SOLID LIQUID REE FRACTIONATION IN THE LATERITIC SYSTEM OF GOYOUM, EAST CAMEROON - THE IMPLICATION FOR THE PRESENT DYNAMICS OF THE SOIL COVERS OF THE HUMID TROPICAL REGIONS/

The REE-Th weathering geochemistry and mineralogy has been investigate d in a lateritic soil cover in relation to the close hydrographical sy stem at Goyoum (East Cameroon). The stutied area, composed of gneissic hills covered by humid tropical forest, belongs to the Sanaga river b asin and corresponds to a transition vegetation zone between rainfores t and savannah. A representative soil catena was taken as a reference zone close to the Sanaga river. The gneissic parent-rock, soil, and gr oundwater samples have been cut off in pits in the lower part of the c atena. Waters were also sampled both in brooks rich in organic matter and in the Sanaga river. All the waters were sampled seasonally during the rainy and dry periods. The water/rock interactions and the export ation of REE and Th have been discussed as a function of the evolution of specific ratios. Considering Th as the less mobile element during laterization, variation of parent rock (PR)-normalized ratios [(Ln/Th) (PR)] vs. depth show that the upper ferruginous horizons are depleted in REE while the basal saprolite presents accumulation zones which are differently located for Ce and the other LREEs. The HREEs are also de pleted. The La accumulations are located near the saprolite/gneiss wea thering front associated with negative Ce-anomalies (Ce/Ce similar to 0.4). The Ce accumulations inducing strong positive Ce-anomalies (Ce/ Ce similar to 4) are located in the upper saprolite beneath the ferru ginous horizons all along the soil catena. The secondary LREE-bearing minerals which reflect LREE fractionation during incipient weathering of the gneiss are hydrous phosphates (rhabdophane, LREEPO4.nH(2)O) and cerianite (CeO2) whose precipitation depends on the Eh-pH changes in the soil profiles. On the other hand, HREE depletion is controlled by the dissolution of xenotime, the major HREE bearer in the parent gneis s. In the <0.45 mu m filtered acidic groundwater (pH similar to 5.5) w hich have flooded the saprolite for a long residence time, Th contents are extremely low (0 < Th < 10 ppt) while LREE contents are high (90 < La < 450 ppt). In these filtered waters, Th seems to be strongly con trolled by minerals (i.e., thorianite) which lead to its immobilizatio n in the soil while LREE remain in the soluble and/or colloidal pools. Moreover, the groundwaters show significant negative Ce-anomalies thr oughout the year (0.1 < Ce/Ce < 0.4. The mobilization of LREE, except redox-active Ce, results from the dissolution and or the mobilization in colloidal form of the secondary phosphates which may present major negative Ce-anomalies as indicated by the bulk soil analyses at the s aprolite/gneiss weathering front. Therefore, the groundwaters have the signature of saprolitic materials. We propose that the strong positiv e Ce-anomalies of the upper saprolite result from ancient weathering p rocesses. These processes would have led to much stronger Ce(III)/Ce(I V) dissolution/reprecipitation conditions than those occurring at the present time in the saprolite. In the <0.45 mu m filtered brook acidic blackwaters, LREE and Th contents are high (100 < Th < 400 ppt; < 400 La 1700 ppt). In this organic matter rich environment, LREE and Th ar e highly mobilized. The colloial pool controls half of the whole LREE- Th load. The geochemical signature of these waters is independent of t he gneissic parent rock and soil LREE contents. There is no Ce-anomaly . This signature is similar to those of other organic-rich rivers drai ning humid tropical ecosystems in the world. In the 0.45 mu m filtered Sanaga waters, LREE and Th contents are lower than in the brooks (15 < Th < 40 ppt; 40 < La < 300 ppt). These concentrations are dependent on the time and on the colloidal composition. However, 90% of these el ements are mainly controlled by the suspended pool. Copyright (C) 1998 Elsevier Science Ltd.