Impact of weathering on REE distribution in soil-saprolite profiles developed on orthogneisses in Borborema Province, NE Brazil

摘要

Rare earth elements (REEs) are a group of elements (from La to Lu) with high economic value and serve mainly as essential raw material for various high-technology items. This study explores mineral weathering and the geochemical evolution of regoliths (quaternary period) derived from orthogneisses (late Cretaceous Period) in four sites in Borborema Province, NE Brazil, providing a new contribution to the understanding of the influence of soil and saprolite attributes on the distribution of REEs in the regolith. We used optical and electron microscopy to depict the weathering of minerals in the rock, saprolite and soil microstructure, focusing on the distribution of REEs in the regolith (Depth: 3.0 m). We also determined the poorly and well-crystallized iron oxide contents, the clay fraction amount and the total organic carbon content to observe how these attributes influence the distribution of REEs in the soil and saprolite. The results allowed the classification of weathering for feldspars, micas and other primary minerals into four stages based on their degree of alteration and the consequent influence on the geochemistry of the regoliths. Advancement of the weathering stages revealed the relative increases in Fe and Ti compared with Ca, Mg, Na and K from the fresh rock up to the soil horizons, and ilmenite and xenotime were the main sources of Yb, Y, Co and P. The patterns of signatures and anomalies were different for light (LREEs), middle (MREEs) and heavy rare earth elements (HREEs) and for regoliths in different metamorphic complexes. The LREEs related well with the clay fraction amount, the poorly crystallized iron oxides and the degree of weathering, as estimated by the mineralogical index of alteration (MIA), while the HREEs related well with the total organic carbon (TOC). In particular, Ce was strongly associated with Mn oxides, possibly due to simultaneous oxidative precipitation.