FACTORS CONTROLLING TUNGSTEN MOBILITY IN W-CU SKARN TAILINGS AT THE CANTUNG MINE, NORTHWEST TERRITORIES, CANADA

As concerns about tungsten’s (W) potential toxicity have only recently emerged, there is lack of literature regarding its fate and transport in the environment. In particular, few studies have addressed tungsten’s mobility in mine tailings. The Cantung Mine in the Northwest Territories, Canada, was a leading global producer of W intermittently between the period of 1962 to 2015. The deposit is a Cretaceous W-Cu skarn hosted within Cambrian carbonates of the Selwyn Basin. Tungsten is mineralized as scheelite (CaWO₄), and is accompanied by a calc-silicate gangue mineralogy, carbonates, and abundant pyrrhotite (Fe_1-xS). The mine hosts five tailings ponds (~6.5 Mt total), as well as ~172,00,0 t of tailings that were deposited directly in the Flat River 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 low temperature W mobility.

In July 2018, eleven surface water samples, nine pore-water samples, and seventeen tailings samples were collected from the Cantung Mine’s tailings. The pore-waters were sampled using suction lysimeters and by centrifuging tailings samples. Tailings samples were collected from the material augered during lysimeter installation. The surface waters and pore-waters range from being acidic to slightly alkaline (pH = 1.92 – 8.05), and have a range of redox conditions (field Eh = -0.39 – 0.63 V). Elemental analysis by high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) yields W concentrations in the waters ranging from 5.83 – 31.63 ppb in the pH range 7 – 8, while all samples in the pH range 2 – 6.5 were below detection limit.

Pyrrhotite in the tailings have variably oxidized, with pseudomorphic replacement by Fe-oxyhydroxides a common texture, along with precipitation of jarosite and other authigenic Fe-bearing phases. The Fe-oxyhydroxides will be investigated further as potential hosts of sorbed W.