Mineralogical signs of scheelite weathering in reducing alkaline mine tailings, and the subsequent release of dissolved tungsten (W) as a major element to the groundwater: Implications for mine waste management.

Tungsten (W) is an emerging contaminant of concern whose high mobility in the environment is of scientific debate. It is also a critical raw material whose mining is expected to increase. Mine waste management aims to create an anoxic environment of neutral pH to limit sulfide oxidation and acid mine drainage. This study evaluates the stability of W-minerals under such geochemical conditions to develop recommendations for W mine waste management. Intact cores of legacy mine tailings from Morkulltjärnen, Sweden, were collected and analyzed using whole rock geochemistry and XRD. Monolayers with minutes amounts of W were generated by hydroseparation and studied using SEM-EDS, automated mineralogy, and microprobe analysis. Scheelite concentrates from the decommissioned processing plant were analyzed with XRF and 7 step sequential extractions. Groundwater was collected from eight wells in the Morkulltjärnen tailings in 2023 and 2024 and analyzed for 71 elements, anions and chemophysical parameters. This is one of the first field studies presenting mineralogical signs of scheelite weathering with tabular morphology, rod-shaped and porous structure, and loss of W from the crystal lattice, leading to very high concentrations (up to 23 mg/L) of dissolved W in anoxic and alkaline groundwater. This shows that standard mine waste management practices are unsuitable for scheelite, and action is needed to limit W mobilization into the surrounding environment from scheelite-rich tailings. Adsorption onto Fe-(hydr)oxides may be effective for controlling W mobilization from legacy mine waste, but in new mines, sulfides and tungstates should be separated in the processing plant and stored under anoxic and oxic conditions, respectively.