Rare earth elements in a historical mining district (south-west Spain): hydrogeochemical behaviour and seasonal variability

Abstract

This work deals with the distribution of rare earth elements (REE) in the abandoned Tharsis mines under different hydrological conditions. High concentrations of REE were observed; mean value of 1747 μg/L. The highest concentrations of REE were recorded during the dry period (DP, mean of 2220 μg/L) due to high evaporation and strong water-rock interactions. However, some sampling points showed the highest REE concentrations during the wet period (WP) due to the washing out of large dumps during intense rainfall. The concentration of REE shows a positive correlation with electrical conductivity (EC) and a negative correlation with pH because more acidic conditions enhance dissolution of minerals. However, the highest concentrations of REE occurred in samples with intermediate levels of metal pollution and EC values. The highest correlations of middle REE (MREE) and heavy REE (HREE) occurred with elements related to hydrothermal mineralisation of Mn and Ni, associated with sulphide deposits. The normalised patterns of the AMD sources showed an enrichment of MREE over light REE (LREE) and HREE in all samples. The use of REE patterns as geochemical tracers confirmed the conservative behaviour of REE in the fluvial network, that is, they are not affected by the precipitation of mineral phases. The quantification of REE released from AMD sources to water bodies reveals that, although the highest concentrations occur during the DP, the main load of REE occurs during the WP, due to the highest discharges, with 6.62 kg/day of LREE, 1.12 kg/day of MREE, and 0.54 kg/day of HREE.