NATURAL ATTENUATION OF ZN AND CD IN CIRCUMNEUTRAL MINE DRAINAGE AT NO CASH CREEK, YUKON, CANADA

The Keno Hill silver district (Yukon, Canada) has been mined intermittently since 1913 and hosts numerous watercourses impacted by historical mine water drainage. Circumneutral drainage from the abandoned No Cash mine, containing approximately 10 mg/L dissolved Zn and 0.2 mg/L Cd, discharges into No Cash Creek where ca. 99% of the dissolved Zn and Cd is removed from solution over the 3 km stream reach. Scanning electron microscopy (SEM), electron microprobe and laser-ablation-ICP-MS inspection of thin sections prepared from No Cash Creek sediments indicated that the Zn and Cd were most prominent in Mn-rich colloform coatings on lithic grains and organic fragments within the sediment.

Based on the SEM inspection, a range of targets were imaged by synchrotron-based micro-X-ray fluorescence (μ-XRF) mapping, and spots (5 x 5 μm) were selected for further analysis using micro-X-ray diffraction (μ-XRD), and Zn and Cd K-edge micro-X-ray absorption near edge structure (μ-XANES) spectroscopy. Multiple μ-XRD spot analyses identified hetaerolite (ZnMn₂O₄) in the Mn-Zn-rich particle coatings, while fitting of associated Zn μ-XANES analyses to model Zn-bearing standards also commonly identified hetaerolite in the Mn-Zn-rich colloform coatings. Hydrozincite (Zn₅(CO₃)₂(OH)₆) and Zn sorbed on (or co-precipitated with) ferrihydrite were other principal hosts of Zn that appear to be present, based on the Zn μ-XANES data collected in the Zn-Mn rich grain coatings. Micro-XRF mapping indicated that Cd was well correlated with Zn, pointing towards similar host phases for these elements, and Cd μ-XANES data suggested that the bulk of Cd was present as sorption complexes, although the nature of the sorbent could not be determined due to the similarity in the model Cd XANES spectra.

The consistent detection of the Zn-bearing minerals hetaerolite (μ-XRD and μ-XANES) and hydrozincite (μ-XANES) in multiple spot analyses suggests that a proportion of Zn in these samples is structurally sequestered. The mineralogical data collected by multiple analytical techniques, coupled with 20 years of water monitoring showing consistent removal of dissolved Zn and Cd along the stream flow path, indicate that attenuation of Zn Cd is an ongoing, stable process at No Cash Creek.