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

The Misima Mine is the pioneer of deep-wafer submarine failings dispos al (STD) and discharges at 112-m depth, well below the zone of major b iological productivity (euphotic zone), onto a steep seafloor slope th at leads directly to a deep ocean basin. The system discharges approxi mately 18,000 tonnes of tailings solids per day and has a mit tank wit h seawater intake from 82-m depth. Prior to discharge, each tailings p art is diluted with approximately seven parts of seawater. Two validat ion surveys have shown that the system complies with regulatory condit ions and meets ambient standards for contaminants well within a submer ged mixing zone that extends to 1200-m radius from the mit tank locati on. Video records obtained by remotely operated vehicle (ROV) show the railings slurry descending the steep slope as a coherent bottom-attac hed density current to at least the limit of the ROV tether at 160-m d epth. Geophysical survey and ocean floor sediment sampling has confirm ed that accumulation of tailings solids are confined to the gently slo ping floor of a deep basin between 1000- and 1500-m water depth. Ocean ographic investigations during the 3-year mine planning and impact ass essment phase showed that the ocean at Misima was permanently stratifi ed, and it was predicted that tailings would not rise to the surface p roviding the outfall was deeper than 100 m. The prediction has been pr oven correct because after 5 years of continuous operation, no tailing s material has ever been observed in the surface waters.