The REE minerals pyroxenites and sövites of Catalão II show a substantial enrichment in REE from the magmatic stage. The activity of carbonate and CO₃^2-/ PO₄^3- ratio have played an important role in the formation of REE-carbonate-phosphates, which are an intermediate form between REE-rich apatite and REE-rich carbonate. The magmatic enrichment in REE and Ti, favored by the presence of a carbonate phase, correlate to a high activity of CO₃^2- and activities of Ti and Na specially in the carbonatite. This feature explains the low enrichment in phosphate (apatite vein) in Catalão II compared to Catalão I, which is rich in apatite vein. In the weathering stage, these minerals interact with the fluid surface and lose some of their REE and Ba. The exchange reactions between pyrochlore and ground water are generally consistent with relatively low pH, low activities of Na⁺, Ca²⁺, F^-and elevated activities of Ba²⁺ /or Ce³⁺. Significant amounts of Ba were derived from barite dissolution with some silicate contribution, whereas the Ce might come, either from dissolved REE-carbonate minerals or from the pyrochlore themselves through selective immobilisation of Ce as Ce⁴⁺.