FEATURE ARTICLE: SUSTAINING THE MINERAL RESOURCES INDUSTRY – OVERCOMING THE TYRANNY OF DEPTH
This article was contributed by Geoscience Australia (September 2007).
Mining in Australia dates back thousands of years with the continent's Indigenous people looking for the best stone to use as hunting and cooking implements. This search for stone continued with the arrival of the First Fleet, when convicts were assigned to cut sandstone blocks from the shores of Port Jackson for the Governor's residence, warehouses, military barracks, prisons and other buildings.
The country's first truly commercial mining venture was at Newcastle in 1799 when coal was exported to Bengal. The coal had been discovered in 1791 by a convict, William Bryant, and led to the establishment of a penal settlement at what was then known as 'Coal River' in 1801. From those humble beginnings, Newcastle has developed into a major metropolitan centre and Australia has become one of the largest coal producers in the world. Production of raw black coal reached a total of 398 million tonnes in 2006, and created exports worth around $23 billion (b).
Overall, mining activity accounts for around 8% of Australia's gross domestic product and has contributed over $500b directly to Australia's wealth during the past 20 years. There are around 320,000 Australians employed in the industry, either directly or indirectly in support industries. Many are in sparsely populated, remote and regional Australia.
To put it in perspective, Australia is the world's largest exporter of black coal, iron ore and gold. It also holds the status of being the leading producer of bauxite and alumina as well as the second largest producer of uranium, lead and zinc; the third largest producer of iron ore, nickel, manganese and gold; the fourth largest producer of black coal, silver and copper; and the fifth largest producer of aluminium.
However, only a handful of major new discoveries have been made in recent years. In an attempt to reverse this trend, mining companies are stepping up exploration efforts in both existing areas of mineralisation and in areas which so far have attracted limited exploration investment. Mineral exploration expenditure in 2006-07 was $1.7b, close to $1b more than in 2002-03. Metres drilled over the same period increased by 64% to 8,455,000 metres.
In the past, available technology meant that mining was limited largely to deposits which were close to the surface with the search for new mineralisation being confined to relatively shallow drilling and observations of near-to-surface geology. But a major change is happening.
The Australian Government through Geoscience Australia is helping to limit the risk associated with mineral exploration by developing a greater understanding of the geological makeup of the continent. The agency has begun a program to look far beneath the surface and look at the geological architecture of the Earth's crust deep below some of Australia's most significant mineral provinces and in areas which geologists believe hold the potential for major mineral deposits.
This approach, which uses techniques such as deep seismic surveys, gravity surveys and airborne electromagnetic surveys can be expected to significantly increase the opportunities for new mineral discoveries beneath Australia's land surface.
Already surveys have revealed complex structures extending to the boundary between the Earth's crust and the mantle, commonly known as the Moho or Mohorovicic discontinuity. In some cases these surveys have extended to around 60 kilometres (km) below the surface to provide vital clues about the geological controls on the distribution of known mineralisation.
In some cases these deep crustal surveys have shown how particular mineral deposits have formed from mineral rich fluids which were forced up to flow along deep fractures in the Earth's crust. The surveys also have revealed similarities between the deep and the near-to-surface geological architecture of areas with known mineralisation and other, targeted survey areas. The discoveries have heightened the likelihood of locating similar mineralisation and provide a guide for exploration companies which could significantly reduce the risk associated with costly drilling programs.
An example is provided near Broken Hill (New South Wales) where an anomaly occurring between 42 and 54 km below the surface helps to explain the previously puzzling difference between 1,720 million year old rocks in the Broken Hill region and less than 600 million year old rocks in the more easterly Darling region. The information will greatly assist explorers to more directly identify locations for other potentially significant mineralisation in the Broken Hill region by focusing exploration in the older rocks.
Through this and similar projects undertaken in the Gawler region of South Australia, the Kalgoorlie/Boulder district in Western Australia, the Tanami Desert on the Western Australian/Northern Territory border, the Mount Isa/Cloncurry/Charters Towers region in north Queensland and gold-bearing regions in central Victoria, mining companies are being presented with new exploration opportunities. Already the results have led to new targeted drilling programs in the Gawler region, north-west Queensland and the Tanami region.
By adding data from these deep seismic investigations to other geological and geophysical information gathered over the past 60 years, geologists will be able to obtain a fuller understanding of the evolution of the continent. The production of predictive maps and three-dimensional imaging indicating regions with a high potential to host mineral deposits can be expected to lead to accelerated exploration investment and investigation by industry of mineral provinces not previously recognised.
This heightened interest combined with the continuing passion among today's miners and the dedication of geologists and other scientists in the various geosciences will ensure Australia has a continuing mining heritage for many years to come. Already the sophisticated program of deep crustal imaging has created opportunities for innovative ways to probe and interpret the Earth's secrets in an effort to discover new lodes to rival the legendary deposits of Broken Hill, Mount Isa, Kalgoorlie/Boulder, Hamersley and Olympic Dam.
The mining industry has come a long way since a long boat ventured along the coast north of Port Jackson and returned with a few lumps of coal. But as new discoveries aided by sophisticated scientific analysis unfold, ore bodies such as Broken Hill and Mount Isa will one day be equated to that long boat and handful of coal.
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