Ore is mineralized rock containing a valued metal such as gold or copper, or other mineral substance such as coal. Open-pit mining involves the excavation of large quantities of waste rock (material not containing the target mineral) in order to extract the desired mineral ore.
After being removed, waste rock, which often contains acid-generating sulphides, heavy metals, and other contaminants, is usually stored above ground in large freedraining piles. This waste rock and the exposed bedrock walls from which it is excavated are the source of most of the metals pollution caused by mining.
After the waste rock is removed and the ore is extracted, the ore must be processed to separate the target mineral from the valueless portion.
So the ore is crushed into finely ground tailings for processing with various chemicals and separating processes to extract the final product. Once the minerals are processed and recovered, the remaining rock becomes another form of mining waste called tailings. Tailings are usually stored above ground in containment areas or ponds.
Mine tailings contain the same toxic heavy metals and acid-forming minerals that waste rock does.
Tailings also contain chemical agents used to process the ores, such as cyanide and sulphuric acid.
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BC Wild, Environmental Mining Council of BC (2006). “Acid Mine Drainage: Mining and Water Pollution Issues“