THE MOUNT PLEASANT TUNGSTEN MINE: USING MICRO-ANALYTICAL TECHNIQUES TO UNDERSTAND MINERAL-WATER INTERACTIONS
Similar to many other sulfide-bearing tailings, the pore-water geochemistry in the near-surface reflects the influence of sulfide-mineral oxidation. In this zone, the pH is relatively low (5.3) and the concentrations of SO4, As and metals are elevated. However, in contrast with many other tailings, the unusual mineralogy at Mount Pleasant leads to some unique geochemical characteristics. For instance, the dissolution of fluorite (CaF2) releases F into the pore water which strongly complexes with Al, enhancing the dissolution of alumino-silicate minerals, and leading to unusually high Al concentrations (up to 152 mg/L) in near-neutral pH pore water. Geochemical modeling suggests that the pore water is near saturation with respect to Al (hydr)oxides but discrete secondary Al phases have not been identified. The tailings display little visual evidence of oxidation with the exception of small amounts of Fe (hydr)oxides. Micro-scale analyses by SEM and EPMA suggest that variable amounts of As, Al, Si, Ca, Cu, Mn, Zn, S, Pb, and Bi are associated with these Fe (hydr)oxides. Goethite (α-FeOOH) and ferrihydrite (~Fe5HO8·4H2O) were identified in these coatings by electron diffraction in the TEM. Nano-scale analyses of discrete grains in the TEM indicate that goethite and ferrihydrite are not pure phases, but include substantial amounts of Al, As and Si (goethite), and Si, Pb, Bi and As (ferrihydrite).
It is commonly believed that at mine-impacted sites aqueous metal concentrations are limited by the precipitation of discrete metal hydroxides. However, in this study detailed mineralogical analyses indicate that substituted phases occur in the tailings and the precipitation of a small number of mineral phases may control the pore-water concentrations of a variety of elements. In this case, it appears that precipitation of Fe (hydr)oxides may limit pore-water concentrations of Al, Si, As, Pb and Bi, in addition to Fe(III). This knowledge suggests that a review of conceptual and numerical models for the prediction of geochemical processes in these systems may be warranted.