Abstract
At the former Faro Lead-Zinc Mine located in the Yukon, Canada, tailings with 40 to 70% pyrite have been weathering under unsaturated conditions since mining ceased in 1992. Detailed field and laboratory investigations conducted from 2018 to 2021 add to the knowledge base on the evolution of tailings solids geochemistry, mineralogy, and porewater chemistry with depth and over time. Standard and more specialized methods, including use of a piston squeezer to extract tailings porewater from the unsaturated part of the tailings were used to characterize the tailings and tailings porewater. Three distinct weathering horizons were identified within the unsaturated tailings. Near the surface and to depths up to 0.75 m, the tailings were highly acidic (pH < 2) with depletion of zinc, manganese, and arsenic from the solids and relatively low concentrations of zinc in the porewater. In contrast, due to the very low pH, this zone showed the highest concentrations of arsenic, cobalt, and nickel in the porewater. Below the near surface horizon and to depths up to 1.0 m, porewater pHs were buffered to pH 2 to 3 resulting in a reddish/orange color and precipitation of iron oxide and sulfate minerals. Mineral precipitates resulted in hardpan formation and textural changes in the tailings. This horizon had the highest copper concentrations in porewater. Below 1.0 m and to the base of the tailings deposit, un-weathered tailings with minimal amounts of accumulated secondary minerals were present. Downward migration of oxidation products resulted in acidic pHs ranging from 3 to 5.5 and the highest concentrations of zinc and manganese in the tailings porewater. Sulfate and iron concentrations were very high throughout the un-saturated and saturated tailings profile ranging from 25,000 mg/L to 180,000 mg/L (median 87,000 mg/L) and 6,500 mg/L to 100,000 mg/L (median 36,000 mg/L) respectively. Sulfate and iron concentrations were higher in the squeezed porewater samples than in groundwater samples from monitoring wells screened within the saturated tailings. This is attributed to slow rates of percolation through the tailings mass which delay the transport of these oxidation products to the saturated zone of the deposit. The results provide insights on the geochemical evolution of sulfide tailings solids and porewater geochemistry and provide measured porewater concentrations for a sulfide rich tailings deposit which has weathered under unsaturated conditions for decades.
