Mineralogy and geochemistry of trace metals and REE in volcanic massive sulfide host rocks, stream sediments, stream waters and acid mine drainage from the Lousal mine area (Iberian Pyrite Belt, Portugal)

Abstract

Acid mine drainage represents a major source of water pollution in the Lousal area. The concentrations of trace metals and the rare earth elements (REE) in the host rocks, stream sediment, surface waters and acid mine drainage (AMD) associated with abandoned mine adits and tailings impoundments were determined, in order to fingerprint their sources and to understand their mobility and water–rock interaction. The results show that the Fe–SO4-rich acid waters vary substantially in composition both spatially and seasonally. These waters include very low pH (mostly in the range 1.9–3.0), extreme SO4 concentrations (4635–20,070 mg L−1 SO42-), high metal contents (Fe, Al, Cu, Zn and Mn) and very high REE contents. The trace metal concentrations decrease downstream from the discharge points either due to precipitation of neoformed phases or to dilution. The North-American shale composite (NASC)-normalized patterns corresponding to sediment from one stream (Corona stream) show a flat tendency or are slightly enriched in light-REE (LREE). The NASC-normalized patterns corresponding to acidic mine waters show enrichment in the middle REE (MREE) with respect to the LREE and heavy REE (HREE). Moreover, the REE concentrations in acidic mine waters are 2 or 3 orders of magnitude higher than those of the surface waters. Changes of REE concentrations and variation of Eu anomaly show two spatially distinct patterns: (a) pond and spring waters with higher REE concentrations (ranging from 375 to 2870 μg L−1), that records conspicuous negative Eu anomaly, and (b) seeps from tailings impoundments corresponding to lower REE concentrations than the first pattern (ranging from 350 to 1139 μg L−1) with typically negative Eu anomaly. The stream water samples collected from the impacted stream during the spring show a low pH (2.8–3.1) and contain high concentrations of Fe and trace elements (up to 61 mg L−1). Also, temporal variations of the REE concentrations were observed in the Corona waters. The results obtained show that the REE concentrations increase during the winter–spring transition. Stream waters draining the Lousal mine area show high REE concentrations, reaching a maximum value of about 2846 μg L−1 (spring). The MREE concentrations are usually enriched with respect to both the LREE and HREE. A decrease in REE concentrations and a pH increase from 3 to 6 was observed downstream of the confluence of a tributary stream. The geochemistry of the waters strongly influenced the mineralogy and geochemistry of efflorescent sulfates precipitated on the banks of the streams polluted by AMD. The mineralogy is dominated by hexahydrite, rozenite, szomolnokite, alunite, gypsum, halotrychite, coquimbite, copiapite and schwertmannite. The REE concentrations determined in the efflorescent sulfates suggest a selective partitioning of the HREE onto Mg–Al-oxyhydroxides.