Dredging and alluvial mining
The international price of gold was fixed for most of the 19th and 20th centuries. As other goods increased in price, gold became relatively less valuable. In the late 1960s the price of gold was allowed to float. It rapidly rose in the late 1970s, making many previously marginal mining proposals economic. This led to a small-scale production boom in the 1980s and 1990s. In Otago and especially on the West Coast two-man operations with an earth-moving digger and a rotating washing screen (known as a trommel) feeding into riffle tables were common. At the Shotover and Kawarau rivers, large diggers retrieved gravel from the river bed and fed it onto floating gold-recovery plants. These covered areas that had been inaccessible or less efficiently worked by earlier dredges. Mechanical diggers with buckets armed with teeth could rip up river beds made of hard-rock and expose the gold trapped in crevices.
Diggers revealed gold-bearing layers that previously had been too deep to mine. In the 1990s, large-scale alluvial opencast mining was carried out at Arahura and Ross on the West Coast, Nokomai in northern Southland, and near Millers Flat and the Tokomairiro River in Otago. All these operations required the removal of huge quantities of overburden (gravel overlying the gold). They were profitable only when the price of gold had risen and large, earth-moving machines were available.
Hard-rock opencast pits
In the late 1970s, hard-rock opencast mining also became economic. In opencast mining a big pit is made as surface rocks are removed to reach the gold-bearing rocks below. It is profitable where there are large, low-grade, hard-rock gold deposits near the surface.
At Martha Hill in Waihī and Macraes Flat in Otago, large opencast pits were dug in the 1990s and 2000s. These accounted for almost all of New Zealand’s recent gold production. Gold-bearing rock was blasted and front-end loaders filled up dump-trucks, which hauled the rock to processing plants. Macraes Flat used trucks with a payload in excess of 190 tonnes that can be filled by a 350-tonne excavator in about 140 seconds.
At Reefton on the West Coast a major opencast gold mine (Globe-Progress) was being developed in 2005. Access roads, silt ponds, and a bridge over the Īnangahua River were under way and the mine is planned to be in operation by the end of 2006. There is estimated to be 17,000 kilograms of gold, and further exploration hopes to reveal further workable resources.
There have been few underground gold mines since the last of the Reefton mines, Waiuta, closed in 1951. Underground mining is expensive and only suited to where there are high-grade gold deposits. In 1981 a new vein adjacent to the former Golden Cross mine near Waihī was discovered. This resulted in the extraction of 20.5 tonnes of gold and 52 tonnes of silver from a rich ore deposit, between 1991 and 1998.
Underground mines were being planned in 2005 at Frasers near the Macraes Flat opencast mine in Otago and at Favona in Waihī. At Frasers, some 65% of the gold occurs in a 7-metre-thick ore band. Once underground mining begins by 2010, this ore deposit is estimated to yield about 1,700 kilograms per year. A discovery in the early 2000s of rich gold–silver quartz veins at Favona, 2 kilometres east of Waihī, was being developed as an underground mine.
Modern mining must comply with the environmental conditions laid out in the Resource Management Act 1991. Past mining often polluted rivers with silt, and many sites were just abandoned. In response to this poor environmental record an anti-mining movement was active on Coromandel Peninsula in the 1980s and 1990s. A 1998 decision by the Thames Coromandel District Council to ban mining in certain areas was overturned by the High Court in 2005. Today’s mining operations must leave the land in the same or better condition as it was before being mined.