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Alluvial
Most of the gold deposits in the South Island were first formed by erosion from glaciers over millions of years, then the rivers carried the gold further and deposited it downstream. While the rocks that are the source of the gold may contain only very small amounts of the metal, this erosion over long periods of time has trapped and concentrated the heavy mineral grains. Over time the gold-bearing rock is eroded and water deposits the gold downstream. Gold is very heavy, and so is deposited in places where the current slows down, on bends, and in depressions in the river bed. These are called alluvial or placer deposits, or placer gold.

Alluvial miners had success in the South Island with gold discoveries at Collingwood, Nelson in 1857, at Gabriels Gully in Otago in 1861 and along the West Coast of the South island in 1864.

The early prospectors with their gold pans and simple sluicing equipment were replaced by larger scale operations that maintained a high level of gold output for the next 60 years. In 1881 the first steam powered dredge the Dunedin was launched at Alexandra on the Clutha River. Dredges would operate in Otago and along the West Coast for the next 60 years and produce most of New Zealand's gold.

The last of the old dredges stopped working in 1982 The Grey River Dredge worked until 2004 The dredge has completed work in its current mining licence area and is being readied to cross the river to a new site.

Alluvial gold is relatively easy to extract from its surroundings as the hard work has already been done over time by natural forces.

The alluvial mining process consists of six stages:
Alluvial
Mining for alluvial gold on the West Coast.

The excavator loads gold-bearing sand and gravel into the plant in the centre of the picture. The material passes through a rotating screen and over a vibrating table to extract the gold.
  excavating the gold-bearing sand and gravel
  transporting this material to a plant to recover the gold
  washing the gravel to remove clay and separate out large boulders
  treating the material to produce a small amount of material rich in gold (called concentrate)
  disposing of the alluvium after the gold has been removed
  rehabilitating the area
AlluvialA hydraulic excavator supplying gravel directly to a transportable plant carries out the first two stages. The recovery plant processes the material. The gold is recovered by physical (as opposed to chemical) methods. Gold is very dense, up to five to seven times heavier than the material which makes up the sand and gravel. then the tailings are replaced and rehabilitated.

The recovery plant is designed to promote the settling out of grains of gold while the lighter material is washed away. The gold pan and sluice widely used by early alluvial miners works on the same principle. Several methods are currently employed.

Riffle tables consist of a box set at a gentle slope and lined with matting such as artificial turf. A grid of metal bars called riffles are set on top of the matting. In the turbulence caused by the sand and water slurry passing over the riffles the lighter particles are washed away and the gold and other dense materials remain trapped in the matting. Course grains of gold can easily be recovered in this way.

Fine grained alluvial deposits require different methods. Some plants use vibrating diaphragms to keep the material moving while water passes through it. The heavy minerals settle through a screen at the base of the unit.

Other plants use centrifuges to separate materials of different density. Heavy minerals are thrown to the side of the centrifuge bowl and trapped in grooves in its lining while the water overflows and carries away the unwanted lighter material
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