Origin of U-mineralizing brines in the Athabasca Basin, Canada
Résumé
High salinity brines are commonly sampled as fluid inclusions in close paragenetic relationship with unconformity-related U mineralizations in the Proterozoic Athabasca Basin. The large range of brine composition results from the mixing of two end-members (NaCl-rich and CaCl2-rich) during or after maximum burial (T from 120 to 180°C) [1]. Here, we address the question of the origin of these brines and their possible interactions with basin and basement lithologies. Chorinities of 5-7 molal, coupled with Cl/Br ratios of 100-400 and negative ä37Cl values, are unambiguously in favour ofa common dominant evaporated seawater origin for both brines, typical of primary brines having past the point of halite precipitation. Such brines either have immediately percolated through the sedimentary pile, or alternatively have been trapped in the evaporitic formation and were expelled afterwards during compaction events. ä18O values of inclusions-hosting quartz and dolomites are compatible with evolved deep basinal brines.The CaCl2-rich brines are thought to result from strong interactions with basement lithologies where they have uptaken uranium, as shown by the very high content detected by LA-ICP-MS. The range of 87Sr/86Sr ratios of inclusionshosting dolomite veins suggests a mixed origin, between a marine origin and a more radiogenic component typical of basement lithologies. D/H values of brines, coupled with ä13C values of dissolved CO2 argue for interactions with organicmatter and D-depleted materials. The isotopic trends mayindicate the involvement of C-rich components in the waterrock processes in relation with the U mineralization.