Following the clues of mineral signatures could allow explorers to cast their nets wider and trigger a new era of discoveries under cover, finds TIM TREADGOLD.
Article from resourceful: Issue 9, March 2016.
An alternative way of thinking about minerals exploration through cover begs the question: why look for a needle when the haystack might be more valuable?
In very simple terms that sums up a project investigating distal footprints – a scientific hunt for the telltale signs of giant mineralised systems formed eons ago that now lie hidden deep beneath the modern Earth’s surface.
Using the latest geochemical and geophysical tools, a team led by CSIRO has embarked on a four-year program to identify the clues that could trigger a new era of discovery.
The initial focus of the study is the Capricorn region of Western Australia (WA), a poorly explored area that lies between two of the world’s best-endowed mineral provinces, the iron ore-rich Pilbara and the precious metal-rich Yilgarn.
"The Capricorn offers the best chance of finding chemical and physical markers left during the formation of giant orebodies," Dr Alistair White, a CSIRO research scientist specialising in mineral and hydrothermal geochemistry says.
"We're looking for the signatures left by major orebodies, often at a considerable distance from the deposit itself, and what sort of tools explorers need to identify the signatures left during mineralising periods that led to the formation of those orebodies."
Rather than trying to pinpoint a particular orebody, which is what most explorers try to do, the 30 geologists involved in the project are looking at the bigger, background picture for the chemical and physical signatures left during the mineralising process.
In that regard, the entire Capricorn region, which covers thousands of square kilometres, is like a giant haystack waiting to be examined for clues pointing to potential orebodies.
One way of looking at the $19 million project is to see it adopting the tactics of a hunter tracking an animal across the outback, looking for telltale signs such as footprints, droppings and damaged foliage, which might lead to the prey.
The big difference in the geological hunt is that the target is underground, perhaps very deep underground, covered in a thick layer of weathered rock and sediments deposited and shifted over billions of years – with the surface cover hampering most explorers.
Finding ways to 'see through' the cover has not been a priority for Australian mining companies until recently because they have been able to focus on outcropping orebodies, including in famous locations such as Broken Hill, Mt Isa and Mt Newman – with a clue to their discovery in the name, either a hill or a mountain, peeping up above the flat surrounding country.
But, with no major new discoveries in decades and with Australia still heavily reliant on its mining industry for export income, the search for hidden orebodies has taken on a greater urgency and knowing where to start looking is the challenge.
In the two years since their work started, the distal footprints team have been focusing on developing the right set of tools to enable them to find the pathways of magma and fluid migration in the layers of ancient rock units that lie under cover. They are, in effect, examining the 'haystack' which is the cover – regolith, sediments and igneous rocks – for signs. In some cases the hunt goes down to the atomic level.
In a way the team, which includes researchers from The University of Western Australia and Curtin University, are following up a proposition first put by one of Australia’s leading exploration geologists, Roy Woodall.
Almost 40 years ago Woodall, when head of exploration at Western Mining Corporation and a key player in the discovery of the deeply buried Olympic Dam copper and uranium orebody in South Australia, said that the key to future big discoveries lay in the ability to "peer beneath the regolith".
In a paper contributed to a 1979 book published to mark the 150th anniversary of white settlement of Western Australia, Mr Woodall wrote that:
"For future exploration to have the best chance of being economically viable, geophysical and geochemical exploration techniques need to be improved and the nature of concealment needs to be better understood.
"The concealment may be shallow residual soils, wind-blown sands, alluvial sheet-wash, thickly variably stripped laterite profiles, alluvium in fossil or current drainage systems, salinas, or tertiary marine sediments.
"Of special relevance to exploration is an understanding of all the parameters influencing the mobilisation, transportation and precipitation of metals in the near-surface zone, so that geochemical surveys can be made more cost-effective and results more confidently interpreted."
Mr Woodall was ahead of his time, partly because mining companies had enough surface outcrops to explore without bothering with the costly business of attempting to peer beneath the cover, and partly because geochemical and geophysical tools in 1979 were primitive compared with what’s available today.
Now, new tools – ranging from measuring gravity and magnetic signatures to testing water in pastoral station wells for molecular indications of minerals – are being harnessed as part of the distal footprints project.
Dr White says the collaborative, multidisciplinary project would seek to deconstruct the Capricorn region to better identify the weak and remote signals of deeply buried ore deposits and enhance the potential for a major future discovery.
"We are at the early stages of looking for Australia’s next generation of mines," he says.
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