Geochemistry of zinc and cadmium in coal waste rock, Elk Valley, British Columbia, Canada

Weathering of mine waste rock can result in high concentrations of zinc (Zn) and cadmium (Cd) in surface and ground water. Design and implementation of mitigation measures requires an understanding of the mineralogical occurrence of Zn and Cd within the waste rock as well as the subsequent weathering and leaching processes. Recent geochemical studies have focused on the characterization of selenium and arsenic leaching from sphalerite and pyrite in waste rock from coal mines in the Elk Valley, British Columbia, Canada. Data collected from these studies provide an opportunity to evaluate the distribution and behavior of Zn and Cd. The abundance and mineralogical association of Zn and Cd were determined in waste rock placed at different times in the dump and leaching of Zn and Cd at the dump-scale was estimated by analyzing porewater and effluent water samples in a rock drain. Mean contents of Zn and Cd in the waste rock samples (n = 260) were 152 mg/kg (range: 48.4-591 mg/kg) and 3.02 mg/kg (range: 0.97-7.56 mg/kg), respectively. Electron microprobe analysis showed Zn and Cd are present in rock samples as primary sphalerite and sorbed onto secondary Fe oxides. The mean concentrations of Zn and Cd in porewater in the dump (n = 121) were 231 mu g/L (range: 4.27-1046 mu g/L) and 0.61 mu g/L (range: <0.01-6.85 mu g/L), respectively. The sorption onto secondary Fe oxides is identified as the sink for aqueous Zn and Cd in the dump. Under the oxidizing conditions that exist throughout much of the dump, Fe oxides will likely be a long-term sink for aqueous Zn and Cd. Mean concentrations of Zn and Cd in the rock drain water were 56.4 mu g/L (range: 1.2-523 mu g/L, n = 213) and 1.53 mu g/L (range: 0.004-3.92 mu g/L, n = 209), respectively. As such, these low concentrations are attributed to dilution by drain waters sourced upstream of the dump and sorption of Zn and Cd onto secondary Fe oxides and calcite precipitated in the drain within 200 m after discharge from the dump. The relative increase of Cd concentration in rock drain water compared to porewater can be because of its lower affinity for the sorption onto secondary Fe oxides and calcite. This study suggests that the mobility of Zn and Cd from the weathering of any low sulfide - high carbonate waste rocks will be limited because the precipitation of secondary Fe oxides and calcite provide a strong internal geochemical control on Zn and Cd mobilization.

Automated summary

The study focused on the mineral distribution and leaching of Zn and Cd in mine waste rock from the Elk Valley coal mines in Canada, demonstrating that secondary Fe oxides and calcite play a significant role in controlling the distribution of Zn and Cd.