NATURAL NUCLEAR FISSION REACTORS: TIME CONSTRAINTS FOR OCCURRENCE, AND THEIR RELATION TO URANIUM AND MANGANESE DEPOSITS AND TO THE EVOLUTION OF THE ATMOSPHERE

The occurrence of manganese and uranium deposits and of nuclear reactors in the Franceville basin is not fortuitous. It results from conditions that could be achieved only during the very specific period of time corresponding to the initial oxygenation of Earth's atmosphere. Our data agree with the model suggesting that the increase of oxygen in the atmosphere happened between 2150 Ma, the time of deposition of the Francevillian black shales devoid of uranium, and 1950 Ma which corresponds to the period when uranium was mobilized in its oxidized state in the FA Formation. The stability of uranium bearing minerals under low PO2 favors their concentration in fluvial and coastal placers. Under anoxic conditions, Mn can migrate over large distances. The fluctuations in sea levels and the local palaeogeographic conditions allowed Mn from distal volcanics to reach the edges of the Franceville basin, in zones where the development of the photosynthetic organisms increased the concentration of oxygen in the water, causing the precipitation of manganese oxides. In these zones the return to more reducing conditions was due to rapid burial possibly triggered by transgression episodes. This promoted the dissolution of the Mn-oxides and the precipitation of Mn-carbonates to form the protore of the Mn ore deposits. The progressive closure of the Franceville basin, marked by the depletion of 13Corg enhanced the contrasts between the oxygenated edges of the basin and the anoxic zones corresponding to the more central parts of the basin.

The mobilization of uranium in the Franceville basin occurred during diagenesis 1950 Ma ago. The migration of uranium is related to oxidation of the sandstone reservoir, alteration of the detrital uranium-bearing minerals and precipitation of iron oxides. This requires a complete recharge of the water and its oxygenation in contact with the atmosphere. We believe that this event marks a significant increase in atmospheric oxygen. The oxidized uranium precipitated when it came into contact with hydrocarbons concentrated in tectonic traps. The occurrence of oxidation-reduction reactions allowed for the first time the formation of very high-grade uranium ore deposits, which led in turn to nuclear fission reactions.