Open pit mining results in huge quantities of waste rock. First, to reach the underground ore deposit huge quantities of rock must be removed. This rock, called overburden, is dumped as waste. Secondly, the copper ore itself is very low grade. The copper ore in Intag has a copper concentration of 0.7%1. This 0.7%, along with 0.03% molybdenum has to be extracted. The other 99.27% is waste. This waste, called tailings, consists of a slurry of water and fine rock particles. To produce 2 million tons of pure copper, hundreds of millions of tons of waste rock will be created.
When the rock body is exposed to the environment toxins that were previously locked safely underground, unable to react with air and water, are released. The rock in Intag contains chalcopyrite and pyrite1. These are acid generating sulfides. When exposed to air they form sulphates, when sulphates contact with water they dissolve into the water as sulfuric acid. The acid releases heavy metals contained in the rock, such as lead and chrome. These metals are toxic at low doses. This process is called acid mine drainage. Once it starts, it is virtually impossible to stop. There are Roman-era mining sites that are 2000 years old that are still producing acid mine drainage.
As acid is generated and heavy metals leach from the rock, the huge piles of waste turn toxic. These toxins infiltrate the ground and contaminate the ground water. Or when it rains, water runs off the waste dumps and into streams and rivers. Modern day regulations state that toxic run off and seepage should be contained by dams and pumps, and liners on the ground. But no method is 100% effective. In heavy rain, some run-off from millions of waste rock will find its way into the environment carrying toxins into water sources. As liners wear, contaminants leak into the soils.
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- Porter Geoconsultancy – geological report of the Junin copper deposit http://www.portergeo.com.au/database/mineinfo.asp?mineid=mn1427
- Basic Concepts of Environmental Geochemistry of Sulfide Mine-Waste
- Acid Mine Drainage article ground truth trekking http://www.groundtruthtrekking.org/Issues/MetalsMining/AcidMineDrainage.html