MOBILIZATION AND REDISTRIBUTION OF REES AND THORIUM IN A SYENITIC LATERITIC PROFILE - A MASS-BALANCE STUDY

REE-Th geochemistry and mineralogy have been studied in a lateritic pr ofile derived from a syenite at Akongo in SW Cameroon. REE and Th mass balance calculations for the host-rock minerals show that at least 70 % of the LREEs and 50% of the HREEs are contained in allanite, apatite , titanite, and epidote and at least 50% of the Th is controlled by th e same accessory minerals which represent about 2 wt% of the unaltered syenite. These accessory phases are destroyed during the first stages of weathering causing most of the REEs and Th to be rapidly released into the soil. Comparison of the variation in the Zr, Ti, and Th conte nt as a function of the apparent density of the different zones of the saprolite shows that Th is the least mobile element. The presence of secondary thorianite (ThO2), the etched surface on zircon grains, and the presence of Ti in secondary cerianite support this geochemical int erpretation. The concentration of thorium was, therefore, chosen as in variant relative to the concentration of the other elements, especiall y the REEs, in mass balance calculations. Most of the REEs are leached in the iron-rich upper horizons (loose nodular horizon, iron crust, a nd top of mottled clay horizon). Where the groundwater table moves (sa prolite and bottom of the mottled clay horizon), the REEs are fraction ated and redistributed. There is a juxtaposition of leached and accumu lation zones with precipitation of LREE aluminous hydrated phosphates. This study supports the existence of two different cycles for the red istribution of elements in the soil: (1) as dissolved ions in the sapr olite horizon, and (2) as individual particles in the upper part of th e profile.