The New Zealand mini-continent is mostly underwater. Much of the sea floor that is close to land is covered with large volumes of sediment shed from New Zealand, while other areas far from land are scoured by powerful bottom currents, which sweep the ocean floor. These conditions provide a range of environments where mineral deposits form. A conservative valuation of this vast but largely unproven mineral estate is $500 billion.
Polymetallic nodules and manganese crusts are mineral deposits that occur over huge areas of sea floor. The minerals crystallise out of sea water, to form little bumps (nodules) or crusts on the sea floor at great depths.
Polymetallic nodules contain a variety of metals. They form in abundance south of New Zealand, where strong bottom currents prevent sediments from building up on the sea floor. Valuable trace elements from sea water can be precipitated directly onto the seabed, forming nodules that are enriched with cobalt, nickel, iron, manganese and copper. They develop extraordinarily slowly: layers are added at the rate of about 10 millimetres per million years.
The region where polymetallic nodules are found extends 1,700 kilometres along the southern base of the Campbell Plateau, at depths of 4,000–5,200 metres. The Campbell Plateau is a large underwater platform south of New Zealand. The estimated weight of nodules in the south-west Pacific, including those around the plateau, is over 1 billion trillion tonnes. Nodule shape, size, and density on the sea-floor varies with the distance from the Campbell Plateau. Cobalt and nickel concentration within the nodules is less than 1%, and there are lesser quantities of silver and molybdenum. Given metal prices in the 2000s it would not be profitable to attempt to mine these minerals. Feasibility studies from the central Pacific show that economic mining requires cobalt and nickel concentrations of 2–3%, and a density of 10 kilograms per square metre of sea floor.
Manganese crusts, also enriched in cobalt, form on the exposed slopes of seamounts – elevated oceanic plateaus and ridges at depths of 1,000–3,000 metres. To be of economic interest, the crusts should have a cobalt content of more than 0.8% by weight, and be located on seamounts more than 20 million years old. On these ancient seamounts sufficient time has elapsed for the minerals dissolved in sea water to precipitate and grow thick crusts of at least 40 millimetres. Manganese crusts, typically less than 30 millimetres thick and with cobalt concentrations of less than 0.5% by weight, are only known to occur on Three Kings Ridge, Bellona Trough and the edges of the Campbell Plateau.
Where underwater vents (hot springs) occur at the summits of underwater volcanoes, they can deposit valuable minerals on the sea floor. The deposits are rich in base metals such as copper, lead and zinc, as well as precious metals like gold and silver.
North-east of New Zealand the Kermadec volcanic arc, comprising at least 40 volcanoes (mostly within the Exclusive Economic Zone), extends over 1,300 kilometres towards Tonga. The site of the most vigorous venting, producing the largest accumulation of mineralisation so far discovered, is known as Brothers caldera volcano. Here, acidic, metal-enriched hydrothermal fluids, up to 300°C, are discharged by black smokers along the caldera wall. Black smokers are underwater hot springs that discharge great plumes of black fluid from individual chimneys.
The hot hydrothermal fluid cools rapidly when expelled into the surrounding cold sea water, causing the precipitation of metals into mineralised zones around the vents. The mineralisation at Brothers includes the minerals sphalerite-pyrite-barite with lesser chalcopyrite and other minor minerals. These minerals contain valuable metals (such as iron, copper, lead and zinc), with less valuable barium and iron. Over half the volcanoes along the Kermadec arc have vent systems, and there is considerable interest in their economic potential. Permits for mineral exploration have already been granted for parts of the Kermadec arc.
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.
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 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
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Hilton, M. J. ‘Management of the New Zealand coastal sand mining industry: some implications of a geomorphic study of the Pakiri coastal sand body.’ New Zealand Geographer 45 (1989): 14–25.