APPLICATION OF ZINC ISOTOPE RATIO MEASUREMENTS TO TWO MINE-WASTE SYSTEMS

Advances in mass spectrometry over the past two decades permit the determination of isotope ratios of metals that are commonly observed in mine-waste systems. We measured Zn isotope ratios and observed fractionation associated with processes occurring within two mine waste disposal areas. Measurements made on the effluent from an experimental waste-rock pile (0.053 wt. % S) at the Diavik Diamond Mine, in the Northwest Territories, Canada, indicate dissolved Zn concentrations in the effluent ranging from 0.4 and 4.7 mg L^-1 (average = 2.2 mg L^-1). The principal mechanisms affecting Zn concentrations are the release of Zn by oxidative dissolution of sphalerite (ZnS) and attenuation by adsorption onto secondary Fe and Al hydroxide solids. Zinc isotope ratios ranged between -0.16 and +0.18 ‰ (average = +0.05 ‰, n = 43) and were consistent with values reported for sphalerite from other deposits. The deviations in isotope ratios ( Δ⁶⁶Zn = 0.36 ‰ ) were significant in comparison to analytical uncertainties (0.06 ‰). However, variations in Zn isotope ratios and concentrations were largely uncorrelated. At the Greens Creek Zn-Ag-Au-Pb mine, near Anchorage, Alaska, measurements of pore-water chemistry, Zn isotope ratios and solid-phase mineralogy were made in test cells containing sulfide-rich tailings augmented with solid-phase organic carbon. Maximum dissolved Zn concentrations ranged from 97 to 320 mg L^-1 in the upper 0.2 m of the cells. Zinc concentrations abruptly declined with depth to < 10 mg L^-1 due to enhanced activity of sulfate reducing bacteria in the reducing zones of the test cells, which resulted in extensive precipitation of Zn sulfide phases. The concomitant release of alkalinity, > 1500 mg L^-1, enhanced the potential for precipitation of Zn carbonate phases. Zinc isotope measurements indicate Δ⁶⁶Zn values of up to ‑0.35 ‰. Precipitation of Zn sulfide phases is anticipated to result in positive δ⁶⁶Zn values. In contrast, precipitation of Zn carbonates leads to increasingly negative δ⁶⁶Zn values. These observations suggest that although precipitation of secondary sulfide phases is the dominant process controlling Zn mobility in the reducing zone, the greater degree of isotopic fractionation associated with the formation of secondary Zn carbonates may have dominated the isotopic signature in the pore-waters.