Geologists are finding it more and more difficult to find ore deposits near the Earth's surface that are suitable for mining because most of them have already been found. They now mostly search for ore deposits below the Earth's surface, sometimes at great depths. 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
Other boxes Box 1. Geological processes and ore body formation
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