Cerium anomalies in lateritic profiles
Abstract
The REE geochemistry and mineralogy have been studied in four lateritic profiles, one derived from a syenite (Akongo, SW Cameroon), the others being developed on a gneissic basement and located along a soil toposequence (Goyoum, E Cameroon). There is a fractionation between LREE and HREE in the lateritic samples during weathering, the weathered residual products being enriched in LREE (from La to Eu) and depleted in HREE (from Gd to Lu); sampled waters are enriched in HREE in relation to the syenite host-rock. A positive Ce-anomaly has been found systematically at the top of the saprolite, beneath a zone of iron oxide accumulation in the laterite. Up to 2000 ppm Ce may be present. In the Akongo profile, cerianite, CeO 2, is present as very fine coatings in non-ferruginous clayey domains. Primary REE-bearing accessory minerals are weathered at the bottom of the profile. Specifically, allanite and apatite are transformed into florencite and rhabdophane but these phases have no Ce-anomaly. All the data are interpreted as the result of the following processes: (1) REE leaching in a reducing environment.
(2) oxidation of Ce 3+ to Ce 4+ in an oxidizing environment. (3) deposition of cerium as cerianite whereas the other REE remain in solution.- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- March 1990
- DOI:
- 10.1016/0016-7037(90)90373-S
- Bibcode:
- 1990GeCoA..54..781B