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

Gossan, disseminated orebody waste, other mining wastes, minesoils and acid mine drainage (AMD) in the abandoned São Domingos mine area (Iberian Pyrite Belt, IPB) have been analyzed for rare earth elements (REE). The main aim is to understand REE mobility during sulphide weathering so that the lanthanide series can be used both as a record of the water–rock interaction and as a tool for identifying impacts of AMD on natural ecosystems. North-American Shale Composite (NASC)-normalized REE patterns corresponding to the disseminated orebody waste are relatively flat (EMREE = − 0.01 ± 0.12). However, NASC-normalized REE distributions in AMD from sulphide oxidation tend to be enriched in middle-REE (MREE) compared to light-REE (LREE) and heavy-REE (HREE). As a consequence, gossan resulting from supergene alteration of massive sulphide presents an evident NASC-normalized MREE-depleted signature. Thus, the overall water–sulphide interaction defines complementary convex (EMREE = + 0.72 ± 0.25) and concave (EMREE = − 0.31 ± 0.12) NASC-normalized patterns in draining waters and oxidation products, respectively. Solutions extracted from minesoils have also NASC-normalized patterns with MREE-enriched signature (EMREE = + 0.62 ± 0.22) similar to AMD. REE 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.