From evaporated seawater to uranium-mineralizing brines: Isotopic and trace element study of quartz-dolomite veins in the Athabasca system

International audienceStable isotope (O, H, C), radiogenic isotope (Sr, Nd) and trace element analyses have been applied to quartz-dolomite veins and their uranium(U)-bearing fluid inclusions associated with Proterozoic unconformity-related UO2 (uraninite) ores in the Athabasca Basin (Canada) in order to trace the evolution of pristine evaporated seawater towards U-mineralizing brines during their migration through sediments and basement rocks. Fluid inclusion data show that quartz and dolomite have precipitated from brines of comparable chemistry (excepted for relatively small amounts of CO2 found in dolomite-hosted fluid inclusions). However, d18O values of quartz veins (d18O = 11& to 18&) and dolomite veins (d18O = 13& to 24&) clearly indicate isotopic disequilibrium between quartz and dolomite. Hence, it is inferred that this isotopic disequilibrium primarily reflects a decrease in temperature between the quartz stage ( 180 C) and the dolomite stage ( 120 C). The d13C values of CO2 dissolved in dolomite-hosted fluid inclusions (d13C = 30& to 4&) and the d13C values of dolomite (d13C = 23.5& to 3.5&) indicate that the CO2 dissolved in the mineralizing brines originated from brine-graphite interactions in the basement. The resulting slight increase in the fluid partial pressure of CO2 (pCO2) may have triggered dolomite precipitation instead of quartz. d18O values of quartz veins and previously published d18O values of the main alteration minerals around the U-ores (illite, chlorite and tourmaline) show that quartz and alteration minerals were isotopically equilibrated with the same fluid at 180 C. The REE concentrations in dolomite produce PAAS-normalized patterns that show some similarities with that of UO2 and are clearly distinct from that of the other main REE-bearing minerals in these environments (monazite, zircon and aluminum phosphate-sulfate (APS) minerals). The radiogenic isotope compositions of dolomite (87Sr/86Sri = 0.7053 to 0.7161 and eNd(t) = 8.8 to 20.3) differ from one deposit to another, reflecting both heterogeneity in the basement geology and variable preservation of the original composition of brines. The previously published 87Sr/86Sri and eNd(t) values of UO2 compare with the most evolved dolomites, i.e. dolomites precipitated from brines that exchanged the most with the basement. This reinforces a close genetic link between dolomites and UO2 deposition and implies that UO2 deposition occurred in a cooling system during the transition from quartz to dolomite formation. The d18O and dD values of the mineralizing brines (d18O= 1& to 8& and dD= 150& to 50&) are considerably shifted from that of their theoretical original values acquired during evaporation of seawater (d18O= 3& and dD= 40&). The positive d18O shift is explained by protracted fluid-rock interaction within the basin and basement rocks. The negative dD shift is attributed to incomplete mixing between the U-mineralizing brines and low dD water. This low dD water was likely produced during the abiogenic synthesis of bitumen by Fisher-Tropsch-like reactions involving CO2 derived from brine-graphite interaction in the basement, and radiolytic H2. The resulting low dD brines have been equilibrated with alteration minerals. This may explain why some alteration minerals yield anomalously low dD values whose significance has long been debated