Mineral resources

"Mineral resources" covers a wide range of economically useful materials from the glamour of precious metals to that everyday essential, water. New Zealand's minerals are varied, but there are few particularly large or rich deposits. Nevertheless, because of material needs the search for new resources and the re-evaluation of known deposits is a continuing process. New technology may require different resources, while new methods and changing economic circumstances may allow the development of previously abandoned deposits.

Metallic minerals
Metals may occur in native (pure) form or combined with other elements as minerals, particularly as sulphides. If the metal can be extracted at a profit the mineral aggregate is called an ore. New Zealand is not particularly rich in metallic minerals, and many potentially metal-bearing rocks are in remote, rugged country, still not fully prospected. Native metals are usually found in association with quartz veins in old, igneous, or metamorphic rocks or in detrital form in terrace, river, or beach sands and gravels.

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Gold and silver tend to occur together in native form and have been mined from quartz lodes in Coromandel, northwest Nelson, Westland and Otago. Rich deposits of gold were also found in river terrace gravels and beach sands in the South Island, and were the basis of the "gold rush" in the late 1860s. These gold-bearing gravels were deposited by ancient rivers which eroded the gold from lodes and transported the grains downstream. The extensive dredge tailings resulting from gold mining are still evident on many river banks in Central Otago and on the west coast of the South Island. Both alluvial and lode gold mining are currently taking place, particularly in Otago and Coromandel. The large open pit mines at Macraes in Otago and Waihi in the Coromandel region are the largest in New Zealand.

Copper has been mined as sulphide ore in Northland, southern Coromandel, and northwest Nelson, and has also been found in association with granite intrusions, serpentinite, and high grade metamorphic rocks.

Lead and zinc in association with gold and silver have been mined from volcanic rocks in southern Coromandel. Other deposits of lead and zinc are known, but not in economic quantities. Minor deposits of antimony, chromium, manganese, mercury,and platinum have been exploited at various times from Coromandel, Nelson, Northland, and Southland.

Uranium has been investigated in breccias near the Buller River, while tungsten ore has been mined from scheelite veins in schist at the head of Lake Wakatipu, at Macraes Flat (northern Otago), and near Blenheim. Nickel, tin, and molybdenum have been investigated but have not been found in economic quantities.

Iron ore was mined from Paleozoic rocks at Onekaka (Nelson area), but by far the greatest resources are in beach and dune sands of Quaternary age along the west coast of the North Island. Here the iron occurs in the form of titanomagnetite, a mineral derived from volcanic rocks. The ironsand is used to make steel near Auckland, and is also magnetically concentrated and shipped overseas from Waipipi and Taharoa. Ilmenite sands near Westport have potential as a source of titanium, a major component of the mineral ilmenite.

Construction materials One of the major needs in New Zealand is aggregate for the construction of roads, as a building material, and to make concrete. Aggregate consists of small fragments of hard rock, and can either be taken direct from naturally occurring river or beach gravels, or derived by crushing solid rock. The main requirements are that the rock be hard, unweathered, and not contain minerals that may cause the rock to disintegrate later, as a result of pressure or chemical reactions. Suitable rocks for aggregate are some volcanic rocks, hard sandstone and limestone, plutonic rocks and some metamorphic rocks. Although many areas of New Zealand have good aggregate supplies, there are problems in obtaining good quality aggregate in areas of soft Tertiary rocks, or very weathered rock. Protection against over-exploitation of aggregate for environmental reasons is desirable in some areas; for instance too much gravel taken from a river bed can alter its course, cause scouring of banks and structures, and may affect groundwater quality. Planning regulations may be necessary to preserve valuable aggregate resources from urban development. Building stone is often imported into New Zealand, although a number of local rock types are suitable. However, some are in remote areas, and transport costs, plus high labour costs in processing the stone, make the use of many New Zealand building stones uneconomic in a limited market. A well known building stone is Ordovician marble from Takaka Hill, near Nelson, and schist from Central Otago makes an attractive ornamental stone. Igneous rocks such as granite make good building stone, while ignimbrite from the North Island is easily worked and is common as a facing material on houses. Tertiary age limestone near Oamaru has been widely used for public buildings, and is still quarried for buildings, monuments and carving. Sand is used for construction, in making concrete and in plastering; suitable deposits are widespread in beaches, dunes, and river beds.

Manufacturing materials A commonly required material for all sorts of manufacturing is clay, and many different types of clay are available for different uses. The ceramic industry uses kaolin and halloysite from Northland for china and earthenware, while other clays are more suitable for brick and tile manufacture. Clay occurs as a weathering product of many different rock types, or as an original deposit, and is widespread throughout the country although large quantities of high quality clay are not common. A naturally swelling variety of clay called bentonite is found on the east coast of the North Island and a non-swelling variety in north Canterbury can be treated chemically to make it swell. Both are used as a drilling mud as well as for other industrial purposes. Diatomite is a siliceous deposit of minute organisms. It is used as a filter or insulator, and is found in a marine Tertiary deposit in Oamaru, as well as in some freshwater Quaternary deposits in the North Island and inland Otago.Pumice is a considerable resource in the centre of the North Island, and is used as an abrasive or insulator. Sand has many uses in manufacturing, particularly for moulding in foundry work and as an abrasive. A particularly pure quartz variety (silica sand) is used in glass manufacture. This type of sand is dredged from Quaternary sands on the sea floor off Kokota Spit in Northland, is dug from coastal terraces of Quaternary age near Glorit (north of Helensville), and is quarried from a lowermost Tertiary sequence at Mount Somers in Canterbury. Limestone and marble with a suitable content of calcium carbonate are used for cement manufacture, in gold processing and in the manufacture of steel, paper, and glass. The main resources are near Whangarei, Te Kuiti, Takaka, and Dunedin, but other deposits of varying quality and age occur throughout the country.

