Yellowknife Geoscience Forum

The Cantung Mine in the western Northwest Territories was a leading global producer of tungsten (W) intermittently from1962 to 2015, and is currently owned by Crown Indigenous Relations and Northern Affairs Canada. The mine hosts five tailings ponds (~6.5 Mt total), as well as ~172,000 t of tailings that were deposited directly on the Flat River floodplain during its first three years of production. The tailings vary in terms of their mineralogy and degree of oxidation, which provides an excellent setting to study factors controlling W mobility in the surface environment. As concerns about the potential toxicity of W have only recently emerged, there is a lack of literature regarding its transport and sequestration in the environment. In particular, few studies have addressed W mobility in mine tailings, which could act as a point source of metal leaching. Furthermore, interest in reprocessing the Cantung tailings to recover tungsten from its primary mineral, scheelite (CaWO₄), warrants the identification of phases that may be secondary hosts.

In July 2018, nine surface water samples, nine tailings pore-water samples, and thirteen tailings samples were collected from the Cantung Mine’s tailings. The water parameters temperature, conductivity, pH, and oxidation-reduction potential were measured in the field using portable meters, and water samples were filtered with a 0.45 μm syringe filter. Unfiltered water aliquots were collected in tandem to compare between dissolved and particulate element concentrations. Water samples have been analyzed by high resolution inductively coupled mass spectrometry, and for the anions SO₄^2-_, NO₃^2-_, Cl­_­­^-_, F^-, and PO₄^2- by ion chromatography. Tailings samples have been examined by scanning electron microscopy, as well as synchrotron-based μX-ray diffraction (μXRD) and μX-ray fluorescence (μXRF) to identify secondary hosts of W.

Samples from Tailings Pond 3 (TP3) are variably altered, and have abundant primary calcite, dolomite, and pyrrhotite, with neutral porewater (pH = 7.05). The Flat River Tailings (FRT) have porewaters with pH between 2.20 – 7.61, and virtually all carbonate and sulfide minerals have dissolved and been replaced by secondary Fe-oxides, gypsum (CaSO₄), and jarosite (KFe₃⁺³(OH)₆(SO₄)₂). Surface waters from TP5 have electrical conductivities (EC) ranging from 1.060 – 2.162 mS/cm with compositions dominated by Ca²⁺, SO₄^2-, and HCO₃^-, whereas FRT surface waters have higher EC (1.027 – 2.890 mS/cm), notably higher concentrations of Fe²⁺ and SO₄^2-, and little to no HCO­₃^2-. Water samples with pH between 7.05 to 8.05 have W concentrations ranging from 5.3 to 26.3 μg/L, whereas all samples below pH 7.05 have concentrations below the detection limit of 300 ng/L. Unfiltered surface water samples have higher concentrations of both W and Fe than the filtered aliquots, suggesting that W is present as dissolved species but is also adsorbed to suspended Fe-bearing phases at neutral pH. Micro X-ray fluorescence of Fe-oxide rims of pyrrhotite grains in TP3 indicate the presence of W, which is also interpreted to be adsorbed. Further analyses are planned to investigate the apparent absence of W in the Fe-oxide phases from the FRT.