RARE-EARTH ELEMENT MOBILITY DURING CONVERSION OF NEPHELINE SYENITE INTO LATERITIC BAUXITE AT PASSA-QUATRO, MINAIS-GERAIS, BRAZIL

In a lateritic bauxite formed by weathering of nepheline syenite at Pa ssa Quatro, Minas Gerais State, Brazil, bauxites on the hill-tops dire ctly develop from the syenite bed-rock, while downslope, a kaolinitic layer occurs between bauxite and syenite. A petrological investigation was performed on undisturbed weathered rock samples collected from a representative upslope pit. The undisturbed weathered rocks were chemi cally analysed for major and trace elements including REE and Zr. Mass balance calculations were applied, and the behaviour of the REE in th e Passa Quatro weathering system was established compared to REE refer ence chondrite and to REE reference parent rock. In the lateritic baux ite, the results suggest that the first stages of weathering induce a volumetric change of 50%, i.e. collapse, with respect to the parent ro ck, and remove REE with a slightly larger loss of the LREE, except Ce, compared to the HREE. In the upper layers, where bauxite is more matu re, a net mass gain in REE is observed relative to the underlying laye rs. This gain takes place during the reduction of the upper layer duri ng the downward progression of the weathering front. Very significant REE losses occurs during the bauxitization processes throughout the up slope profile. In addition, the downslope kaolinitic system is demonst rated to be depleted in REE in the same proportions as the upslope bau xite. We propose that the REE exported in solution from the whole weat hering mantle have enriched neighbouring watershed sediments.