Nd isotopes track rare earth element sources in acid mine drainage, Appalachian Basin, USA

Acid mine drainage (AMD) has been proposed as a potential source of strategic rare earth elements (REEs). To simulate AMD-rock interactions and determine the source of energy-critical REEs, we conducted experiments using overburden, coal (Pittsburgh Coal), and underclay samples from an AMD generating mine in the Appalachian Basin. Sulfuric acid (0.05N) leached < 10% of whole rock total REEs from all samples except the underclay, which released 35%. Unlike the relatively flat North American Shale Composite (NASC) normalized whole rock REE patterns, sulfuric acid leachates of these units show middle rare earth element (MREE) enrichment patterns similar to what is observed in AMD. Measured ƐNd(0) values for the whole rocks range from -11.2 to -12.0. Most leachates for those rock units have more radiogenic ƐNd(0) values between -5.8 and -9.3, similar to that of AMD from the Pittsburgh Coal. When corrected to a late Carboniferous depositional age (300Ma), the ԐNd(T) range for all samples (whole rocks and leachates) narrows to -9.9 to -6.2 for all samples. These data, together with geochemical and petrographic analysis, suggest that the MREE enrichment of AMD results from preferential leaching of a readily dissolvable, MREE-rich, high Sm/Nd mineral phase such as carbonate or phosphate accessory minerals