Looking for clues to our mineral wealth

Key text

There is a catch, though. The black smokers are more than a kilometre below sea level, making their extraction an immense technological challenge. But perhaps even more interesting is their discovery in the first place. How did scientists know where to look? The answer lies in the understanding of the processes that lead to the formation of ores – rocks rich enough in minerals that they can be mined.

Geological processes can concentrate minerals

Elements that we are interested in mining (eg, gold, silver, copper and iron) usually occur in the Earth's crust in relatively low concentrations. At these low concentrations, the elements are almost impossible to extract from the surrounding rocks. In some regions, geological processes have increased the concentration of the elements. Geologists search for these higher concentrations of valuable minerals, known as ore bodies.

There are four geological processes that can concentrate minerals: igneous, sedimentary, metamorphic and weathering (Box 1: Geological processes and ore body formation). The first three are often linked to another geological process, plate tectonics (Box 2: Plate tectonics).

Plate tectonics, igneous processes and the formation of black smokers

According to the theory of plate tectonics the Earth's crust is divided into six large plates and a number of smaller ones. At their boundaries they are either moving towards each other (converging), moving apart (diverging) or sliding laterally past each other. Where these movements occur, the immense forces involved produce large earthquakes and spectacular volcanoes.

Volcanoes occur in a pattern across the world – a pattern closely associated with the distribution of plate boundaries. When volcanoes erupt, they spew out molten – melted – material called magma. As the magma cools it crystallises into igneous rocks.

Here is a description of how the black smokers in the Bismarck Sea form. In Papua New Guinea, the Indian-Australian plate and the Pacific plate are colliding. The resultant volcanic activity at the plate margins heats sea water that has seeped into the seabed to temperatures as high as 400°C. Elements in the magma such as sulfur and gold are dissolved in the extremely hot water. The mineral-rich water then flows back up into the ocean through cracks in the seabed. When it comes into contact with the cool water, sulfide and sulfate minerals solidify into tiny black particles that make the water appear black. These particles fall back down towards the ocean floor, sometimes forming chimney-like tubes – black smokers – around the cracks in the seabed, through which the mineral-rich black water continues to flow.

The term 'black smoker' is certainly an apt one; they have also been called 'satanic mills' because the landscape on the ocean floor resembles that of a polluted industrial city in the 1800s, with vast fields of chimneys belching out dirty 'smoke'.

Scientists have been able to film the formation of black smokers using submersible vessels at depths of up to 2 kilometres. They have found the rocks to be rich in antimony, arsenic, cadmium, copper, gold, mercury, silver and zinc.

Mining black smokers

Mining companies have suggested that it may be technically and economically viable to mine the black smokers near New Britain. Entire chimneys could be dragged up from the ocean floor, loaded onto barges and shipped off to processing facilities. Compared to mining on dry land it may even be cheaper, since there would be no soil and rock (overburden) to remove before getting to the ore body. In addition, geologists think the black smokers may be virtually a renewable resource, since once they were harvested they would re-form, as igneous processes continued on the ocean floor.

Unusual life forms are found around black smokers

The environment around black smokers forms the habitat for a number of highly specialised animals. Species of tube worms, bivalves, gastropods and crustaceans are capable of surviving in complete darkness, under extreme pressures and at water temperatures that range from 10°C to 400°C. These organisms survive by eating bacteria that use hydrogen sulphide as their primary energy source.

The importance of increased understanding of geological processes

Scientists would not have known to look for these mineral-rich black smokers had it not been for an increasing understanding of ore body formation and advances in the exploration of the sea floor. Previous work by Australian and other scientists demonstrated the way in which some ore bodies, now on dry land, had originally formed when they were part of the ancient sea floor. This led them to speculate that similar ore bodies may also be located at sites of current volcanic activity along plate boundaries. Continuing research into igneous and other geological processes will probably reveal more clues to the location of mineral ores currently hidden from view (Box 3: Discovering Australia's mineral deposits).

Related Nova topic:

Prospect or suspect – uranium mining in Australia

Box 1. Geological processes and ore body formation

Igneous processes

Igneous processes that concentrate minerals are often associated with regions where plates converge and collide. As two plates converge, mountains are formed. Friction between the two plates causes the Earth's crust to 'melt' 15-30 kilometres below the surface and to form large magma chambers deep underground. The magma begins to cool and crystallise, allowing the more dense metal oxide minerals and some sulfide minerals to sink to the bottom of the magma chamber, where they form layers.

The 'black smokers' described in the key text show how igneous processes can concentrate minerals.

