Going with the flows

Materials World magazine
,
4 Aug 2015
By imaging the Yilgarn Craton, researchers mapped the craton’s 3D architecture to explain why komatiites are localised in specific belts.

A new way of mapping ancient lava streams in the Earth’s crust could have huge implications for discovering new ore bodies. Guy Richards reports.

In the constant search for new and economically viable ore bodies, exploration geologists rely on an array of techniques for mapping the ground under them and, in this age of declining quality of fresh deposits, they are likely to accept all the help they can get, especially at the initial stage of identifying prospective regions. 

To that end, research published in Proceedings of the National Academy of Sciences and the Geological Society of London’s Special Publications series demonstrates a way of mapping the Earth’s crust that can be applied at the scale of cratons – huge, pre-Cambrian and still largely under-explored regions of the modern-day continents. The research was led by Dr David Mole, who carried out the study as part of his PhD at the Centre for Exploration Targeting at the University of Western Australia. He now works for Australia’s national science agency, the CSIRO.

Mole and his team focused primarily on understanding the distribution of nickel-copper-PGE deposits hosted in komatiites – ultra-hot (about 1,600ºC) lava flows produced by flood basalt-style volcanoes more than 2.7 billion years ago – in the Yilgarn Craton in Western Australia. A number of these deposits form some of the largest nickel-copper ore bodies ever found. 

At the craton scale, however, it was not fully understood how the komatiite volcanism was localised. ‘We knew they formed from mantle plumes, which are radial features, but the distribution of komatiites was not radial,’ Mole says. ‘In fact, they tend to occur in linear belts parallel to the major terrane boundaries. This led us to the idea that they may be localised by the lithosphere [the rigid, outermost shell of the Earth] itself.’

The team sampled crustal rocks (mainly granites) from across the Yilgarn, analysed them for isotopes of hafnium (Hf) and neodymium (Nd) and contoured the results over the area. As Mole explains, ‘The founding work for this project was the mapping of Nd isotopic data by [national geological survey body] Geoscience Australia, which highlighted a number of old and young crustal age domains in the Yilgarn which had not been known before, and could not have been identified by any other method. The use of Nd isotopes and the contour mapping technique was vital for this. 

‘Our project took this technique but added the use of Hf isotopes. These work in a very similar way but we are able to extract more temporal information, whereas the Nd system is fairly restricted in age. This allowed us to plot maps showing crustal architecture through time, through a series of ‘time-slices’. This has not been done before.’  

What the team found was that the major, high-temperature, high-flux komatiite events, and their ore deposits, occurred on the edge of isotopically old crust. ‘It appears that the older crust is also thicker, and that mantle plumes, when impinged on this older crust, were diverted to the edges, where the magma then ascended and erupted as komatiites. The internal regions of the older blocks show more typical, lower temperature volcanism and at lower frequency,’ says Mole.

This, he adds, demonstrates that the isotopic mapping technique is able to pick up the hidden architecture of the early continents, and that major komatiite sequences and their ore bodies are localised at the edge of the older continental blocks. ‘Subsequently, if we map out the changing isotopic character of the crust through time, we can identify where the major continental margins were at given times, and hence select regions prospective for nickel-copper exploration based on this information,’ he says.

In later research, the team also investigated correlations between isotopic architecture and deposits of gold and iron ore, and found that there are consistent relationships between crustal architecture and age, and the clustering of these deposit types. While prior work, by research geologist and consultant Dr Graham Begg, shows that nickel-copper-PGE systems are focused at the edges of major cratons, Mole says his team has shown that the same process can operate within cratons. 

In addition, he says, ‘Since lithospheric and crustal architecture are fundamental first-order controls on fluid – magma or other – transfer and pathways from source to ore-forming environment, it is likely that the fundamental architecture we reveal by using isotopic mapping has implications for all in-situ, hypogene mineral systems.’ 

Although the technique can be used on its own to identify prospective regions for various mineral systems, Mole stresses that it is much more powerful when used in conjunction with other geological and geophysical information. 

He says, ‘Geological mapping is always going to be the first port of call in an area, whether it is performed on the ground, using magnetics or a combination of both. In many areas, however, especially those which are relatively unexplored, deep weathering profiles obscure much of the bedrock geology, and a lot of the bedrock mapping must be done by interpreting magnetic data – as is the case for much of the Yilgarn – so the technique also has significant applications for exploration under cover.’

As with all exploration tools, it is a method for reducing the search box and zooming in on the target area. ‘It is designed to be applied in the initial stages of exploration, or even tenement acquisition,’ Mole explains. ‘Ideally it would be a pre-competitive dataset collected by the government or regional geological surveys, just as geophysics is now, and made available to companies to aid in tenement acquisition and area selection. On an individual mine basis, once mineralisation is found, smaller-scale methods using detection-based techniques such as drilling take over. However, the isotopic mapping could be used to extend prospective zones and direct regional drilling programmes.’

In theory, the technique could be commercialised. The original cost per sample to collect the isotopic data needed was about AUS$1,300 (£645/€890), and around 100 samples were needed for the area covered. However, that total of around AUS$100,000 could be brought down, as the team had to use two laser mass spectrometers for the analysis whereas the same work can now be done with one – although this doesn’t account for the man-hours involved, so if analysis was carried out in-house by a mining company, labour costs would have to be included. 

Mole points out, though, that ‘the cost of this programme may seem like a lot, but with current diamond drilling costing about AUS$200/m, its cost – which covers half a craton – is equivalent to that of a single AUS$500m diamond drill hole.’ 

Further research could be on the cards here as well, and Mole says there are three ways that might help refine the technique and increase understanding of the crust’s prospectivity:

  • Add oxygen isotopes to the protocol to help determine whether the crust developed in multiple evolution stages, and to allow investigation into the interaction of the crust with high- and low-temperature fluids. Since fluids are essential in the formation of gold, iron and copper-gold-silver systems, understanding regional variations in fluid activity may help identify regional hot-spots and major fluid pathways. 
  • Expand the current isotope maps of Canadian and Australian cratons to define crustal blocks at higher resolution and therefore the more prospective zones. 
  • Map key areas at very high resolution to test the scale down to which it is applicable. 

A range of mining companies declined to comment on the utility of this research – although one platinum miner did say it is doing something that could be considered in a ‘similar ball-park’ – but Bill Williamson, Senior Lecturer in Applied Mineralogy at the University of Exeter’s Cambourne School of Mines, UK, is impressed with it. Sample from a komatiite-hosted Ni-Cu-PGE mine, showing the base of a mineralised komatiite lava flow, A is the underlying basalt with evidence of melting by the komatiite, B is the nickel sulphide ore that pools at the base of the overlying  lava flow.

He says, ‘This work provides fascinating insights into Earth’s evolution as well as being an excellent example of how academic geological research can have very real industrial applications. The research team has a number of interesting ideas to enhance the utility of the 4D mapping technique, possibly incorporating other isotope systems and complementary geological datasets, and to test the method in cratons elsewhere around the world.

‘The outcome, on this occasion, is a new and relatively inexpensive exploration tool that is likely to reduce the amount of costly field reconnaissance, geophysics and drilling required to identify new deposits.’ 

The mining industry has been slashing its spending over the past few years, with the most visible cuts being to exploration budgets, which have more than halved in the past three years among the majors. The advent of this new technique could, therefore, prove timely.