Résumé - Abstract

 What do we know (and not know) about the mobility of critical metals in mine waste?

Heather E. Jamieson¹, Anežka Borčinová Radková¹, Brent Kazamel¹, Sean Des Roches¹, Matthew Leybourne¹,  Michael B. Parsons² and Hendrik Falck³

¹ Queen’s University, Kingston, ON Canada

²Natural Resources Canada, Dartmouth, NS, Canada

Northwest Territories Geological Survey³

Abstract

Critical metals are elements that are vital for modern technology, yet there is a real or perceived risk of disruption in their supply. Growing global demand for products using these metals, such as electric cars, cell phones and wind turbines, is leading to an increase in the mining of these resources. Critical metals are commonly mined as the main target commodity (e.g., rare earth elements (REEs), niobium and tungsten), but others are extracted as by-products of the mining of major mineral commodities (cobalt, indium, germanium). Very little is known about the environmental mobility of many critical metals, particularly how they are leached from primary host minerals, mobilized under different conditions of pH and redox, and potentially sequestered in secondary phases. Weathered mine wastes provide an excellent opportunity to study mineral-water interactions involving critical metals.

 This presentation will focus on the environmental mobility of tungsten, antimony and rare earth elements (REEs). In mine waste, the mobility of antimony is highly dependent on which secondary minerals are stable. Tripuhyite (FeSbO₄) is relatively insoluble, whereas brandholzite ((Mg[Sb(OH)₆]₂•6(H₂O) may be a temporary and ephemeral secondary host. Our research has shown that scheelite (CaWO₄), the major ore mineral of tungsten, is stable after more than 50 years of exposure to acid drainage. Low concentrations of tungsten are mobile in pH-neutral surface water as dissolved species and adsorbed to colloidal iron oxyhydroxides. Most primary minerals hosting REEs are relatively insoluble, but REEs can still be transported in surface and ground-waters when adsorbed to iron oxyhydroxide and organic colloids. The presence of even trace amounts of sulfide minerals in host rocks is important in creating micro-environments that mobilize REEs .

 Our emerging understanding of the geochemical behaviour of critical metals in the near-surface environment is important for guiding waste management decisions at existing and new mines. However, there are few guidelines for the maximum concentration of these elements in water, soil or sediment – how much is too much? To resolve this, we need toxicological data that are relevant to the mine waste environment to combine with geochemical data and modeling results to help inform the environmentally responsible production of critical metals in Canada.

1.    Introduction

Critical minerals are those defined as essential for modern technology, but for which the mineral supply is also vulnerable to interruption (Graedel et al. 2014). Canada’s first-ever critical minerals list was published in March 2021 (Table 1). Although lists vary somewhat between jurisdictions, there are many commonalities, and those elements considered in this paper, namely antimony, tungsten and the REEs, are included in the EU and US lists as well as Canada’s (EU 2020; USGS 2018). These elements were chosen because Canada has been an important producer (tungsten and antimony) or has the potential to be a producer (REEs). Here we report recent field-based studies that provide insights on how these elements are likely to be mobilised in the mine waste environment. These three commodities are typically not by-products of other metal production and thus the environmental impact of mining antimony, tungsten and REEs can be evaluated independently from that of other commodities.

Table 1. Canada’s critical minerals list (Natural Resources Canada 2021)

Environmental mobility can be defined as how easily an element moves to or from solid to aqueous form. This mobility can involve dissolution/precipitation, adsorption, co-precipitation and complexation reactions that are influenced by changes in pH, redox and temperature in the mine waste environment. The solubility of the primary host mineral under prevailing conditions in waste storage facilities is the primary control on the release of the element of concern. For mine waste management, elements of concern hosted in a solid form are typically much easier to manage because of reduced mobility.

Increased amounts of mine tailings and waste rock containing antimony, tungsten and REEs can be expected as mining of these commodities increases to meet anticipated demand. Understanding whether these elements will remain in their primary ore minerals, dissolve, be sequestered in secondary precipitates or adsorbed on other phases is important to predict the concentrations that may be released to drainage water. This information is useful for designing waste management strategies and can be combined with toxicity data to provide a framework for developing water quality guidelines for environmental monitoring of critical metal mines.

