DEEP-WATER STD AT THE MISIMA GOLD AND SILVER MINE, PAPUA, NEW-GUINEA

The Misima Mine is the pioneer of deep-wafer submarine failings disposal (STD) and discharges at 112-m depth, well below the zone of major biological productivity (euphotic zone), onto a steep seafloor slope that leads directly to a deep ocean basin. The system discharges approximately 18,000 tonnes of tailings solids per day and has a mit tank with seawater intake from 82-m depth. Prior to discharge, each tailings part is diluted with approximately seven parts of seawater. Two validation surveys have shown that the system complies with regulatory conditions and meets ambient standards for contaminants well within a submerged mixing zone that extends to 1200-m radius from the mit tank location. Video records obtained by remotely operated vehicle (ROV) show the railings slurry descending the steep slope as a coherent bottom-attached density current to at least the limit of the ROV tether at 160-m depth. Geophysical survey and ocean floor sediment sampling has confirmed that accumulation of tailings solids are confined to the gently sloping floor of a deep basin between 1000- and 1500-m water depth. Oceanographic investigations during the 3-year mine planning and impact assessment phase showed that the ocean at Misima was permanently stratified, and it was predicted that tailings would not rise to the surface providing the outfall was deeper than 100 m. The prediction has been proven correct because after 5 years of continuous operation, no tailings material has ever been observed in the surface waters.