| Tailings generated from a two-year mining operation (1983 --- 1985) at the Mount Pleasant Tungsten Mine, located in southern New Brunswick, were investigated in this study. The deposit is host to a unique minerai assemblage that includes minerais such as wolframite [(Fe,Mn)WO4], arsenopyrite [FeAsS], Itillingite [FeAs2], sphalerite [ZnS], molybdenite [MoS 2], bismuthinite [Bi2S3], fluorite [CaF2 ], quartz [SiO2], topaz [Al2SiO4(F,OH) 2] and micas. The primary objective of this research was to understand the mineral/water interactions that contribute to the geochemical evolution of the tailings. In order to satisfy this objective, aqueous geochemical data were collected from the tailings pore water at discrete depth intervals (31 to 45 cm increments) at four locations, and detailed mineralogical investigations were conducted on the near-surface tailings using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron microprobe (EMP), and transmission electron microscopy (TEM).;The tailings are not heavily oxidized, but iocalized areas of oxidation are visible in the near-surface tailings. Secondary coatings that occur in close proximity to oxidized sulfide minerais (arsenopyrite, sphalerite, and tennantite [Cu10(Fe,Zn)2As4S13]) and other primary grains were investigated in detail at the nanoscale. The data indicate that the secondary coatings are composed of a number of discrete phases that are not discernable at the micrometre scale. Some phases that were identified include various Fe oxyhydroxides, Fe---As---O-rich phases, wulfenite [PbMoO4], a Bi---Pb---As---0 phase, an In-O phase, and secondary sulfides. As a result of mineral/water interactions such as adsorption and coprecipitation, many of these minerals were compositionally complex. In many instances, the precipitation of these compiex phases is not predicted by current geochemical models, but it is evident that these phases have an important influence on the geochemical evolution of this system.;In general, the near-surface pore-water is characterized by lower pH values (minimum 4.8) and elevated sulfate (maximum 2378.3 mg/L) and metal concentrations compared to the pore-water at depth. These trends are consistent with the oxidation of sulfide-bearing minerais. A unique characteristic of the near-surface pore-water chemistry is the presence of elevated Al and F concentrations, up to 258.7 and 274.7 mg/L, respectively. The formation of strong Al---F complexes promotes the dissolution of fluorite and aluminosilicate minerais and leads to elevated levels of Al and F in the near-neutral pH pore water. Another distinctive characteristic of the tailings pore-water chemistry is the presence of aqueous W, which is a result of the dissolution of wolframite. In general, aqueous W concentrations are lower in the near-surface pore water compared to concentrations at depth, a trend which is consistent with the adsorption of oxyanions (e.g., W042-) onto mineral surfaces at low pH. |
