Story: Marine minerals
Page 2 – Phosphates, ironsands and sands
Chatham Rise phosphates
The Chatham Rise is a large underwater plateau lying to the east of New Zealand. Extensive phosphate deposits occur along 400 kilometres of the crest of the Chatham Rise, at depths of 400 metres or less. The phosphate occurs in nodules with a diameter ranging from 2 millimetres to over 150 millimetres. They are patchily distributed, with layers reaching 0.7 metres below the sea floor. The highest potential for mining lies between 179° and 180° E longitude, where nodule abundance averages 66 kilograms per square metre. This amounts to an estimated resource of 100 million tonnes.
The potential use of phosphate nodules as a slow-release fertiliser for farmland has been recognised for over 30 years. This resource could supply New Zealand farms for 50–100 years, although the Chatham Rise phosphate content is 8–10% lower than that of imported phosphates. There have been successful trials of crushed phosphate nodules, especially on hill-country farms. A prospecting licence for the deposit was held briefly in the early 1980s, but current economics and mining technology make full-scale mining risky.
Ironsands and other placer deposits
Placer deposits are heavy minerals (like gold) that have become concentrated in sands or gravels while other minerals less resistant to transport have washed away. Titanomagnetite ironsands (which contain both titanium and iron oxides) are a prime example. They occur as beach sands, coastal dunes, and near-shore deposits along the west coast of the North Island between Kaipara Harbour and Whanganui.
The total resource is greater than 850 million tonnes. Mines at Waikato North Head and Taharoa have produced around 1.2 and 1.4 million tonnes of titanomagnetite concentrate respectively since the 1970s. The concentrate is used for steel production at Glenbrook, south of Auckland, and exported to Asia. Ironsand deposits, with localised concentrations of greater than 10% titanomagnetite, occur on the inner shelf offshore from Auckland, and in the northern and southern Taranaki bights.
Ilmenite is another mineral that contains titanium and iron. Coastal ilmenite deposits are known from the South Island’s West Coast, with the two largest deposits at Barrytown and Greymouth comprising a total of around 12.4 million tonnes. This can be processed to produce high-purity rutile (titanium dioxide), which is used as a paint pigment. No deposits have been mined in New Zealand. Beach sands have concentrations of 10–25% ilmenite, while dunes generally have less than 6%. There are also localised concentrations of gold. Offshore, the ilmenite concentrations are uneconomic – generally less than 0.17%. Localised ilmenite deposits also occur in beach and dune sands of eastern Coromandel beaches and Matakana Island in the Bay of Plenty.
Other near-shore placer deposits (mainly gold) occur off Otago, Southland, the West Coast, and Coromandel. Eroded sediments from gold-bearing rocks have accumulated in river valleys and beach deposits of the last glaciation. These were drowned when the sea reached its present level, 6,000–7,000 years ago.
Offshore sand deposits
Offshore sand deposits are an important resource in the Northland and Auckland regions, where the price has risen because of waning quarry volumes and urban regulations that limit the supply. Historically a number of east coast beaches between East Cape and North Cape have had limited sand extraction. Since the early 1990s commercial operators have suction-dredged sand from the sandbars off Mangawhai Heads and Pākiri Beach (north of Cape Rodney), extracting 165,000 cubic metres per year over 10 years.
Although in terms of volume th