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

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 behavior of REE during alteration. Oxygen and carbon isotope compositions of the Matongo carbonatite and the carbonatitic dykes have a magmatic signature, with 7.2<delta O-18 (vs. SMOW) <8.5 parts per thousand. and -4.7<delta C-13 (vs. PDB) <-5.4 parts per thousand in agreement with the Sr isotopic composition. The oxygen isotope composition of zircon and ilmenite megacrysts (delta O-18(Zr)=4-4.7 parts per thousand., delta O-15(lim)=-4.3-1.5 parts per thousand, respectively) also point to a magmato-hydrothermal origin of the forming fluids. Some samples of the Matongo carbonatite and the carbonatitic dykes, with high delta O-18 values (delta O-18 = 8.6-21.8 parts per thousand), 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 delta O-18 and delta C-13 values (delta O-18 = 16.3-24.7 parts per thousand. and delta C-13 = -4.7-0.2 parts per thousand), 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. (C) 2015 Elsevier B.V. All rights reserved.