Mobilization and mechanisms of retardation in the Oklo natural reactor zone 2 (Gabon)—inferences from U, REE, Zr, Mo and Se isotopes

Thermodynamics of formation of coffinite, USiO 4

“…Stable U(IV) minerals, which could form as secondary phases, would impart lower uranium solubility to such systems. Thus, knowledge of coffinite thermodynamics is needed to constrain the solubility of U(IV) in natural environments and would be useful in repository assessment.In natural uranium deposits such as Oklo (Gabon) (4,7,11,12,14,17,18) and Cigar Lake (Canada) (5, 13, 15), coffinite has been suggested to coexist with uraninite, based on electron probe microanalysis (EPMA) (4,5,7,11,13,17,19,20) and transmission electron microscopy (TEM) (8, 15). However, it is not clear whether such apparent replacement of uraninite by a coffinite-like phase is a direct solid-state process or occurs mediated by dissolution and reprecipitation.…”

“…The potential release of radionuclides into the environment strongly depends on the availability of water and the subsequent corrosion of the waste form as well as the formation of secondary phases, which control the radionuclide solubility. Coffinite (1), USiO 4 , is expected to be an important alteration product of SNF in contact with silica-enriched groundwater under reducing conditions (2)(3)(4)(5)(6)(7)(8). It is also found, accompanied by thorium orthosilicate and uranothorite, in igneous and metamorphic rocks and ore minerals from uranium and thorium sedimentary deposits (2,4,5,(8)(9)(10)(11)(12)(13)(14)(15)(16).…”

“…In natural uranium deposits such as Oklo (Gabon) (4,7,11,12,14,17,18) and Cigar Lake (Canada) (5,13,15), coffinite has been suggested to coexist with uraninite, based on electron probe microanalysis (EPMA) (4,5,7,11,13,17,19,20) and transmission electron microscopy (TEM) (8,15). However, it is not clear whether such apparent replacement of uraninite by a coffinite-like phase is a direct solid-state process or occurs mediated by dissolution and reprecipitation.…”

Fate of trace elements during alteration of uraninite in a hydrothermal vein-type U-deposit from Marshall Pass, Colorado, USA

“…As for the concentration of trace elements in uraninites in previous studies, Hofmann and Eikenberg (1991) reported up to 3.65 wt% of As 2 O 3 and 0.90 wt% of WO 3 ; Leroy and Holliger (1984) measured 0.28 wt% of MnO. The concentrations of MoO 3 , SeO 2 , ReO 2 , and MgO were 0.32 wt%, 0.22 wt%, 0.32 wt%, and 0.21 wt%, respectively, (Min et al, 2005); 0.24 wt% of V 2 O 3 and 0.12 wt% of TiO 2 (Bros et al, 2003). Fayek et al (1997a) reported 0.19 wt% of As 2 O 3 ; 0.39 wt% of CuO; and 0.4 wt% of MnO.…”

Redistribution of REE, Pb and U by supergene weathering studied from in-situ isotopic analyses of the Bangombé natural reactor, Gabon

“…However, the interpretation was not supported by isotopic data. To address this issue, in-situ isotopic analyses of the micro-minerals found in the peripheral rocks of the reactors have been performed by secondary ion mass spectrometry (SIMS) (Janeczek and Ewing, 1996b;Bros et al, 2003;Horie et al, 2004;Hidaka et al, 2005). SIMS has been widely applied for the in-situ isotopic analyses of geological samples for two decades.…”