Fluid inclusion evidence of the differential migration of H 2 and O 2in the McArthur River unconformity-type uranium deposit (Saskatchewan, Canada). Possible role on post-ore modifications of the host rocks

Abstract

A fluid inclusion study was performed in the vicinity of the McArthur River unconformity-type uranium deposit,Saskatchewan, Canada. Euhedral quartz veins and breccias were sampled from the contact of the uranium orebody to a distance of 50 m from it. Raman analyses of the fluid inclusions vapor phase indicated the presence of H 2 and O 2 with various proportions. Fluids located at the immediate vicinity of the uranium ore contain nearly exclusively O 2. At 1 m from the ore, the oxygen proportion relative to other gases rapidly decreases, whereas H 2 increases and occasional traces of CH 4 occur. Additional C 2H 6 as well as CO 2 appear at about 10 m from mineralization, in the basement. Finally, only occasional traces of H 2 were detected in fluid inclusions located at 50 m from the mineralization, in the sandstones. Thus, fluids that circulated at the contact of the orebody became oxidizing, whereas fluids circulating at distance from it were more reduced, due to the combination of water radiolysis and differential migration of H 2 and O 2. This repartition of gases probably induced indirectly significant modifications in the host rock after the ore formation, such as hematization or graphite alteration and subsequent gas production. In addition, the migration of H 2 at long distance from the orebody should represent an potential prospecting tool.