Hypersaline fluids generated by high-grade metamorphism of evaporites: fluid inclusion study of uranium occurrences in the Western Zambian Copperbelt |
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| Authors: | Email author" target="_blank">Aurélien?EglingerEmail author Clément?Ferraina Alexandre?Tarantola Anne-Sylvie?André-Mayer Olivier?Vanderhaeghe Marie-Christine?Boiron Jean?Dubessy Antonin?Richard Marc?Brouand |
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| Institution: | 1.UMR GeoRessources 7359, CNRS-CREGU,Université de Lorraine,Vand?uvre-lès-Nancy Cedex,France;2.AREVA, BU Mines,Paris,France |
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| Abstract: | In the Pan-African Lufilian belt (Western Zambian Copperbelt), uranium mineralizations, preferentially scattered in kyanite ± talc micaschists (metamorphosed evaporitic sediments) or concentrated along transposed quartz veins provide an opportunity to (1) understand the time/space relationship between the ore minerals and the deformation of the host rocks, (2) identify the different fluid events associated with specific stages of quartz deformation and (3) characterize the ore fluid geochemistry in terms of fluid origin and fluid/rock interactions. In the U occurrences studied in Lolwa and Mitukuluku (Domes region, Western Zambian Copperbelt), two mineralizing stages are described. The first generation of ore fluids (53–59 wt% CaCl2, 13–15 wt% NaCl; N2–H2 in the gas phase of fluid inclusions) circulated during the high-temperature quartz recrystallization, at 500–700 °C. This temperature is in agreement with the P–T conditions recorded during the crustal thickening related to continental collision at ca. 530 Ma. LA-ICPMS analyses show the presence of uranium within this fluid, with a concentration mode around 20 ppm. The second generation of ore fluid (21–32 wt% NaCl, 19–21 wt% CaCl2; CO2–CO in the gas phase of fluid inclusions) percolated at lower temperature conditions, at the brittle–ductile transition, between 200 and 300 °C. This temperature could be related to the exhumation of the high-grade metamorphic rocks at ca. 500 Ma. The formation of H2 and CO is interpreted as the result of radiolysis in the presence of dissolved uranium in the aqueous phase of these fluid inclusions. Finally, a late fluid (14–16 wt% NaClequiv) circulated in the brittle domain but seems unrelated to U (re-)mobilization event. |
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