Factors controlling tungsten mobility in W[sbnd]Cu skarn tailings

Abstract

Few studies have addressed the mobility of tungsten in mine waste, which could act as a point source for metal leaching of this potentially toxic metal. This study examines the behavior of tungsten in acidic floodplain tailings and circumneutral impoundment tailings at the Cantung Mine in the Northwest Territories, Canada. In the Cantung deposit, tungsten is present as scheelite (CaWO4), copper is hosted in chalcopyrite (CuFeS2), and gangue mineralogy consists of abundant pyrrhotite (Fe1-xS), silicate minerals, and carbonate minerals. Samples of surface waters, tailings porewaters, and solid tailings samples were collected from Cantung Mine. For surface waters, both filtered (0.45 μm) and unfiltered aliquots were collected to compare dissolved and particulate element concentrations.Tungsten concentrations range from 5.8 to 30.0 μg/L in water samples with pH values between 7.1 and 8.1, whereas samples below pH 7.1 have concentrations below the detection limit of 0.1 μg/L. Tungsten and iron concentrations are both on average 1.7 times 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 at neutral pH. Tailings were analyzed by scanning electron microscopy (SEM) paired with automated mineralogy software (MLA), synchrotron-based μX-ray diffraction and fluorescence (μXRD-XRF), and partial leach digestions with inductively coupled plasma mass spectrometry (cold and hot hydroxylamine hydrochloride, aqua regia, Li-metaborate fusion, and 4-acid digestion with HNO3-HClO4-HF-HCl). Samples from the impounded tailings and the highly oxidized Flat River Tailings distributed on the floodplain (FRT) have similar scheelite content (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 identified goethite (FeOOH) and lepidocrocite (γ-FeOOH) as products of pyrrhotite oxidation. Rare rims on pyrrhotite from the tailings impoundment have μXRD spectra consistent with hematite (Fe2O3) and maghemite (γ-Fe2O3). The μXRF maps of the hematite-maghemite rims have prominent tungsten peaks, which represent included scheelite grains and possibly structurally incorporated tungsten. The hydroxylamine leaches yield higher tungsten concentrations in tailings samples from the circumneutral impoundments than samples from the acidic 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. These results indicate that scheelite has limited solubility in neutral or acid mine waters, and the mobility of the small amount of tungsten released is governed by the present of iron oxyhydroxide minerals, the transport of colloidal material in surface water, and the stability of the depositional environment.