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Geology; August 2000; v. 28; no. 8; p. 743-746; DOI: 10.1130/0091-7613(2000)28<743:ELREEM>2.0.CO;2
© 2000 Geological Society of America
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Extreme light rare earth element mobilization by diagenetic fluids in the geological environment of the Oklo natural reactor zones, Franceville basin, Gabon

Michel Cuney1 and Régis Mathieu1

1 Unité Mixte de Recherche G2R-7566, Université Henri Poincaré, BP239, F-54506 Vandoeuvre-lès-Nancy Cedex, France

The anomalously high Th/La ratio (~1.14) of the Early Proterozoic silicified sandstones of the Franceville basin (Gabon), compared to Archean and Proterozoic metasedimentary rocks (Th/La ~0.27), results from extreme light rare earth element (REE) migration during diagenesis. Monazite, which represents the main light REE-bearing phase in the sandstones, was altered by diagenetic brines at 140 °C and 1 kbar. The alteration phase is a microcrystalline Th-silicate phase, indicating low Th solubility 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-silicate phase indicates that about 76% of the light REEs were leached out, corresponding to a global amount of 2.01 x 109 metric tons at the scale of the FA Formation in the Franceville basin. Uranium was also leached during 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.

Key Words: monazite • alteration • rare earth element • diagenesis • migration • Oklo




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