Waste rock is simply rock that contains no ore. It is removed, along with ore-bearing rock before and during mining. A lot of rock waste has to be removed to produce the benches (the flat ledges around the inside of an open pit) and slopes that are needed to make the sides of the open pit safe. At Northparkes, for every tonne of ore that has been mined, about 2 tonnes of waste rock also has also been removed.
Waste rock was also excavated when building the decline (tunnel) down to the underground ore-body.
This diagram is a cross-section of the E26 ore-body, and shows the shape of the open-pit that was excavated. Compare the amount of waste-rock (pinkish-brown colour) that had to be removed with the amount of ore (green colour) that was recovered.
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The low grassy mound at the back of this open cut pit (red arrow) is a 10 m high soundbund, made from waste rock extracted from the pit during mining of the copper-rich ore. |
Despite its name, most waste rock is not wasted at Northparkes, but is used in a way that helps improve the environment at and near the mine. For example, some waste rock has been piled into 20 metre high walls along the northern side of the open cut pits. These walls are called soundbunds and they help reduce noise coming from the pits, which might disturb neighbouring farms to the north of the mine site. Waste rock has also been crushed and used in the construction of roads around the mine site. |
Other waste rock is piled into heaps, which are covered with top soil and planted with grasses and shrubs. This improves the appearance of the waste rock heap, and helps prevent erosion. When mining at Northparkes is finished, the waste rock heaps will be re-shaped to resemble small hills, and replanted with native vegetation.
Tailings are the waste products left after the copper and gold-rich concentrate has been removed from the crushed and ground ore.
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Flat surface of one of the Northparkes tailings dams, and the pipes through which tailings are pumped from the processing plant. |
Tailings consist of very finely powdered rock, water and chemicals used during the mineral separation flotation process. At Northparkes, it is thickened to mud-like slurry (55% solids) before being pumped through plastic pipes to two specially constructed tailings dams, which will each have walls 20m high and an area of 144 hectares. |
The walls and base of the tailings dams are lined with a barrier of impermeable clay, designed to stop water leaking into the ground, where it might contaminate ground-water. In the ground around the tailings dams pipes have been sunk into the water table (called piezometers). From these piezometers, ground-water samples can be extracted and tested to make sure there is no leakage from the base of the tailings dam. The tailings dams walls are also regularly checked to make sure there is no leakage.
When mining is finished, the tailings dams will be covered with layers of impermeable clay and top soil, and replanted with native vegetation.
