Origin of Rare Earth Elements in Acid Mine Drainage: Mineralogical Insights from the Iberian Pyrite Belt (SW Spain)

The Iberian Pyrite Belt (IPB), located in the SW of the Iberian Peninsula, is one of the largest polymetallic massive sulfide provinces in the world. Historical mining activity in the area has left a significant legacy of mine residues, including 90 abandoned mines and more than 1.000 ha of waste rock dumps and tailings. Therefore, large volumes of Acid Mine Drainage (AMD) are produced due to oxidation of pyrite-rich residues exposed to atmospheric conditions, which end up in the Tinto and Odiel rivers and subsequently into the ‘‘Ría de Huelva’’ estuary (SW Spain), polluting these water environments. During the passive treatment of AMD with Dispersed Alkaline Substrate (DAS) technology, sequential precipitation of Fe³⁺ as schwertmannite and Al as basaluminite occurs, and Rare Earth Elements (REE) are preferentially concentrated within the Al-rich precipitate layer. This could be an interesting alternative source of REE, given that AMDs of the IPB are enriched in middle REE (MREE) and heavy REE (HREE). A rough estimation of the REE potential of these AMD sources, based on 40 DAS plants operating in the Odiel basin with variable content of REE, will be the production of 11 kton/year of basaluminite containing 21 ton/year of REE₂O₃ with grade of 0.19%. However, the origin of REE in AMD is not well understood. This work examines the concentration and pattern of REE in AMD, ore bodies, and country rocks in two representative mining areas of the IPB: Perrunal and Poderosa mines. Leaching experiments were conducted on sulfide ores and host rocks under simulated AMD formation conditions, and the results were compared with the AMD formed in these two mining areas. The preliminary results indicate that the host rocks (felsic and mafic volcanics and shales) are the primary source of REE in the AMD. A mineralogical and chemical study of the country rocks in Perrunal and Poderosa mines reveals that secondary phosphates and carbonates contain the highest REE content, which are also soluble under acidic conditions. REE-rich monazite-type is systematically present in felsic volcanics and shales in both mining areas. HREE-rich xenotime-type is also present in most felsic volcanics, while REE-rich carbonates (mainly parasite-type) are present in carbonate-rich shales from the Perrunal mine. Other minor REE-bearing minerals, such as apatite and zircon, have been identified in the host rocks. However, due to their lower abundance and solubility under acidic conditions, they are not considered an important source of REE in the AMDs of the studied mining areas. Finally, the petrographic evidence shows a selective leaching of these REE-bearing phosphates and carbonates which highly supports their involvement as the main source of REE in AMD.

Acknowledgements: This work is part of the I+D+i TRAMPA project (PID2020-119196RB-C21), funded by MCIN/AEI/10.13039/501100011033/.