Factors Controlling Tungsten Mobility in W-Cu Skarn Tailings from the Cantung Mine, Northwest Territories, Canada

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Kazamel, Brent
Keyword
Tungsten , Environmental Geochemistry , Mine Waste , Tailings , Cantung Mine , Scheelite
Abstract
Few studies have addressed the mobility of tungsten in mine waste, which could act as a point source for metal leaching. This study addresses the behaviour of tungsten in tailings at the Cantung Mine in the Northwest Territories, Canada. In addition to this study, this thesis includes a literature review regarding the environmental geochemistry of tungsten. In the Cantung tailings, tungsten is present as scheelite (CaWO4; 0.1 – 0.7 wt.%), copper is hosted in chalcopyrite (CuFeS2; 0.1 – 1.5 wt.%), and gangue mineralogy consists of abundant pyrrhotite (Fe1-xS; 0.2 – 49 wt.%), calc-silicate minerals (26 – 62 wt.%), and carbonate minerals (0 – 30 wt.%). In July 2018, nine surface water samples, five tailings pore-water samples, and thirteen tailings samples were collected from the Cantung Mine’s tailings. Water samples have been analyzed by high resolution inductively coupled mass spectrometry (HR-ICP-MS), yielding tungsten concentrations ranging from 5.3 to 26.3 μg/L, exclusively in samples with pH values between 7.05 – 8.05. Tungsten and iron concentrations are both on average 1.6 x higher in unfiltered aliquots compared to filtered aliquots, suggesting that tungsten is transported as dissolved species but is also adsorbed to suspended Fe-oxyhydroxide minerals. Tailings were analyzed by scanning electron microscopy (SEM) paired with automated mineralogy software (MLA), synchrotron-based μXRD-XRF, and partial leach extractions. The scheelite content of Tailings Pond 3 (TP3) and the FRT are similar (0.15 wt.% and 0.21 wt.%, respectively), and scheelite shows no evidence of alteration. Synchrotron-based μXRD of Fe-oxyhydroxide minerals in the FRT identify goethite (FeOOH) and lepidocrocite (γ-FeOOH), whereas μXRD spectra of pyrrhotite rims from TP3 do not match Fe-minerals, with the exception of rare rims that match hematite (Fe2O3) and maghemite (γ-Fe2O3). The μXRF maps of the hematite-maghemite rims have prominent tungsten peaks, which represents included scheelite grains and possibly structurally incorporated tungsten, likely formed during ore processing. The hydroxylamine leaches yield higher tungsten concentrations in tailings samples from the impoundments than samples from the FRT, suggesting the tailings impoundments have more tungsten that is associated with poorly crystalline and amorphous Fe-oxyhydroxide phases than the FRT. Over time, labile-hosted tungsten in the FRT may have been washed down the Flat River during Fe-oxyhydroxide recrystallization and high energy flooding events.
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