Mining activities
A range of different mining techniques have been developed to obtain these valuable resources.
Types of mining
Underground mining – occurs when minerals are deep below the ground. This is the most widespread method used for mineral extraction in Aotearoa, and involves digging a main shaft, with parallel shafts allowing the maximum extraction of minerals. The impacts to streams from this type of operation are frequently limited to those from transport and facilities associated with the mine, rather than direct mineral extraction. However, acid mine drainages have been observed in some cases where underground mines have collapsed or closed.
Hydro-mining – involves using high pressure water to blast against the face of seams to remove coal and carry it to a dewatering plant or to the surface. This technique improves the efficiency of coal extraction, but also exacerbates the potential for impacts to the environment from mine drainage into surrounding soils and streams.
Opencut or opencast mining – is generally cheaper than underground mining and a greater proportion of the mineral deposit can be extracted. Opencut mining involves the removal of surface topsoil, vegetation, and rock to allow excavation of shallow underground mineral seams.
Alluvial or placer mining – is also employed in Aotearoa, particularly for gold mining in the South Island. Alluvial mining involves excavation of stream beds, often down to the bedrock, to sift out gold washed into streams from surrounding mountains. This method significantly changes the stream bed, water quality measures and impacts on any mahinga kai present.
Seabed mining – (also known as blacksand mining, ironsand mining or dredging) is conducted with suction pipes, pumping sand mixed with seawater from the seabed. Iron ore is magnetically separated from the sand, whilst other minerals are extracted by sieving, before returning the residue back to the sea.
Development of mines
Mining often requires the clearance of large areas of forest, both for the mine itself and for roads and other infrastructure associated with the mine. When forests are cleared, habitat is lost through the removal of riparian vegetation , especially around stream margins. This can lead to exposure of underlying soils and increased erosion, thereby increasing sediment into waterways and a decline in the number and types of mahinga kai present.