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

The δ18O and δD values of the mineralizing brines (δ18O = −1‰ to 8‰ and δD = −150‰ to −50‰) are considerably shifted from that of their theoretical original values acquired during evaporation of seawater (δ18O = ∼−3‰ and δD = ∼−40‰). The positive δ18O shift is explained by protracted fluid-rock interaction within the basin and basement rocks. The negative δD shift is attributed to incomplete mixing between the U-mineralizing brines and low δD water. This low δD 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 δD brines have been equilibrated with alteration minerals. This may explain why some alteration minerals yield anomalously low δD values whose significance has long been debated.