Sedimentary processes

Rivers transport and deposit sand and mud that were formed by the weathering of rocks in the Earth's crust. These sediments are deposited in layers, forming sedimentary rocks. The rocks often have a banded pattern of alternate layers of sand and mud.

Thick layers of sediments can build up in river delta or sea-bed depressions. These sedimentary basins form when tectonic forces stretch the crust, creating shallow depressions.

The remains of plants and animals are often washed into sedimentary basins. Large deposits of such organic material may be converted into fossil fuels such as coal and petroleum after being subjected to heat and pressure over very long periods of time. The coal-rich Sydney (NSW) and Bowen (Queensland) sedimentary basins formed during a period of tectonic activity that began almost 300 million years ago. Petroleum deposits containing oil and gas also occur in sedimentary basins on the northwest shelf in Western Australia and the Cooper and Eromanga Basins in South Australia.

Metamorphic processes

The immense forces generated by converging and colliding tectonic plates can cause the Earth's crust to buckle and fold, exerting so much pressure and heat that new minerals form in the rocks. Minerals formed in this way can be concentrated, forming ore bodies.

Weathering processes

Weathering and erosion play a role in exposing ore bodies. Australia's mountains were formed so long ago that weathering – or erosion – over many millions of years has worn them down, often removing material many kilometres in depth. Old magma chambers, particularly in Western Australia, are now near the surface, giving access to rich nickel and gold deposits.

Acting together

Ore formation is usually a product of a complex combination of processes and each deposit is usually different in detail from others. Metallic minerals formed during igneous activity can become concentrated in sedimentary layers and later concentrated further by metamorphic and weathering processes.

The ore-body at Broken Hill in western New South Wales is a good example of this. Metal sulphides that formed by volcanic activity millions of years ago were concentrated in layers of fine-grained sediment on the sea floor. As the oceanic plate carrying these sediments slid under another plate, the sediments were scooped up and chemically changed into new minerals by intense heat and pressure. Thus, the coarse-grained ores of lead and zinc mined at Broken Hill are the products of igneous, sedimentary, and metamorphic processes.

Related sites

• Science at sea (Scientific American, Explorations)

• Ocean Drilling Program (Joint Oceanographic Institutions, Inc.)

Box 2. Plate tectonics

The Earth is a rocky planet 12,700 kilometres in diameter. Deep in the centre of the Earth lies the core, which has a diameter of about 6900 kilometres. The core probably consists mostly of iron and nickel. The temperature in the inner part is estimated to be about 4000°C. The pressure there is intense since the weight of the rest of the planet is pushing down on it.

The core is surrounded by a rocky layer called the mantle, about 2900 kilometres thick, which constitutes about 80 per cent of the planet's volume. Due to the heat and pressure, the minerals in the mantle can move slowly, rather like thick putty. In some places the top part of the mantle is partly molten. Above the mantle lies the lithosphere, the outermost shell of the Earth. The lithosphere is about 100 kilometres thick and is rigid and strong. The upper part of the lithosphere is the Earth's crust.

There are two types of crust: oceanic and continental. The oceanic crust consists of lava flows that are about 5 kilometres thick. They form at the mid-ocean ridges where the lava wells up from the interior. The crust spreads away from the ridges as it forms and dives back into the mantle when it collides with a continent or another plate. As it sinks back down, the pressure converts the minerals in the rock to denser ones, like garnet. This makes the slab of rock heavier, so that it sinks deeply into the mantle. This seems to be the main driving force for plate tectonics. The sea floor spreads away from the mid-ocean ridges because it is dragged back down into the interior, pulled rather than pushed.

As the thin oceanic crust dives down beneath the continents, the high temperatures 100 kilometres below the surface cause parts of it to melt. The mixture of molten rock (magma) and water that is formed erupt at the surface as spectacular explosive volcanoes, like Mt St Helens. These volcanoes not only add ore deposits but also contribute to the thick continental crust. In contrast to the dense oceanic crust, the continents are typically about 40 kilometres thick and are made up of lighter rocks like granite. The lighter, thicker continental crust elevates the continents above the level of the ocean basins.

The Earth's surface is made up of moving plates

Although it feels solid enough, our planet's rocky surface, on land and under the sea, is a restless jigsaw of slowly moving pieces.

The fact that the lithosphere is rigid, and that the mantle can move a little, is important in explaining this. During the 1960s geologists came to realise that parts of the lithosphere are in constant motion relative to one another and that they carry the continents with them. These moving parts are called plates.

Each plate is about 100 kilometres thick. The plates move extremely slowly, creeping along at a rate of about 1-12 centimetres per year. Although slow, such movements are driven by great forces and dramatic events occur where two plates are pushed together or pulled apart.

