Brine-rock interaction in the Athabasca basement (McArthur River U deposit, Canada): consequences for fluid chemistry and uranium uptake

P>The nature of uranium source rocks transport conditions and deposition processes are still highly controversial for world class unconformity related U deposits. This article presents the first detailed chemistry of brines associated with the giant McArthur River U deposit Canada. LA ICP MS analysis of individual fluid inclusions suggests mixing between a Na Ca Mg K Sr Ba brine and a Ca Mg Na K Sr Ba brine. The brines share a common origin (evaporated seawater) and show evidence for contrasting interaction with basement rocks. The Na rich brine lost Mg and K in alteration haloes around U ores while the Ca rich brine results from Na Ca exchange and Sr Ba gain. U concentrations (0.3 530 mu g g 1) are anomalously high compared with usual basinal fluids this indicating that U uptake occurred within basement rocks. The two brine end members have mixed within the main U deposit area which could be one of the major driving forces for U deposition.