Looking for clues to our mineral wealth
Box 1 | Geological processes and ore body formation
Igneous, sedimentary, metamorphic and weathering processes can all act to concentrate minerals, either on their own or in combination.
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.
Boxes
Box 2. Plate tectonics
Box 3. Discovering Australia's mineral deposits
Related sites
Science at sea (Scientific American, Explorations)
Ocean Drilling Program (Joint Oceanographic Institutions, Inc.)
Posted July 1998.