The movements and collisions of plates account for the existence of folded mountain ranges, earthquakes, volcanoes and continental drift. Over millions of years, the movement of plates can make entire continents split, come together or drift apart. When looking at a map of the world, you might have noticed that the outlines of some continents suggest that they once could have fitted together.

Why the plates move

The Earth's plates move because of the heat inside it. Within the mantle, convection currents circulate, slowly mixing its material. The slow rising and falling of these currents goes on continuously. In the process, parts of the lithosphere are moved apart by the sideways movement of the currents underneath. The convection currents in the mantle also bring heat to the surface.

Earthquakes

When plates try to slide past each other, friction between them stops their movement at first. Tremendous strains then build up. Eventually, the friction is overcome and the plates suddenly snap past each other, moving by a few metres at a time. Earthquakes are the result.

Mountain building

Where the movement of plates has caused land masses to collide (although in slow motion), huge mountain ranges are pushed up over millions of years, like wrinkles on a tablecloth. The Himalayas were formed in this way as the plate carrying India slowly pushed into the one carrying Asia. These plates are still moving together, at the rate of about 11 centimetres per year, causing the surface to buckle and the ridges of the Himalayas to continue rinsing.

Gondwanaland

About 600 million years ago, all the major land masses were assembled in a supercontinent called Pangaea. By about 100 million years ago a northern supercontinent, made up of what is now most of North America, Greenland and Eurasia, was completely separated from a large southern supercontinent called Gondwanaland. As well as much of present-day Australia, Gondwanaland was made up of what is now Antarctica, India, South America, Africa and Arabia, and a mass that later became New Zealand. At times, various parts of this land mass were below the ocean.

Present-day Australia

Because of the movement of the plates, Gondwanaland gradually broke up. India, South America, Africa and Arabia started to separate at different times and fan out northwards.

About 90 million years ago a rift in the land started to develop between Australia (attached to what is now New Guinea) and Antarctica. By about 65-45 million years ago, the two areas were clearly separating, as Australia started its long drift northwards and Antarctica stayed almost stationary near the pole. By 35 million years ago the break was complete, and deep water separated the two continents.

Related sites

• When the Earth moves – seafloor spreading and plate tectonics (Beyond Discovery, National Academy of Sciences, USA) (A PDF file of the complete article is available.)

• Layers of the Earth (Volcano World, USA)

• Plate tectonics (Wheeling Jesuit University, USA)

• This dynamic Earth: Historical perspective (US Geological Survey)

Box 3. Discovering Australia's mineral deposits

In Australia, ore deposits are especially deep. Millions of years of surface weathering on this old continent have built up a regolith – a blanket of fragmented rock, soil and sand covering the bedrock. The regolith covers about 70 per cent of Australia's surface and mineral deposits are buried under it, sometimes to depths of hundreds of metres. Other countries (such as Canada) have mineral deposits that are easier to find because glacial action during the last ice age scraped away the overlying regolith.

Mineral exploration

Searching for buried deposits by random underground drilling programs is as effective as searching for a needle in a haystack. Successful mineral exploration requires a well-planned program to pinpoint likely areas of buried mineral deposits. An exploration program involves the work of a team of geologists, geophysicists and geochemists.

Geologists use ground-mapping techniques to identify features seen on satellite images and aerial maps of large tracts of the continent. These features help determine past environments. For example, one feature might indicate the presence of an ocean floor, millions of years old, that has been uplifted, eroded and altered. Any layers of metal sulfide may have contained have now been concentrated deep below the surface and would be a likely place to begin an exploration.

When a mineral exploration team identifies a promising site, geophysicists measure the gravity, magnetics and electrical properties of the rocks. Any measurements that differ from those of the surrounding rocks are called anomalies, and could indicate the presence of a mineral deposit. For example, some metal ore deposits are more magnetic than the Earth's normal magnetism. Magnetometers can be used either on the ground or in an aircraft to measure magnetic anomalies. (Deposits might also have a higher specific gravity or density than the surrounding rocks so that they give anomalous gravity readings.)

One of the most useful geophysical tools is airborne electromagnetic (AEM) technology. The depth of Australia's regolith has been an incentive for some very innovative research and Australia currently leads the world in AEM. A low flying aircraft is fitted with a special transmitter and a sensitive receiver, called a 'bird', is towed behind.

Rocks containing ore deposits often have different electrical conductivity than rocks without them. Ore deposits produce different responses to electrical pulses emitted by the aircraft's transmitter. The responses are picked up by the very sensitive receiver in the 'bird' and recorded. This allows deposits as deep as 300 metres beneath the surface to be identified, mapped and made into 3-D computer models. This technique greatly increases our chances of finding deeply buried deposits.

