Mineral/water interactions in tailings from a tungsten mine, Mount Pleasant, New Brunswick

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Abstract

The pore-water geochemistry and mineralogy of tailings derived from a granitic tungsten deposit were characterized by collecting pore-water samples at discrete depth intervals throughout the tailings for the analysis of major and minor element concentrations. Mineralogical samples from the oxidation zone were analyzed by X-ray diffraction, scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDS), electron microprobe (EMP) combined with wavelength dispersive X-ray spectroscopy (WDS), and transmission electron microscopy (TEM). The oxidation of sulfide minerals in the near-surface tailings leads to a decrease in pore-water pH and elevated SO4, As, and metal concentrations. The unusual mineralogy of this deposit, compared with that of commonly studied base-metal and gold deposits, results in several unique geochemical characteristics. The dissolution of fluorite releases F into the pore water; the F forms strong complexes with Al and enhances the dissolution of aluminosilicate minerals within the oxidation zone. As a result, high Al concentrations (up to 151.7 mg/L) are detected in the near-neutral pore water in the oxidation zone. The combined dissolution of aluminosilicates and carbonate minerals maintains the pH near 10 in the pore water at depth. Elevated concentrations of W (up to 7.1 mg/L) are detected in the pore water throughout the tailings, likely as a result of the dissolution of wolframite. Consistent with geochemical model calculations, results from SEM/EDS, EMP/WDS and TEM/EDS analyses indicate that secondary minerals, which occur as orange-brown coatings on grains of primary-minerals, are Fe oxyhydroxides. Examples of these secondary minerals display a fibrous habit at high resolution in the TEM. One of these minerals, which contains substantial amounts of Al, As, and Si as impurities, was identified by selected-area electron diffraction (SAED) analyses to be goethite. Another mineral contains relatively high amounts of Si, Pb, Bi, and As, and SAED analyses suggest that the mineral is two-line ferrihydrite.

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.

References (58)

  • I. Lebrón et al.

    Calcite nucleation and precipitation kinetics as affected by dissolved organic matter at 25°C and pH > 7.5

    Geochim. Cosmochim. Acta

    (1996)
  • I. Lebrón et al.

    Kinetics and mechanisms of precipitation of calcite as affected by PCO2 and organic ligands at 25°C

    Geochim. Cosmochim. Acta

    (1998)
  • G. Lee et al.

    Removal of trace metals by coprecipitation with Fe, Al and Mn from natural waters contaminated with acid mine drainage in the Ducktown Mining District, Tennessee

    Appl. Geochem.

    (2002)
  • R.G. McGregor et al.

    The solid-phase controls on the mobility of heavy metals at the Copper Cliff tailings area, Sudbury, Ontario, Canada

    J. Contam. Hydrol.

    (1998)
  • G.A. Waychunas et al.

    Surface chemistry of ferrihydrite: Part 1. EXAFS studies of the geometry of coprecipitated and adsorbed arsenate

    Geochim. Cosmochim. Acta

    (1993)
  • J.A. Wilkie et al.

    Adsorption of arsenic onto hydrous ferric oxideEffects of adsorbate/adsorbent ratios and co-occurring solutes

    Colloids Surf. APhysicochem. Eng. Aspects

    (1996)
  • T.A. Al et al.

    The geochemistry of mine-waste pore water affected by the combined disposal of natrojarosite and base-metal sulphide tailings at Kidd Creek, Timmins, Ontario

    Can. Geotech. J.

    (1994)
  • C. Amrhein et al.

    Calcite supersaturation in soil suspensions

    Soil Sci.

    (1993)
  • J.R. Atkinson et al.

    Geology of Mount Pleasant tungsten, New Brunswick

    Can. Mining J.

    (1981)
  • J.W. Ball et al.

    WATEQ4F—User’s Manual With Revised Thermodynamic Data Base and Test Cases for Calculating Speciation of Major, Trace and Redox Elements in Natural Waters

    (1991)
  • J.M. Bigham

    Mineralogy of ochre deposits formed by sulfide oxidation

  • J.M. Bigham et al.

    Iron and aluminum hydroxysulfates from acid sulfate waters

  • P.T. Black

    Tin, tungsten, molybdenum mineralization, Mount Pleasant area, New Brunswick

    Can. Mining J.

    (1961)
  • H.C. Bruun Hansen et al.

    Monosilicate adsorption by ferrihydrite and goethite at pH 3–6

    Soil Sci.

    (1994)
  • A.S. Campbell et al.

    Si incorporation into hematite by heating Si-ferrihydrite

    Langmuir

    (2002)
  • L. Carlson

    Aluminum substitution in goethite in lake ore

    Bull. Geol. Soc. Finland

    (1995)
  • C.C. Davis et al.

    Modeling silica sorption to iron hydroxide

    Environ. Sci. Technol.

    (2002)
  • N.M. Dubrovsky et al.

    Geochemical evolution of inactive pyritic tailings in the Elliot Lake uranium district

    Can. Geotech. J.

    (1985)
  • D.A. Dzombak et al.

    Surface Complexation ModelingHydrous Ferric Oxide

    (1990)
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