Origin of rare earth elements in acid mine drainage traced by strontium and neodymium isotopes

Acid mine drainage (AMD) generated from coal and sulfide mining has attracted much interest due to the high concentrations of rare earth elements (REE) observed. However, the origin of REE in AMD and particularly the mechanism of enrichment in medium REE remain uncertain. The combined study of Sr and Nd isotopes can be used to trace the processes that control the mobility and fractionation of REE in aquatic systems such as AMD. This work reports for the first time worldwide isotopic data of Sr and Nd in AMDs from sulfide mining (Iberian Pyrite Belt, SW Spain) and compare with those in host rocks. Additionally, leaching experiments have been carried out to simulate the water - rock interactions found in these acidic systems. The results obtained strongly suggest that the origin of REE in AMDs from metal mining may be related to the preferential dissolution of shales and felsic rocks. The suitability of Sr and Nd isotopic signatures to trace the REE source in AMD-affected systems has been proven, it provides a new environmental and geochemical prospecting tool which could be useful in different geological water - rock interaction systems.