Evidences of ion-adsorption clay-type rey enrichment in a weathered crust of the sÃo francisco granite (SÃo paulo, Brazil)

Abstract

The rare earth elements and yttrium (REY) ion-adsorption clay deposits (IAC) are important in the global supply chain of these metals, and originated by the weathering of biotite and/or muscovite syenogranite carrying REY weatherable minerals, with enrichment of REY occurring by adsorpotion in neoformed clay minerals, mainly kaolinite and halloysite. Evidences of REY IAC enrichment is reported, for the first time, in the 6,1 to 7,8-m-thick weathering profile on the São Francisco Granite (SFG-southeastern Brazil), a late-tectonic metaluminous to peraluminous greisenized syenogranite. Petrography and SEM/EDS analysis indicates that the parent rock, a peraluminous syenogranite, hosts REY-(fluor)carbonate minerals originated by alteration process. X-ray diffraction of the weathering profile, with soil and saprolite horizons, indicates the presence of neoformed kaolinite and halloysite. Chemical analysis of the weathered profile indicates REY content modification along the weathering horizons. The enrichment factor presents REY loss in horizons with positive cerium anomalies and a high chemical index of alteration (CIA>90) (soils and the saprolite of the P1 weathering profile). Meanwhile, REY gain was observed in saprolite horizon with negative cerium anomalies and lower chemical alteration index (CIA<90 and >70) (saprolite of the P3 weathering profile). Mass balance calculation demonstrates the REY mobility and mass gain in the weathering profile in similar conditions to the Asian REY-IAC deposits.