Alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the Oklo-Okelobondo and Bangombe natural nuclear reaction zones (Franceville basin, Gabon)

Large-scale light rare earth element (LREE), uranium, lead and phosphorus m igration has been evidenced in the FA Lower Proterozoic sandstones of the F ranceville basin (Gabon) hosting Oklo natural nuclear reaction zones (RZ) i n relation with extensive accessory mineral alteration by highly saline dia genetic brines (28.7 wt.% NaCl eq. to 30 wt.% CaCl2 eq.) at about 140 degre esC 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 (T h/La similar to 0.27) to altered sandstones (Th/La similar to 1.14) shows t hat about 76% of the LREE was leached. This corresponds to the leaching of 2.01 X 10(9) metric tons at the scale of the FA formation in the Francevill e 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 b y oxidizing brines with leaching of U together with LREE and P. It correspo nds to the leaching of 9.06 x 10(6) metric tons of uranium. This amount of uranium largely exceeds the known uranium reserves from the Franceville bas in. In zircon crystals, the cores are generally homogeneous, weakly fractured a nd 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 com position corresponds to the pure end-member (Zr,Hf)(SiO4), poor in Th and U (Th/U similar to 1). At the contrary, their rims, which present several gr owth zones with cracks fillings, are enriched in REE, P, Th and U with high er Th/U ratios (5-10). Both altered monazite and altered zircon contain galena as numerous inclusi ons 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 (m aximum 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, produc ing a porous and distorted crystal structure which has allowed a later inco rporation of Pb. Galena inclusions in altered zircons located in the vicini ty 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. ph isotopic evolution models indicate a crystallization age sometime after 1000 Ma, both for galena located close to and far from U mineralizat ions and reactor zones, which maybe synchronous with a regional extension e vent contemporaneous with intrusion of dolerite dyke swarms, between 1000 a nd 750 Ma, at the scale of the Franceville basin. The present study also illustrates the different retention capacities of ac cessory mineral for elements representing analogs of the radiotoxic nuclide s in the relatively extreme natural conditions created by the circulation o f moderately hot and chloride-rich fluids during the diagenesis of a sedime ntary basin. (C) 2001 Elsevier Science B.V. All rights reserved.