2.    ANTIMONY

Antimony is used mainly for lead alloys and in fire retardants. Although current production is primarily from China, Canada has one of the world’s largest deposits at the Beaver Brook mine in Newfoundland and Labrador (Schwarz-Shampera 2014). There have been multiple reviews on the behaviour of antimony in the environment, including a recent one focused on antimony in mine waste (Borčinová Radková et al. 2021 and references therein). Antimony has been identified as a carcinogen and is considered a priority pollutant by the EU and the USEPA (Obiakor et al., 2017). Antimony is regulated in drinking water with a maximum allowable concentration (0.006 mg/L in Canada and the USA) lower than that for arsenic (0.01 mg/L). Soil quality guidelines in Canada are 20 mg/kg for residential areas, and 40 mg/kg for industrial areas (Borčinová Radková et al. 2021).

Antimony is not only present in waste from mines where it is the primary commodity but is also found in many precious and base metal deposits. However, antimony is rarely regulated in mine waste effluents. This may be because antimony commonly accompanies arsenic, although typically at lower concentrations, and is generally thought to behave in a similar way to arsenic. Measures designed to manage arsenic in mine waste may or may not be effective in managing antimony. The environmental mobility of antimony is highly dependent on the stability of secondary minerals in the mine waste environment.

Stibnite (Sb₂S₃) is the most common ore mineral, although antimony may also be present in many sulfosalt and oxide minerals (Borčinová Radková et al. 2021). Examples of primary and secondary antimony minerals in mine waste are shown in Figure 1.

Figure 1. Back-scattered electron images of antimony minerals in mine waste (Borčinová Radková et al. 2021)

(A) Tripuhyite grain in tailings from Pezinok Sb-Au deposit, Slovakia.

(B) Sb-Ca mineral with pyrochlore structure from Au-Sb deposit, Medzibrod, Slovakia.

(C) Porous goethite with 0.9% Sb from Beaver Brook Sb tailings, Newfoundland and Labrador

(D) Sénarmontite from soil contaminated by Sb mining from Dubrava Sb deposit, Slovakia.

(E) Tabular crystals of brandholzite from Beaver Brook Sb tailings, Newfoundland.

(F) Fe oxides with 9.2% Sb forming rim on arsenopyrite (Azp) from Beaver Brook Sb tailings, Newfoundland and Labrador

(G) Roméite and tripuhyite forming veinlets as tetrahedrite (Ttd) oxidation products from the Cu-Ag historical mine site Špania Dolina-Piesky, Slovakia

(H) Tripuhyite-like Sb-Fe oxide with 21.3% Sb with needle-shaped micron size crystals from Beaver Brook Sb tailings, Newfoundland.

Majzlan et al. (2016) pointed out that statements in the literature regarding the environmental mobility of antimony are contradictory. This is likely due to the differences between field conditions where insoluble antimony secondary minerals, particularly tripuhyite (FeSbO₄) may form, and those where antimony is hosted in more soluble secondary phases. Figure 2 shows how the presence of iron in an aqueous environment with elevated antimony can stabilize tripuhyite. 

Figure 2. Eh-pH diagrams for Sb. (a) The Sb-O-H system. SSb = 10^-10, 25 °C. Adapted from Takeno (2005); (b) the Sb-S-H₂O system. SSb = 10^-8, SS = 10^-3, 25°C. Adapted from Filella et al. (2002); (c) the Sb-Fe-H₂O system. SSb = 10^-6, SFe = 10^-6, 25°C. Areas of dissolved Sb species (mol/L) separated by dashed lines, and Sb species shown in italics. Adapted from Leverett et al. (2012).

Because it is stable over a wide range of pH and Eh, can accommodate a range of Fe/Sb and is relatively insoluble, tripuhyite is considered an effective solid phase to sequester antimony in mine wastes. Adding iron to antimony-bearing mine water might be used to facilitate precipitation of tripuyhite as a remediation strategy. Our recent research has suggested that the kinetics of tripuhyite precipitation should be investigated to test this approach. Antimony-bearing waste rock from the Špania Dolina-Piesky Cu-Ag mine site in Slovakia, which had been exposed to the atmosphere for centuries, contains tripuhyite as well as other secondary antimony minerals (Fig. 1G). Recently exposed tailings from the Beaver Brook mine contain a tripuhyite-like mineral that forms acicular clusters (Fig 1H) that may be a precursor of crystalline tripuhyite (Borčinová Radková et al., 2017).

More soluble hydrous antimony secondary minerals may precipitate in mine waste if conditions are appropriate. Borčinová Radková et al. (2020) showed that evaporation of standing water on the tailings surface at Beaver Brook resulted in the precipitation of brandholzite (Mg[Sb(OH)₆]₂•6(H₂O); Fig 1E). When this phase was present (2015), it accounted for a large proportion of the solid antimony in the near-surface tailings, but under wet, rainy conditions (2013), brandholzite is not present (Fig 3).

