Atypical and typical zinc geochemistry in a carbonate setting, Sä Dena Hes Mine, Yukon Territory, Canada

Abstract

The Sä Dena Hes Mine produced Pb and Zn concentrates from a polymetallic Manto deposit in 1991 and 1992. Ownership of the mine was assumed by Sä Dena Hes Operating Corporation (Teck Cominco Ltd and Pan-Pacific Metal Mining Corporation) in 1994. Sphalerite and galena ore bodies with negligible Fe sulphide were processed by conventional flotation methods resulting in carbonate-rich tailings. This paper presents evaluations of atypical and typical Zn geochemistry under non-acidic conditions using laboratory testing, mineralogy and simple modelling. Atypical behaviour is shown by drainage containing 40 mg/l Zn from a short adit. The Zn load contained in the mine drainage is not apparent in a spring-fed stream a few hundred metres down-gradient. Results of an attenuation test to mimic field conditions showed that Zn was removed to concentrations of less than 0.001 mg/l. Spherical aggregates containing 40% Zn were found in the column residues using scanning electron microscopy. The aggregates are likely a Zn hydroxide or basic carbonate based on the Zn concentration, though the exact mineralogical form was not determined. Similarly, elevated Zn concentrations (up to 56 mg/l) have been found in pH-neutral porewaters near the water table in the tailings deposit. Zinc concentrations generally decrease to below 1 mg/l just below the water table and below 0.1 mg/l in seepage from the tailings dam. However in this case, simple dilution modelling and thermodynamic equilibrium modelling of the tailings porewater chemistry showed that the decrease in Zn concentrations could be attributed to the formation of Zn carbonate. Electron microscopy confirmed the presence of smithsonite (ZnCO 3 ) in the tailings; in addition Zn also occurred as a trace element in Mg–Mn–Ca carbonate phases.