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: |
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Mining for Alluvial Gold |
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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. |
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