Reduction kinetics of aqueous U(VI) in acidic chloride brines to uraninite by methane, hydrogen or C-graphite under hydrothermal conditions: Implications for the genesis of unconformity-related uranium ore deposits
Résumé
The formation of hydrothermal uranium ore deposits involves the reduction of dissolved U(VI)((aq)) to uraninite. However, the nature of the reducing agent and the kinetics of such a process are currently unknown. These questions are addressed through dedicated experiments performed under conditions relevant for the genesis of unconformity-related uranium (URU) deposits. We tested the efficiency of the following potential reductants supposed to be involved in the reaction: H-2, CH4, C-graphite and dissolved Fe(II). Results demonstrate the great efficiency of H-2, CH4 and C-graphite to reduce U(VI)((aq)) into uraninite in acidic chloride brines, unlike dissolved Fe(II). Times needed for H-2 (1.4 bar), CH4 (2.4 bar) and C-graphite (water/carbon mass ratio = 10) to reduce 1 mMof U(VI)((aq)) in an acidic brine (1 mLiCl, pH approximate to 1 fixed by HCl) to uraninite at 200 degrees C are 12 h, 3 days and 4 months, respectively. The effects of temperature (T) between 100 degrees C and 200 degrees C, H-2 partial pressure (0.14, 1.4, and 5.4 bar), salinity (0.1, 1 and 3.2 m LiCl) and pH at 25 degrees C (0.8 and 3.3) on the reduction rate were also investigated. Results show that increasing temperature and H-2 partial pressure increase the reaction rate, whereas increasing salinity or pH have the reverse effect. The reduction of uranyl to uraninite follows an apparent zero-order with respect to time, whatever the considered electron donor. From the measured rate constants, the following values of activation energy (Ea), depending on the nature of the electron donor, have been derived: Ea(C-graphite) = 155 +/- 3 kJ mol(-1), Ea(CH4) = 143 +/- 6 kJ mol(-1), and Ea(H2) = 124 +/- 15 kJ mol(-1) at T \textless 150 degrees C and 32 +/- 6 kJ mol(-1) at T \textgreater 150 degrees C. An empirical relationship between the reaction rate, the hydrogen partial pressure, the uranyl speciation, and the temperature is also proposed. This allows an estimation of the time of formation of a giant U ore deposit such as McArthur River (Canada). The duration of the mineralizing event is controlled both by the U concentration in the ore-forming fluids and the dynamics of gaseous reductants input, and not by the kinetics of U(VI)((aq)) reduction itself. Focused flow of mobile electron donors (H-2, CH4) along quasi vertical fractured zones into U(VI)((aq))-bearing oxidized fluids may explain the large volume and high concentrations of uranium in the URU deposits.