Spatial and temporal evolution of Cu–Zn mine tailings during dewatering

Abstract

Abstract The Ruttan Cu–Zn mine produced about 50 mT of fine-grained tailings over 30 a. Since the closure of the mine in 2002, the tailings have been systematically dewatered through trenches draining into the open pit and underground workings. This study evaluated the evolution of tailings that were underwater until 2002, and also tailings that had been exposed to oxidizing conditions for more than 20 a. Acid generation is dominated by the oxidization of the abundant pyrite and pyrrhotite comprising 25 wt.% of tailings with Zn being mobilized from the sphalerite remaining after beneficiation of the ore. Little Cu is being mobilized partly due to the armouring of remnant chalcopyrite by primary quartz, and also by preferential absorption of Cu rather than Zn on secondary Fe oxy-hydroxides. A very fine grained fraction of unoxidized sulfides is a likely cause of an initial pulse of acid generation, and metal and SO 4 2 - release occurring at the onset of dewatering. Metals are temporarily attenuated from waters associated with the tailings, either absorbed on Fe oxy-hydroxide precipitates or as evaporite hydroxy sulfate minerals at the surface of the tailings. While some secondary phases are stable, evaporites are only temporary metal sinks as they redissolve in wet weather conditions. Trace amounts of calcite provide little buffering capacity resulting in rapid acidification of pore and surface water. The pH of pore water and shallow groundwater decreased first to the Al oxy-hydroxide buffer at 4.5 and then stabilized at values of 2–3 being controlled predominantly by the dissolution of solid Fe oxy-hydroxides. The metal contents of the ground and surface water are still increasing but the Ruttan Lake reservoir that receives drainage water from the tailings is maintaining a constant composition.