Although James Hector, the first director of the New Zealand Geological Survey, gave an impressive list of metal ores that had been discovered by 1870 in New Zealand, only three metals were successfully mined in 2005 – gold, silver and iron. Some of the known metal occurrences are too small to be mined, while other material is cheaper to import.
The main aluminium ore is bauxite, a mixture of aluminium-rich clay minerals, usually formed by intense tropical weathering. Small amounts of bauxite are known near Kerikeri in Northland, but have never been mined.
New Zealand annually produces about 250,000 tonnes of aluminium at the Tīwai Point smelter in Southland from bauxite imported from Weipa in Northern Queensland, Australia. It is more economical to process the ore in New Zealand than Australia because of the availability of electricity from the Manapōuri hydroelectric scheme. However, the smelter will close in 2021.
The main chromium mineral is chromite, which occurs as lenses and pods in ultramafic rocks such as dunite and serpentine. About 5,000 tonnes were mined in the Dun Mountain area near Nelson between 1858 and 1866, but the amount present is very small.
Compounds of copper are found at many localities, often identified by the distinctive green-blue staining of malachite (copper carbonate). New Zealand’s first underground mine opened on Kawau Island in the 1840s, from which a total of 2,500 tonnes was produced. Although other small copper mines have been opened in Northland, near Woodville and at Dun Mountain, only small amounts of copper have been produced.
A little goes a long way
As an apprentice geologist in the 1930s, Harold Wellman spent a fortnight mapping and sampling an area near Otama in Southland where copper had been reported. His boss, Eric Macpherson, quickly realised that only a tiny amount of copper was present, telling Wellman that: ‘A pennyworth of copper will stain a mountainside.’ 1
The mineral cinnabar (mercury sulfide, HgS) occurs in rocks associated with extinct and active hot springs in Northland, especially around Puhipuhi and Ngāwhā. A total of 88 tonnes of mercury was obtained from workings in Northland between 1890 and 1945, but subsequent prospecting and drilling failed to find deposits that could be economically mined.
Cassiterite (tin oxide, SnO2), the main tin mineral, is found in some granites and some stream gravels. It was discovered in the remote Tin Range in Stewart Island in the 19th century, and about a tonne of alluvial tin was mined between 1888 and 1894.
The main titanium-rich mineral found in New Zealand is ilmenite (iron titanium oxide, FeTiO3), which occurs as extensive black sand deposits along the West Coast of the South Island. The ilmenite grains have been derived from erosion of schist in the Southern Alps and concentrated by wave action on the beaches. Although large resources of ilmenite are known near Barrytown and Westport, the cost of extracting the titanium is currently uneconomic.
Titanium-bearing black sand is found along the western side of New Zealand, but the sand minerals vary. South Island black sand is made of ilmenite, whereas that in the North Island is titanomagnetite.
Scheelite (CaWO3), the main ore of tungsten, has been found in quartz veins in a number of localities, often in association with gold. It has the unusual property of fluorescing in ultraviolet light, which is used to identify it when prospecting. Small mines have been worked at Macraes and Glenorchy (Otago) and Wakamarina (Marlborough), but the recovery of tungsten is uneconomic at current prices.
A worldwide shortage of tungsten during the Second World War led the government to take over two of the scheelite mines in Glenorchy. Once the price dropped in 1944–45 the mines reverted to private ownership.
There was great excitement when uranium was discovered in the lower Buller Gorge in 1955. Despite considerable prospecting, only small, low-grade deposits have been found, and it has never been mined.