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Large-scale light rare earth element (LREE), uranium, lead and phosphorus migration has been evidenced in the FA Lower Proterozoic sandstones of the Franceville basin (Gabon) hosting Oklo natural nuclear reaction zones (RZ) in relation with extensive accessory mineral alteration by highly saline diagenetic brines (28.7 wt.% NaCl eq. to 30 wt.% CaCl2 eq.) at about 140°C and 1 kbar. Monazite is the most severely altered accessory mineral in the coarse-grained sandstones of the basal FA formation. Detrital monazite crystals are altered to Th–OH silicate microcrystalline phase with very low concentrations of U and LREE. The Th/La ratio increase from non-altered (Th/La∼0.27) to altered sandstones (Th/La∼1.14) shows that about 76% of the LREE was leached. This corresponds to the leaching of 2.01×109 metric tons at the scale of the FA formation in the Franceville basin. Similarly, the Th/U increase from monazite (Th/U=18.6) to the Th-silicate phase (Th/U=88.7) is interpreted as a result of an alteration by oxidizing brines with leaching of U together with LREE and P. It corresponds to the leaching of 9.06×106 metric tons of uranium. This amount of uranium largely exceeds the known uranium reserves from the Franceville basin. In zircon crystals, the cores are generally homogeneous, weakly fractured and well preserved as attested by the Archean ages (2867±24 and 2865±51 Ma) obtained by ionic microprobe analysis on zircon of the FA Formation, respectively, from the marginal and central parts of the basin. Their composition corresponds to the pure end-member (Zr,Hf)(SiO4), poor in Th and U (Th/U∼1). At the contrary, their rims, which present several growth zones with cracks fillings, are enriched in REE, P, Th and U with higher Th/U ratios (5–10). Both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings. For example, in zircon, the Pb of galena crystals (3–23 wt.%) largely exceeds the amount of Pb (maximum 0.1 wt.%) that would have been produced in situ by radioactive decay in this mineral. Nearly all the lead were introduced into altered zones of accessories. Dissolution of accessory minerals occurred at 2000 Ma, producing a porous and distorted crystal structure which has allowed a later incorporation of Pb. Galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions. Altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non-radiogenic Pb isotopic compositions, confirming the external origin of lead. Pb isotopic evolution models indicate a crystallization age sometime after 1000 Ma, both for galena located close to and far from U mineralizations and reactor zones, which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms, between 1000 and 750 Ma, at the scale of the Franceville basin. The present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride-rich fluids during the diagenesis of a sedimentary basin.