Pounamu (greenstone, New Zealand jade) is an attractive rock found in parts of Westland, Otago and Fiordland. It is used in the manufacture of ornaments and jewellery.

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Agricultural materials (fertilisers) The main use of limestone is for agricultural lime, as New Zealand soils tend to be rather acid. There are many limestone deposits throughout the country suitable for this purpose. Dolomite contains magnesium carbonate and is used to enrich soils with a low magnesium content (it is also used in glass manufacture). It is found in an Ordovician marble near Collingwood in the north of the South Island. The application of superphosphate is helped by adding serpentine. Serpentinite is found in Northland, southern Waikato, Nelson and western Otago. Phosphate has been quarried from Tertiary sediments south of Dunedin, and other minor occurrences have been noted in association with glauconitic sandstone. The feasibility of mining nodular phosphorite from the sea floor on the Chatham Rise has been explored. Sulphur associated with volcanic activity is found in the Taupo-Rotorua area, where there is a large, but low grade deposit at Lake Rotokawa. Sulphur was mined from White Island until a volcanic eruption in 1914 made the exercise too dangerous.

Mineral fuels The search for oil and gas has been carried out since the late 1800s. Prospects looked good in the early days as there were numerous reports of oil seeps and gas blows, but most investigations came to nothing. The only commercial field was in New Plymouth where a few barrels of oil per day were produced from 1934 to 1972. A big breakthrough was made in the early 1960s when the Kapuni gas field was discovered, and a few years later the large Maui field was found offshore. Both these gas fields are contained within a non-marine coal measure unit of Eocene age. Drilling onshore in Taranaki and, more recently, Southland, has explored the oilfield potential of these regions. Attractive prospects in offshore basins in the Tasman Sea, Hawke Bay, the Canterbury Basin and the Great South Basin have been evaluated but so far there has been no production from these areas. The target horizons, those thought most likely to be oil bearing, are generally beds of Late Cretaceous-early Tertiary age. Oil shale is a hard mudstone which when crushed and heated in a retort releases oil; deposits have been investigated in Central Otago and Southland.

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Investigation of coal resources in New Zealand has been important from the early days of colonisation. Bituminous coal is produced only from the West Coast coalfields of Greymouth, Buller, and Reefton, which are of Late Cretaceous to Eocene age. The sulphur and ash content of the coals vary considerably in these West Coast coalfields, and the coals are suited to a variety of uses. Large deposits of sub-bituminous coal are present in the North Island, particularly the Waikato area, where the Huntly, Maramarua, Rotowaro, and Glen Massey fields have been the main producers. Ohai and Kaitangata have been major coal producing areas from Cretaceous sub-bituminous deposits in the southern South Island. Very large lignite reserves underlie parts of Southland and Central Otago, and further large sub-bituminous coal reserves have been proved from Hamilton to north Taranaki.

Groundwater This is a major and increasingly important resource, even in a high-rainfall country like New Zealand. Many industries including manufacturing, cropping and agriculture, depend on the availability of water for their success. Water is a necessity of life, and when rainwater or surface water are not sufficient, groundwater can be utilised. For instance, the water supply of many large urban areas, including Christchurch, Napier, Hastings, and Palmerston North, is entirely from groundwater reserves.

Water is present in all rocks below a certain level (the water table), and is constantly replenished by rain and surface water percolating through the ground. A rock layer carrying water is called an aquifer. When a well is drilled into an aquifer the water is usually pumped to the ground surface, but sometimes water (confined under pressure) will rise to the surface without being pumped and is then called artesian. The best-known and largest groundwater systems are those underlying alluvial plains in Hawkes Bay, Wairarapa, Wairau, Waimea, and Canterbury. The aquifers are gravel or coarse sand layers of Quaternary age, which are recharged from rivers flowing across the plains and from rain in the mountains. These groundwater systems support thriving farming and horticultural industries. The water within the groundwater system has collected over thousands of years, but can become depleted if drawoff exceeds recharge. Without due care, groundwater may become contaminated by sea water intrusion near the coast, sewage, fertiliser, or industrial spillage.

A special type of groundwater provides electricity and heating - geothermal steam. In areas of recent volcanic activity where the ground is still hot, groundwater temperatures may exceed boiling point. When wells are drilled into the water-bearing strata, the superheated water is discharged under great pressure. The system is recharged by rain and surface water percolating through the hot rocks. A steam field at Wairakei is exploited for electricity generation, other fields nearby and in Northland have the potential for development. Geothermal steam is used directly by industry at Kawerau, while many factories and homes use hot water for central heating in Rotorua and Taupo. Hot water springs and pools occur in many places throughout the country, usually associated either with recent volcanic activity, or with active faulting. Over-use can deplete geothermal fields, and may cause ground subsidence.