Figure 3. Distribution of antimony in Beaver Brook antimony mine tailings, determined by automated mineralogy. Conditions in 2013 were wet, while those in 2015 were dry.  Adapted from Borčinová Radková et al. (2020)

Antimony is commonly present in carbonate-quartz vein hosted gold deposits, commonly called orogenic gold deposits. We have examined arsenic and antimony solids phases in multiple studies of tailings, sediments and soils at or near Giant and Con Mines, Yellowknife, Canada. Antimony is present in primary phases (stibnite, sulfosalts), as a minor element incorporated in roaster-generated waste products (maghemite, arsenolite), and is also associated with iron oxyhydroxide minerals (Fawcett and Jamieson 2011; Bromstad et al. 2017; Schuh et al. 2018; Paudyn 2021). Tripuhyite has not been identified, likely due to the iron-poor environment.

 3.    TUNGSTEN

There are few substitutes for tungsten’s industrial applications. Hard metal alloys, specialty steels and ammunition all take advantage of the metal’s hardness and high melting point (Brown and Pitfield 2014; Kazamel et al. 2021a). Concerns about the toxicity of tungsten emerged in the early 2000’s when clusters of childhood leukemia in the southwestern United States were linked to concentrations of tungsten in groundwater, but research on tungsten toxicity and ecotoxicity is still in its infancy (Kazamel et al. 2021a). There are few guidelines for tungsten in Canada, the United States, or the European Union. A soil quality guideline of 2.4 mg/kg was recently established by the USEPA to protect groundwater. In Russia, the maximum allowable concentration is 0.05 mg/L for drinking water and 0.0008 mg/L for protection of aquatic life (Kazamel et al. 2021a). Canada has former and potential tungsten producers in New Brunswick, Nova Scotia, Yukon, and the Northwest Territories.

We have recently investigated the environmental mobility of tungsten at Cantung mine in the Northwest Territories (Kazamel et al. 2021b, in press). This is a skarn-hosted scheelite (CaWO₄) deposit that operated intermittently between 1962 and 2015. Previous work had shown that although almost all the impounded tailings are classified as PAG (potentially acid-generating) due to the presence of pyrrhotite, there has been relatively little oxidation of the sulfide minerals (Jamieson et al. 2019; Kazamel 2020). However, a smaller quantity of tailings deposited directly on the floodplain during the early years of mining has oxidized and contains acidic (pH = 2.2 to 5.5) pore water. Figure 4 indicates the typical mineral proportions in the impounded tailings (upper photograph and pie chart) compared to the flood plain tailings (lower photo and pie chart). The pyrrhotite and carbonate minerals have been almost entirely consumed during oxidation and replaced by goethite and gypsum.

Figure 4. Typical tailings mineralogy of the impounded tailings (above) and tailings deposited on the floodplain (below). Adapted from Kazamel 2019.

Scheelite is commonly regarded as having a very low solubility and the tungsten concentrations measured in surface water at Cantung are low (Figure 5). Tungsten behaves as an anion in solution (WO₄^2- in most waters). It is expected to adsorb to iron oxyhydroxide mineral surfaces at lower pH values. Figure 5 demonstrates how the tungsten concentration in surface water at Cantung depends on pH and iron concentration. The highest tungsten concentrations are found in neutral to alkaline surface water on tailings ponds. Unfiltered surface waters have higher tungsten and iron concentrations than those filtered to 0.45 µm, suggesting the tungsten is associated with iron oxyhydroxide particles. The surface waters associated with the floodplain tailings have a wide range of pH values, and tungsten is above the detection limit only in the samples with pH > 7.1, and the unfiltered water samples have slightly higher concentrations. The floodplain tailings surface waters with the highest iron concentrations have low pH values and no detectable tungsten. Finally, the river waters sampled at several locations in the mine area have tungsten concentrations below the detection limit. These results suggest that if higher values of tungsten were released to the river, a portion of the load would be adsorbed to suspended iron oxyhydroxide particles.

Figure 5. Tungsten and iron concentrations in surface waters at Cantung Mine. Note log scale for iron. Adapted from Kazamel (2019)

Secondary tungsten minerals in mine waste have rarely been reported. Kazamel et al. (2021b) suggest that meymacite (WO₃·2H₂O) or tungstite (WO₃·H₂O) are the most likely to form due to their simple stoichiometry, and a yellow coating on scheelite (inferred to be tungstite) has been observed in skarn tailings in Yxsjöberg, Sweden (Hällström et al., 2018b). Some pyrrhotite grains in the Cantung tailings have iron-rich rims that were identified as hematite and maghemite using synchrotron microdiffraction, and likely formed during roasting. Tungsten associated with these rims may have been incorporated into iron oxide minerals during ore processing or be associated with very fine-grained inclusions of scheelite.

