Geological Issues at the OK Tedi Mine Keith McGregor Sophie Price

Presentation Order The Region Mine Development and Geology Tailings Dam Problems Environmental Impacts Conclusion

The Region Papua New Guinea gained independence from Australia in 1975 GDP $510 Ok Tedi Mine in the Star Mountains in the poor Western Province close to the Indonesian border. High rainfall –Coastal areas ~1.5m/year –Inland areas ~10m/year

Mine History 1963, Government Patrol makes first contact with ‘Min’ people of the Star Mountains, notice traces of copper , Kennecott Copper Corporation survey area using a diamond drilling program, large reserves of gold and copper found under Mt Fubilan , Australian company BHP forms an agreement with the Papua New Guinea government to assess the feasibility of developing mining operations , BHP proposals accepted and construction begins in April 1981

Geology of Mine

Mineral Resource Leached cap contained 88 million tonnes of material with 1.75g/tonne (155 tonnes) of gold 290 million tonnes of sulphide copper porphyry with 0.59g/tonne (170 tonnes) of gold and 7400g/tonne (2,150,000 tonnes) of copper 33 million tonnes of copper porphyry deposits with 0.99g/tonne (33 tonnes) of gold and 13650g/tonne (450,000 tonnes) of copper

Mine Proposal Total resources –~350 tonnes of gold –~2.6 million tonnes of copper These resources were worth over $10 billion at early 1980’s prices However, to get this, 1.1 billion tonnes of mine waste would be produced Papua New Guinea Government insisted that a permanent tailings disposal system be in place before mine operations started

Dam Site m Limestone hard rock cap 1000m Mudstone Rocks dipped at angle of 8-10 o Eastern bank slope 9 o Western bank slope o

Dam Construction Dam critical for commencement of mining operation Fast-track approach used Geological site investigations occurred concurrently with detailed design and construction Shear strength of mudstone over-estimated No detailed geological model made –Only one cross-section produced Excavation of dam foundations started 4 th November 1983

Landslide On 6 th December 3.4 million cubic metres slipped 5m down into the excavation from the Eastern slope On 6/7 th January a further 34 million cubic metres slipped 20m into the dam site, halting construction Excavations removed material supporting base of slope, triggering landslide Site declared no longer suitable for dam

Impact of Landslide An alternative solution had to be found to allow mine to go online The Papua New Guinea Government allowed an interim tailings dump to be reviewed in 1988 Alternative tailings dam too expensive and unfeasible due to unstable geology Government allowed waste mining material to be deposited into local rivers by setting a high ‘acceptable particulate limit’ of 940mg/l This amounts to 90 million tonnes of waste a year

Environmental Impact of Mine Huge sediment level in river caused the OK Ma river bed to rise Flooding and vegetation die-back over a area of 1,400 sq km Reduction in fish numbers by 70-90% As a result of these problems, the mine company was sued by local people and had to pay nearly £300 million for relocation and compensation

Conclusions The lack of an appreciation of the total geology of the site can be seen to be critical to the occurrence of the landslide A proper survey and the production of detailed geological models would have led to a better design or earlier investigations into alternatives Poor geological knowledge of the site led to huge costs later in the project both financially and environmentally