Extreme light rare earth element mobilization by diagenetic fluids in the geological environment of the Oklo natural reactor zones, Franceville basin, Gabon

The anomalously high Th/La ratio (similar to 1,14) of the Early Proterozoic silicified sandstones of the Franceville basin (Gabon), compared to Archea n and Proterozoic metasedimentary rocks (Th/La similar to 0,27), results fr om extreme light rare earth element (REE) migration during diagenesis, Mona zite, which represents the main light REE-bearing phase in the sandstones, was altered by diagenetic brines at 140 degrees C and 1 kbar, The alteratio n phase is a microcrystalline Th-silicate phase, indicating low Th solubili ty at these conditions. Light REEs are simultaneously leached out together with P and U. The increase in Th/La from detrital monazite to residual Th-s ilicate phase indicates that about 76% of the light REEs were leached out, corresponding to a global amount of 2.01 x 10(9) metric tons at the scale o f the FA Formation in the Franceville basin. Uranium was also leached durin g monazite alteration and may have contributed significantly to the genesis of the high-grade uranium deposits of the Franceville basin that host the natural nuclear reaction zones.