by the co-variation of δ18O and δ13C values (δ18O = 16.3 to 24.7‰ and δ13C = -4.7 to 0.2‰), implying the involvement of a mixed H2O-CO2 fluid. As a whole, the circulation of fluids in the URAPC was initiated during magmatic emplacement and the geometry of this circulation was controlled by the syn-emplacement crustal scale shear zones. Element mobility, one expression of which being the mineralization features described here, follow the same scheme.
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Sophie Decrée, Philippe Boulvais, Corentin Cobert, Jean-Marc Baele, Gilbert Midende, et al.. Structurally-controlled hydrothermal alteration in the syntectonic Neoproterozoic Upper Ruvubu Alkaline Plutonic Complex (Burundi): Implications for REE and HFSE mobilities. Precambrian Research, Elsevier, 2015, 269, pp.281-295. ⟨10.1016/j.precamres.2015.08.016⟩. ⟨insu-01186043⟩

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