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.
Depending on the type of mining, a number of chemical contaminants may be used during various stages of operation, including mercury, cyanide, and arsenic which can significantly impact on water quality. Mining can also create considerable amounts of waste material (overburden and tailings) which must be disposed off. Once excavated, soil and rock increases by 50% in volume, so backfilling of mines still leaves substantial quantities of excess overburden. Tailings consist of ground-up rock, water and chemical residues that remain after minerals have been removed from the ore. These need to be safely stored to prevent long-term damage to the environment from leaching, land instability, and other hazards.
Tailings dams are structures built for the purpose of impounding this waste material. These dams may generate environmental and safety risks for many years, including the potential for dam failure and contamination of land and water by acid leaching. The amount of risk will depend on the surrounding geology, dam design and construction, and the nature of the tailings. It can take many years after a mine closes for the tailings to consolidate (compact and solidify).
Mining requires the use of large machinery. In addition to the direct disturbance from excavation of mines, there are also environmental issues associated with compaction of soils increasing sediments into streams from erosion and the possibility of oil spills (chemical contaminants).
Dredging of the riverbed is used in some gold mining operations, while seabed mining involves offshore dredging of sand near the coast. Both cause considerable disruption to aquatic habitats on the beds of a river or the sea, as large amounts of substrate are removed and filtered before any left over sediment is returned. In addition to substrate disturbance, all aquatic vegetation is cleared along the area that is dredged. Flow may also be temporarily diverted and directed elsewhere to expose streambeds during alluvial mining operations. This may have significant short and long term effects on aquatic invertebrate communities and mahinga kai.