Looking for clues to our mineral wealth
Key text
This topic is sponsored by the Australian Geodynamics Cooperative Research Centre and the Australian Government's National Innovation Awareness Strategy.
In late 1997, Australian scientists announced the discovery of what they believe to be the richest gold deposit ever found. The gold is contained in 'black smokers' – volcanic chimneys found on the ocean floor.
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Extensive fields of volcanic chimneys known as 'black smokers' have been found in the Bismarck Sea near the island of New Britain in Papua New Guinea. Reports suggest that the black smokers contain gold at concentrations five to ten times higher than those found in the West Australian goldfields.
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).
Boxes
1. Geological processes and ore body formation
2. Plate tectonics
3. Discovering Australia's mineral deposits
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Posted July 1998.