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The combination of structural, geochemical and palaeotopographic data proves to be an efficient tool to understand fluid transfers in the crust. This study discriminates shallow and deep fluid reservoirs on both sides of the brittle-ductile transition under an extensional regime and points out the role of major transcurrent fault activity in this palaeohydrogeological setting. Palaeofluids trapped in quartz and siderite-barite veins record the transfer of fluids and metal solute species during the Neogene exhumation of the Sierra Almagrera metamorphic belt. Ductile then brittle-ductile extensional quartz veins formed from a deep fluid reservoir, trapping metamorphic secondary brines containing low-density volatile phases derived from the dissolution of Triassic evaporites. During exhumation, low-salinity fluids percolated within the brittle domain, as shown by transgranular fluid inclusion planes affecting previous veins. These observations indicate the opening of the system during Serravalian to early Tortonian times and provide evidence for the penetration of surficial fluids of meteoric or basinal origin into the upper part of the brittle-ductile transition. During exhumation, synsedimentary transcurrent tectonic processes occurred from late Tortonian times onwards, while marine conditions prevailed at the Earth's surface. At depth in the brittle domain, quartz veins associated with haematite record a return to high-salinity fluid circulation suggesting an upward transfer fed from a lower reservoir. During the Messinian, ongoing activity of the trans-Alboran tectono-volcanic trend led to the formation of ore deposits. Reducing fluids caused the formation of siderite and pyrite ores. The subsequent formation of galena and barite may be related to an increase of temperature. The high salinity and Cl/Br ratio of the fluids suggest another source of secondary brine derived from dissolved Messinian evaporites, as corroborated by the δ34S signature of barite. These evaporites preceded the main sea-level drop related to the peak of the salinity crisis (5.60-5.46 Ma). [ABSTRACT FROM AUTHOR]

Fluid inclusions from two quartz samples of the McArthur River and Rabbit Lake unconformity-related uranium deposits (Athabasca Basin, Canada) were analysed by synchrotron X-ray fluorescence ( SXRF) and X-ray absorption near-edge spectroscopy ( XANES) to shed light on (i) the detailed chemistry of the fluids having transported the uranium and (ii) the speciation of uranium in these fluids. The analysed samples contain variable proportions of NaCl-rich or CaCl2-rich (25-35 wt% salts) fluid inclusions that homogenise into the liquid phase between 120 and 200°C. For unknown reason, all of the CaCl2-rich fluid inclusions decrepitate under X-ray beam after a few seconds, precluding any SXRF or XANES measurement. SXRF on 12 homogenised NaCl-rich fluid inclusions from the Rabbit Lake sample shows that the fluid inclusions contain appreciable amounts of Br, Fe, Sr, transition metals (Mn, Ni, Cu, Zn), Pb, U and rare earth elements ( REE) (La, Ce), with concentrations being relatively homogeneous among fluid inclusions. Within the complex McArthur River sample (numerous fluid inclusions + randomly distributed solids), statistical analyses of hyperspectral SXRF images were performed using the non-negative matrix approximation ( NNMA) method. This strategy allows distinguishing the three domains contributing to the overall signal: (i) the quartz matrix, which notably contains significant amounts of Cr, Cu and Pb; (ii) the fluid inclusions characterised by high amounts of Br, Fe, Sr and transition metals; and (iii) La-Ce ± Fe solids. Part of the U and REE are spatially associated with distinct optically invisible solids within the quartz matrix. XANES on four McArthur River sample fluid inclusions at room temperature and at 150°C (fluid inclusion trapping temperature) as well as in solid and liquid U( IV) and U( VI) standards, respectively, shows that the uranium has remained in the form of U( VI) from trapping to present in the fluid inclusion. [ABSTRACT FROM AUTHOR]