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

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 mu m) 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 parts per thousand. relative to IRMM 3702, and were strongly dependent on the pH (r(pH-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 parts per thousand). Desorption processes resulted in enrichment of Zn-66 in the leachate reaching a maximum offset of +0.32 parts per thousand compared to the proposed sphalerite isotope signature. Over a period characterized by pH = 6.1 +/- 0.6, isotope ratios were significantly more enriched in Zn-66 with an offset of approximate to 0.23 parts per thousand 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. (C) 2014 Elsevier B.V. All rights reserved.