Neodymium isotopes track sources of rare earth elements in acidic mine waters

Abstract

Acid mine drainage (AMD) has been proposed as a potential source of strategic rare earth elements (REEs) due to its high concentrations (>1000 µg/L) of dissolved REEs, as well as the middle rare earth element (MREE) enriched pattern exhibited by both coal mine- and metal mine-derived AMD, in which some of the most energy-critical REEs are significantly concentrated. However, the source of REEs and the origin of the MREE enrichment observed in AMD discharges is poorly understood; suggestions include dissolution of MREE-enriched surface coatings, fractionation by colloids, solid-liquid exchange reactions, incongruent dissolution of pyrite, and selective leaching of MREE-enriched phases. Filtration experiments on acidic to circumneutral AMD (unfiltered to <0.002 µm) indicate that REEs are in a dissolved state. Thus, colloids are not a significant carrier of REEs in these samples and are not responsible for the observed MREE-enriched patterns. The similarity of MREE-enriched patterns exhibited by AMD to those of laboratory sulfuric acid (simulated AMD) leachates of corresponding overburden samples suggest that these lithologies are a major contributor of REEs to AMD. Isotopes of strontium (87Sr/86Sr) and neodymium (143Nd/144Nd) in AMD discharges are distinct from whole-rock values of associated lithologies, but fall within the range of values of the leachates. Whole rocks, leachates, and AMD define a trend on a Sm-Nd isochron diagram consistent with the ∼310 Ma depositional age of the coal. These data suggest that MREE enrichment of AMD results from preferential leaching of a readily dissolvable, MREE-enriched mineral phase, most likely apatite or a related phosphate phase, that formed during or shortly after deposition of the coal and associated units.