Behaviour of major and trace elements and fractionation of REE under tropical weathering of a typical apatite-rich carbonatite from Brazil

Abstract

Detailed petrographical, mineralogical and geochemical studies have been carried out on a weathering profile developed on the phosphate-rich carbontite of Juquiá(Brazil). Four weathering trends could be distinguished by means of the relationships between primary (apatites, dolomite, phlogopite, magnetite and barite) and secondary (apatites, gorceixite, wavellite, calcite and Fe Mn-oxyhydroxides) minerals. Local isovolumetric and isoelemental weathering processes allowed us to estimate chemical mass balances within some weathering trends. Except for Al in one profile, all the major and trace elements are mobile and transferred as a result of weathering of the primary minerals. The REE patterns of each primary/secondary mineral show that the weathering processes fractionate LREE and HREE. Relationships between REE content and the water/rock ratio may explain the various REE contents and distribution curves exhibited by the secondary minerals. Precipitation of secondary minerals in open microsystems (i.e., veins and fissures) appears to be associated with REE depletion and fractionation. In contrast, crystallization of secondary minerals within the intragranular micropores may result in a significant preservation of the REE content and distribution of the primary minerals being replaced. Moreover, poorly crystallized and flaky secondary minerals act as traps for REE.