High contents of rare earth elements (REES) in stream waters of a Cu-Pb-Zn mining area

Stream waters draining an old mining area present very high rare earth element (REE) contents, reaching 928 mug/l as the maximum total value (SigmaREE). The middle rare earth elements (MREEs) are usually enriched with respect to both the light (LREEs) and heavy (HREEs) elements of this group, producing a characteristic roof-shaped pattern of the shale Post-Archean Australian Shales-normalized concentrations. At the Fenice Capanne Mine (FCM), the most important base metal mine of the study area, the REE source coincides with the mine tailings, mostly the oldest ones composed of iron-rich materials. The geochemical history of the REEs released into Noni stream from wastes in the FCM area is strictly determined by the pH, which controls the REE speciation and in-stream processes. The formation of Al-rich and mainly Fe-rich floes effectively scavenges the REEs, which are readily and drastically removed from the solution when the pH approaches neutrality. Leaching experiments performed on floes and waste materials demonstrate that Fe-oxides/oxyhydroxides play a key role in the release of lanthanide elements into stream waters, The origin of the roof-shaped REE distribution pattern as well as the peculiar geochemical behavior of some lanthanide elements in the aqueous system are discussed. (C) 2002 Elsevier Science Ltd. All rights reserved.