Evidence of rare earth elements origin in acid mine drainage from the Iberian Pyrite Belt (SW Spain)

Acid mine drainage (AMD) is a worldwide pollution problem of watersheds. In addition to toxic metal(oid)s and acidity, many elements of economic interest are released into the environment, which make AMD a potential strategic secondary source of these elements such as rare earth elements (REE). Despite the importance of these metals, their origin in AMD is still uncertain. Recent hypotheses suggest preferential leaching of REE-enriched minerals as a possible source. Leaching tests with H2SO4 have been developed to simulate the interaction under AMD formation conditions with sulfide bodies and host rocks from two representative mining areas in the Iberian Pyrite Belt: the Perrrunal and Poderosa mines (SW of Spain). The REE patterns and Ce and Eu anomalies of the rock leachates have confirmed the geochemical relationship between the AMD and certain country rocks (felsic and mafic volcanics, and shales). A detailed chemical and mineralogical study has confirmed the existence of a diversity of minerals with high concentrations of REE. Thus, the minerals with the highest REE contents are also those with the fast dissolution kinetics under acid conditions: REE phosphates (monazite and xenotime type) and carbonates (parisite type). Finally, petrographic evidence of the selective leaching of these minerals clearly supports these minerals as the main source of REE in the AMD.