Zinc stable isotope fractionation upon accelerated oxidative weathering of sulfidic mine waste

- Controlled oxidative weathering was applied on Kidd Creek mill tailings.
- For the first time δ66Zn was determined in the leachate of the weathering cell.
- δ66Zn provided additional information on zinc sources and mobilization.

Accelerated oxidative weathering in a reaction cell (ASTM D 5744 standard protocol) was performed over a 33 week period on well characterized, sulfidic mine waste from the Kidd Creek Cu-Zn volcanogenic massive sulfide deposit, Canada. The cell leachate was monitored for physicochemical parameters, ion concentrations and stable isotope ratios of zinc. Filtered zinc concentrations (< 0.45 μm) in the leachate ranged between 4.5 mg L− 1 and 1.9 g L−1-potentially controlled by pH, mineral solubility kinetics and (de)sorption processes. The zinc stable isotope ratios varied mass-dependently within + 0.1 and + 0.52‰ relative to IRMM 3702, and were strongly dependent on the pH (rpH-d66Zn = 0.65, p < 0.005, n = 31). At a pH below 5, zinc mobilization was governed by sphalerite oxidation and hydroxide dissolution-pointing to the isotope signature of sphalerite (+ 0.1 to + 0.16‰). Desorption processes resulted in enrichment of 66Zn in the leachate reaching a maximum offset of + 0.32‰ compared to the proposed sphalerite isotope signature. Over a period characterized by pH = 6.1 ± 0.6, isotope ratios were significantly more enriched in 66Zn with an offset of ≈ 0.23‰ compared to sphalerite, suggesting that zinc release may have been derived from a second zinc source, such as carbonate minerals, which compose 8 wt.% of the tailings. This preliminary study confirms the benefit of applying zinc isotopes alongside standard monitoring parameters to track principal zinc sources and weathering processes in complex multi-phase matrices.