Story: Marine minerals
Page 1 – Nodules, crusts and vents
Mineral wealth beneath the sea
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.
Volcanic vent deposits
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.