 Sediments containing high levels of copper and other heavy metals sourced from the Ok Tedi Mine in Papua New Guinea are a potential pollution risk for the far northern Great Barrier Reef and Torres Strait regions.Some 67% of corals died in the reef's worst-hit northern section, the ARC Centre of Excellence for Coral Reef Studies report said.The runoff problem is exacerbated by the loss of coastal wetlands which act as a natural filter for toxins and help deposit sediment.