The Okelobondo natural fission reactor, Southeast Gabon; geology, mineralogy, and retardation of nuclear-reaction products

Nuclear fission reactors occur in the Oklo-Okelobondo uranium deposit in southeast Gabon. The Okelobondo reactor zone (RZOKE, approximately 310 m depth) was the last reactor zone to be excavated before mining was terminated in December 1997. RZOKE is situated at the base of a 2.5 m deep and 2.7 m wide reactor synform located between the FA sandstone and hydrothermally altered black shales--argile de pile--of the FB Formation. The reactor synform developed by hydrothermal dissolution of the FA sandstone during criticality at ca. 2 Ga. The core of RZOKE is up to approximately 55 cm thick and contains < or =90 vol% uraninite (< or =91.24 + or - 0.91 wt% UO (sub 2) and < or =7.22 + or - 0.53 wt% PbO) embedded in a matrix of Si-rich illite (6.08 to 7.91 Si per formula unit [p.f.u.]). The argile de pile (typically < or =60 cm) consists mainly of donbassitic chlorite (7.84 to 8.07 octahedrally coordinated Al p.f.u.) cut by fine illite veinlets. Fe-rich chlorite (0.708 to 1.427 Fe p.f.u.) is abundant at the edges of the reactor zone. Organic matter in both the reactor zone and near-field surroundings played an important role in the enhancement of porosity and stabilization of U during formation and operation of RZOKE. Fissiogenic Zr, Ce, Nd, and Th (daughter of (super 240) Pu and (super 236) U generated by neutron-capture reactions) are well retained in uraninite and (U,Zr)-silicate. The (U,Zr)-silicate formed during local migration of Zr, Sr, U, Pu, and lanthanide fission products during reactor criticality. Fissiogenic Ru, including (super 99) Ru--a daughter of (super 99) Tc, was mainly retained as ruthenium arsenide (+ or - Pb, Co, Ni, and S). Although RZOKE generally appears well preserved, partially dissolved galena and uraninite, as well as the presence of accessory anglesite(?) and lead-uranyl sulfate hydroxide hydrate, suggest that recently observed deep, oxidized groundwaters have begun to interact with RZOKE.