Hypersaline fluids generated by high-grade metamorphism of evaporites: fluid inclusion study of uranium occurrences in the Western Zambian Copperbelt

Abstract

International audienceIn the Pan-African Lufilian belt (Western Zambian Copperbelt), uranium mineralizations, preferentially scattered in kyanite +/- talc micaschists (metamor-phosed 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; N-2-H-2 in the gas phase of fluid inclusions) circulated during the high-temperature quartz recrystallization, at 500-700 degrees 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 degrees C. This temperature could be related to the exhumation of the high-grade metamorphic rocks at ca. 500 Ma. The formation of H-2 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

Raman spectra of water in fluid inclusions: I. Effect of host mineral birefringence on salinity measurement

“…. Pluri‐centimetric kyanite porphyroblasts were found in phlogopite schists bearing syn‐metamorphic to post‐metamorphic uranium and copper mineralizations (Roan group, western Copperbelt Zambia) . Millimetric rhombs of calcite were found in calcitic veins oblique to marble foliation of the Kamariza schists (Laurion, Greece).…”

CO 2 flow during orogenic gravitational collapse: Syntectonic decarbonation and fluid mixing at the ductile-brittle transition (Lavrion, Greece)

“…Fluid production and flow in the Earth's crust can be reconstructed through careful microstructural and geochemical investigations of veins and fluid inclusions (e.g Küster and Stöckhert, 1997;Vapnik and Avigad, 2004;Eglinger et al, 2014;Diamond and Tarantola, 2015). Besides H 2 O, CO 2 is a dominant volatile phase commonly observed in metamorphic fluids (Roedder, 1984;Crawford and Hollister, 1986;Diamond, 2001;Kerrick and Connolly, 2001;Yardley and Bodnar, 2014).…”

Capabilities of sequential and quasi-simultaneous LA-ICPMS for the multi-element analysis of small quantity of liquids (pl to nl): insights from fluid inclusion analysis

“…n ¼ number of fluid inclusions analysed. 54 For a similar 15 mm multi-phase uid inclusion, the liquid and solid phases are properly discriminated during TOFMS measurement, with the signals of the majority of the detected isotopes displaying specic shapes during acquisition. See Table 1 for the analytical conditions.…”

“…Table 3 Chemical compositions of two-phase fluid inclusions from sample BP-66-210 (Mont Blanc Massif, French Alps) analysed by LA-ICPQMS, LA-ICPSFMS and LA-ICPTOFMS. 54 However, some signals are mixed and could indicate a partitioning of the elements between different phases. (Table 1), this corresponds to ca.…”