Rare earth element geochemistry of sulphide weathering in the São Domingos mine area (Iberian Pyrite Belt): A proxy for fluid-rock interaction and ancient mining pollution

The EMREE parameter is proposed to measure the curvature in the MREE segment, and its size is quantified as the normalized maximum vertical difference between the polynomial curve fitting of the MREE region and its theoretical Y-axis position in the absence of enrichment or depletion (EMREE > 0 for MREE-enriched signatures; < 0 for MREE-depleted signatures; and = 0 for horizontal patterns). The São Domingos stream, although it has been highly affected by AMD, flows into the Guadiana river that has an estuarine system where pollution is considerably attenuated due to the mixing, according to the metal geoaccumulation indexes currently used in the literature. However, sediments of this estuary were also analyzed and reflect MREE-enriched signatures (EMREE = + 0.25 ± 0.03), which demonstrate that this apparently non-polluted estuarine system is being certainly affected by historical mining activities from the IPB. The EMREE index is more sensitive to recognize curved MREE-signatures than other normalized ratios such as (La/Gd)NASC, validating the use of REE patterns as a proxy for environmental pollution by AMD.