Mineral/water interactions in tailings from a tungsten mine, Mount Pleasant, New Brunswick
Introduction
The Mount Pleasant tungsten mine is located ∼60 km south of Fredericton, New Brunswick, Canada (Fig. 1). During operation, from 1983 to 1985, the mine produced 1 million tonnes of ore, and ∼2000 tonnes of 70% WO3 were recovered (McCutcheon et al., 2001).
Numerous papers have discussed the geology and mineralogy of the Mount Pleasant deposits (Black, 1961; Petruk, 1964; Parrish, 1977; Atkinson et al., 1981; McCutcheon, 1990; Sinclair and Kooiman, 1990). In general, the W-Mo deposits at Mount Pleasant are associated with intrusive rocks, particularly granites (McCutcheon et al., 2001). The two areas that contain Mo and W mineralization are commonly referred to as the Fire Tower Zone and North Zone (Atkinson et al., 1981). The Fire Tower Zone, which was developed during mining operations, contains three Mo-W ore bodies (Atkinson et al., 1981). The North Zone has four Mo-W zones, a Bi zone, a deep Sn zone, and six near-surface Sn/base-metal bodies (Atkinson et al., 1981).
The principal gangue minerals associated with the Fire Tower Zone include arsenopyrite, fluorite, löllingite, quartz, and topaz (McCutcheon et al., 2001; see Table 1 for a list of chemical formulas for minerals mentioned in the text). Although Parrish (1977) catalogued >80 minerals that have been identified at Mount Pleasant and suggested that it is difficult to give a mineralogical breakdown of a typical sample because of the complexity of the deposit, he provided the following as a mineralogical description of the Fire Tower Zone: 70% quartz, 10% topaz, 5% fluorite, 4% micas/clays, 4% chlorite, 4% K-feldspars, and 3% opaque minerals. The opaque minerals include arsenopyrite, sphalerite, wolframite, molybdenite, bismuth, galena, and tin minerals.
Many hydrogeochemical studies of tailings systems have focused on tailings derived from copper, nickel, lead, zinc, and gold deposits (e.g., Dubrovsky et al., 1985; Blowes et al., 1991; Al et al., 1994; Jambor and Blowes, 1994; McGregor et al., 1998a; McGregor et al., 1998b; Johnson et al., 2000). It is well understood that the oxidation of sulfide-bearing minerals in tailings piles leads to significant changes in the pore-water geochemistry and the tailings mineralogy over time. Although these changes are highly site dependent, extensive research has been conducted on these types of tailings piles, which has led to a good understanding of the geochemical processes that occur. In contrast, the geochemical conditions in tailings derived from granitic mineral deposits, such as Mount Pleasant, are not so well known.
As noted by Parrish (1977), the Mount Pleasant deposit is unique because of its large variety of minerals. From a geochemical perspective, the mineral assemblages present at Mount Pleasant provide an opportunity to explore the hydrogeochemical changes that occur in the tailings as a result of the weathering of these minerals. For instance, it is expected that sulfide oxidation is an important geochemical process at Mount Pleasant, similar to that which occurs at gold and base-metal sites. However, at Mount Pleasant, highly reactive Fe-sulfides, such as pyrite and pyrrhotite, are not abundant and it was anticipated that this would influence the geochemical evolution of the tailings compared with other sites. The presence of unusual minerals, such as fluorite, in the Mount Pleasant tailings was also expected to have a significant effect on the geochemical evolution. For example, it was hypothesized that the dissolution of fluorite in the tailings would lead to increased aluminosilicate weathering due to the formation of strong Al-F complexes. In addition, the geochemical behaviour of other minerals, such as wolframite and löllingite, in tailings is unknown.
As the primary minerals react within the tailings it is expected that secondary minerals will also form. Bigham (1994), Jambor 1994, Jambor 2003, (Bigham et al. 1996), and Bigham and Nordstrom (2000) have demonstrated the importance of identifying these secondary minerals to fully understand the mineral/water interactions that are occurring in mine-waste environments. Many previous studies of secondary minerals from mine tailings have used traditional techniques such as optical microscopy, X-ray diffraction (XRD), and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDS) to identify these minerals (e.g., McGregor et al., 1998a; McGregor et al., 1998b; Johnson et al., 2000). Although the results from these analyses are indispensable for understanding the type and composition of secondary minerals, bulk-sample XRD is not sensitive to minerals at low abundance. Furthermore, the analysis of samples using techniques such as SEM/EDS may result in a bulk analysis of mixed secondary minerals because of the low spatial resolution of the instrument with respect to the scale of the discrete mineral grains. The use of analytical transmission electron microscopy (TEM) in combination with optical microscopy, SEM, and XRD offers advantages because samples can be analyzed for chemical composition and morphological characteristics at the nano-scale. This advantage is particularly important for secondary minerals, which commonly occur as nano-scale coatings. Several recent studies have utilized TEM to investigate mineral precipitates that form in mine-related environments (Bigham et al., 1996; Hochella et al., 1999; Al et al., 2000), and similar techniques are used in this study.
The main objective of this study was to understand better the mineral/water interactions that occur in the tungsten-mine tailings system by collecting both aqueous geochemical and mineralogical data. More specifically, the objectives were to characterize the geochemical conditions in the tailings pore water, identify aspects of the geochemistry that relate to the unique mineralogy of this deposit, and investigate the secondary mineralogy in the oxidation zone of the tailings.
Section snippets
Hydrogeology
Hydraulic data were collected from the tailings using piezometers (Fig. 1). The piezometers were made from polyethylene tubing (1.6 cm O.D.) and a screen, 12 cm long, was cut directly into each tube. The screens were covered with nylon material that was held in place with stainless steel wire, and the bottom of each tube was sealed with silicone. After installation, the piezometers were purged several times to eliminate fines. The piezometers were used in single, shallow (1 m) installations to
Groundwater Flow
The tailings from the milling process were deposited as a slurry at the shoreline of an artificial pond on the mine site. The exposed tailings were flooded after mining ceased by raising the water level in the pond (Fig. 1). In 1997, the dam that surrounded the pond was breached and the water level was lowered by several metres, thus exposing the raised portion of the tailings to atmospheric oxygen.
Currently, the tailings remain partly saturated and groundwater flow is from the northeast to
Conclusions
Trends in pore-water geochemistry in the near-surface tailings from the Mount Pleasant tungsten mine reflect the effects of sulfide-mineral oxidation. The pH is relatively low and the concentrations of SO4, As and metals are relatively high in the surficial oxidation zone. The presence of fluorite in the tailings seems to have had an important influence on the pore-water geochemistry. The dissolution of fluorite leads to the release of F into the pore water, resulting in the formation of strong
Acknowledgments
Funding for this research was provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada. The authors acknowledge the in-kind contribution from Fibics Inc. (Ottawa, Canada) for the FIB preparation of the TEM samples used in this study. The authors would also like to thank Dr. Eggleston, Dr. Jambor and an anonymous reviewer for their valuable comments on an earlier version of the manuscript.
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Associate editor: C. M. Eggleston