Given the concern regarding tungsten toxicity, and the paucity of published studies of its behaviour in mine waste, it seems clear that additional research on the environment mobility of tungsten in tailings from different types of tungsten deposits is needed.

4.    RARE EARTH ELEMENTS

Rare earth elements have similar geochemical properties, or the properties change gradually and smoothly over the series (with the notable exceptions of Ce and Eu), and we can generally consider their environmental mobility to be similar. However, their industrial uses are quite distinctive, with specific elements in the series used for magnets, alloys, catalysts, ceramics and other applications (Wall 2014). Very little is known about the toxicity of REEs. Like tungsten, REEs have a reputation of being relatively immobile although they are more mobile at low pH. There are several advanced exploration projects of REEs in Canada, and these include substantial reserves of both carbonatite- and silicate-hosted deposit types.

The former St. Lawrence Columbium mine near Oka, Quebec provided an opportunity to investigate how mineral-water reaction controls the mobility of REEs in carbonatite waste rock exposed to weathering for several decades (Figure 6).

Figure 6. Waste rock at St. Lawrence Columbium mine: A. General view showing dominance of sövite (coarse-grained calcite-rich carbonatite); B. Loose calcite amongst deeply weathered sövite waste rock;  C. Rare example of oxidized sulfide-bearing sövite.

From 1961 to 1977, the mine produced niobium (formerly called columbium) from the Oka carbonatite complex. Des Roches (2020) used automated mineralogy, electron microprobe, and laser-ablation ICP-MS analysis to demonstrate that most of the REEs in the waste rock are hosted in apatite, with lesser amounts in calcite and pyrochlore (Figure 7). Rare REE-enriched sövite contains parasite, synchysite, ancylite and britholite.

Figure 7. Distribution of REE minerals in various waste rock types at the St. Lawrence Columbium Mine, Oka, Quebec. Adapted from Des Roches (2020)

Shake flask tests were used to determine which waste rock type would release the highest concentration of REEs over 24 hours during which 300 g of crushed waste rock was shaken with  900 mL of ultra-pure water. As expected, leachate from the REE-enriched sövite had a higher total REE concentration (0.4 µg/L) than the REE-poor sövite and loose calcite (both leachates were less than the detection limit of 0.005 µg/L). However, a minor rock type, which contained pyrite weathered to iron oxyhydroxide minerals on the boulder surface (Figure 6), produced 4.62 µg/L REE_tot, more than 10 times as much as the REE-enriched sövite sample. Des Roches (2020) combined field observations estimating the relative amount of different rock types in the waste pile with the shake flask test results to predict the contribution of each rock type to three elements of potential concern in the drainage water (Figure 8). The sulfide-bearing rock type, although estimated at less than 1% of the entire waste rock pile, is predicted to be the source of most easily-released REEs. This is likely due to a local decrease in pH due to sulfide oxidation, and partial dissolution of REE-bearing minerals. These results are consistent with those of Edahbi et al. (2018) who used long-term humidity cell tests conducted on mine wastes from the Montviel carbonatite-hosted REE deposit in Quebec to show that the amount of REEs in the leachate was directly proportional to the sulfide content of the waste rock.

Figure 8. Estimated contribution of elements of concern to drainage from waste rock at St. Lawrence mine. Adapted from Des Roches (2020)

5.    Conclusions

Growing global demand for critical metals to support low-carbon energy initiatives and other aspects of modern technology is likely to result in more mines extracting these resources. Industry, regulators, communities, and consumers all need to understand the environmental implications of these mining and processing activities. Geochemical and mineralogical research is providing new information on the environmental mobility of critical metals. Our understanding of antimony is sufficiently advanced that the optimal remediation measures for specific mine waste conditions can be designed and regulatory limits defined. Tungsten is not easily released from scheelite but is expected to be higher in concentration in neutral to alkaline waters and may be transported as adsorbed anions on suspended particles of iron oxyhydroxide minerals. Rare earth elements are most likely to be released from carbonatite mine waste at low pH, even where those conditions occur in micro-environments associated with sulfide mineral oxidation. In all cases, toxicity data relevant to mine waste environments (the question of how much is too much?) is also needed to guide environmental assessments and monitoring activities.  References

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