Geochemists can determine the composition of what lies below the Earth's surface by sampling soil. Soil at the surface can carry a chemical signature of what lies below, because of the movement of chemicals through the rise and fall of the water table. For example, chemical testing of soil (and possibly of plants growing in the area) can show higher than normal concentrations of metals that have been carried up from ore deposits below.

Positive geochemical results from surface sampling are followed by a drilling program. Because of the great expense, drilling is only carried out when the area is very likely to contain substantial mineral deposits. Just where to drill is determined by all the data that has been recorded and mapped on the computer.

Drilling produces either rock fragments, or 'cores' of rock for geochemical sampling to determine whether the mineral deposit contains worthwhile concentrations of ore minerals. The final step is to determine whether the deposit can be mined economically.

Related sites

• Exploration techniques (Geoscience Australia)

• Cooperative Research Centre for Australian Mineral Exploration Technologies

Activities

• Minerals Council of Australia
• Oresome froth – an on-line resource for students to investigate a mineral processing plant.
• Minerals downunder – students investigate mineral formation, mineral exploration, mining processes and environmental management.

• The American Museum of Natural History
  • Black smokers

• Newton's Apple, USA
  • Gold mine: How is gold found in the ground?

Further reading

• New resource assessment for Dampier and Rankin(by Andrew Barrett)
  Updates forecasts for the discovery of hydrocarbons in the Carnarvon Basin.

• Nickel sulphide metallogenic provinces of Australia: resources and potential (by Subhash Jaireth, Dean Hoatson, Lynton Jaques, Mike Huleatt, Mitch Ratajkoski)
  Describes a web-based information and mapping system to help find nickel sulphide resources.

• Going for gold beneath the Tanami (by Bruce Goleby)
  Describes a project to find mineral deposits in the Tanami.

Useful sites

A range of resources, including a glossary and teaching resources for rocks and minerals, plate tectonics and earthquakes and volcanoes.
http://volcano.oregonstate.edu/education/index.html

Minerals downunder (Geoscience Australia)

Links to a number of fact sheets, including 'Gold' and 'Exploration techniques'.
http://www.australianminesatlas.gov.au/education/down_under/index.html

Factsheets (Minerals Council of Australia)

Links to a series of factsheets about the major mineral commodities in Australia.
http://www.minerals.org.au/education/secondary/secondary_resources/factsheets

Australia's mineral exploration (Australian Government’s Department of Education, Science and Training)

A report by the Prime Minister’s Science, Engineering and Innovation Council highlighting Australia's place in the global mineral industry.
http://www.dest.gov.au/sectors/science_innovation/publications_resources/profiles/mineral_exploration.htm

Black smokers (American Museum of Natural History)

Describes what black smokers are and how they form.
http://www.amnh.org/nationalcenter/expeditions/blacksmokers/black_smokers.html

Australians sweep up black smoker chimney (The Geological Society, UK)

Describes the discovery of a chimney-like tube from a black smoker by a CSIRO research vessel.
http://www.geolsoc.org.uk/template.cfm?name=blacksmoker

Hydrothermal vents (University of Delaware, USA)

A simple explanation of hydrothermal vents (black smokers are the hottest of the vents).
http://www.ocean.udel.edu/deepsea/level-2/geology/vents.html

Hydrothermal plume studies (Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, USA)

An overview of hydrothermal plumes (eg, black smokers) and what researchers can learn from them. Click on 'black smokers' in the text or go to www.pmel.noaa.gov/vents/PlumeStudies/BlackSmokers.html to see a black smoker vent. http://www.pmel.noaa.gov/vents/PlumeStudies/plumes-whatis.html

Glossary

continental drift. The very slow movement of the continents on their underlying plates. See also plate tectonics.

mineral. A naturally occurring, inorganic substance. It can be in the form of a chemical element or a compound which has a distinctive chemical composition and therefore predictable chemical properties. Examples of minerals are bauxite, diamond, gold, tin, and salt.

ore. A natural mineral aggregate, especially one that is mined to extract minerals.

plate tectonics. The theory that the Earth's surface is made up of huge plates that have moved very slowly during geological history, and continue to move, thus changing the position of continent and oceans. The plates are about 100 kilometres thick and move at a rate of about 1-12 centimetres per year. (See also continental drift.)

water table. The top level of water in the ground that occupies spaces in rock or soil and lies above a layer of impermeable (non-porous) rock. When the water table rises about ground level a spring, lake or wetland is formed.