Story: Rock, limestone and clay
Page 9 – Phosphate, sulfur and pumice
As New Zealand lacks phosphate deposits, it needs to be imported. However, a small phosphate deposit was quarried at Milburn in Otago. The rock was burned, and then crushed and chemically treated. The phosphate was used as a fertiliser, replacing imported manures. The major working periods were 1920–24, with 140,000 tonnes produced, and 1942–44, with a further 53,000 tonnes of output. But the deposit was not large and the quality not high enough to meet New Zealand agriculture’s requirements.
The yellow mineral sulfur is found in the geothermal areas of the central North Island and Bay of Plenty. It is used to make sulfuric acid, for the manufacture of superphosphate fertiliser. Sulfur production has never been a big industry and most supplies are imported.
Domestic deposits are small and the geothermal environment where sulfur is mined is very corrosive on machinery. In 1917 the New Zealand Drug Company mined 4,392 tonnes of sulfur for use in a chemical works at Rotorua. The company employed workers to dig sulfur from fumaroles. Attempts were also made to mine sulfur from White Island in 1898–1902 and 1912–14; altogether, 11,200 tonnes were extracted. In 1993, 6,600 tonnes were mined at Tikitere near Rotorua, but in the early 2000s there was no production.
Pumice is a very light volcanic rock that floats. When the central North Island volcanoes erupted they spurted out pumice, which is riddled with holes that were filled with gas when it formed. Rivers have also eroded and carried down pumice to create alluvial deposits near Hamilton and on the Hauraki Plains. In the late 1800s it was used as an insulation material in refrigeration. The Firth Pumice Company on the banks of the Waikato River exported this lightweight, floating rock to Australia.
Today pumice sand is dredged from the Waikato River near Mercer and quarried from rock in the central North Island. It is used as a drainage material in construction, and as an additive in potting mixes. Some stonewashed jeans are made from denim that has been washed with New Zealand pumice.