Search

International audience

International audience; Hydrogen (H2)-rich fluids are observed in a wide variety of geologic settings including gas seeps in serpentinized ultramafic rocks, sub-seafloor hydrothermal vents, fracture networks in crystalline rocks from continental and oceanic crust, and volcanic gases. Natural hydrogen sources can sustain deep microbial ecosystems, induce abiotic hydrocarbons synthesis and trigger the formation of prebiotic organic compounds. However, due to its extreme mobility and small size, hydrogen is not easily trapped in the crust. If not rapidly consumed by redox reactions mediated by bacteria or suitable mineral catalysts it diffuses through the rocks and migrates toward the surface. Therefore, H2is not supposed to accumulate in the crust. We challenge this view by demonstrating that significant amount of H2may be adsorbed by clay minerals and remain trapped beneath the surface. Here, we report for the first time H2content in clay-rich rocks, mainly composed of illite, chlorite, and kaolinite from the Cigar Lake uranium ore deposit (northern Saskatchewan, Canada). Thermal desorption measurements reveal that H2is enriched up to 500 ppm (i.e. 0.25molkg−1of rock) in these water-saturated rocks having a very low total organic content (

WOS:000361007300002; International audience; 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.