Combining imaging techniques at multiple scales is building a clearer picture on how orebodies form. This knowledge is helping Northern Star Resources explore for more gold below Jundee mine. TIM THWAITES reports
Gold has two important properties that make it ideal for currency: it’s scarce and it’s unreactive. But those same two characteristics also make it difficult to find.
CSIRO researchers are using the latest, sensitive analytical and imaging techniques over a range of scales to understand the chemical environment in which gold has been deposited and relate that back to larger-scale indicators of its presence that are easier to detect.
The gold we mine comes from reservoirs deep down, and has been concentrated in the Earth’s crust by fluids moving through fractured rock. But just where and in what form it is deposited depends on a complex interplay of the physical and chemical conditions the metal encounters.
“We use a range of techniques to look at the actual molecular processes that occurred in, and around, the gold deposition,” CSIRO group leader, Mark Pearce, says.
“So we help mining and exploration companies to explore efficiently and make decisions based on a more thorough understanding of their deposit.”
Working with Northern Star Resources to understand more about the Jundee gold deposit
Gold miner, Northern Star Resources, is the current owner and operator of the Jundee mine about 500 kilometres north of Kalgoorlie. Northern Star came to CSIRO seeking an explanation for a curious phenomenon they had noticed in their deposit.
In their assay data, the gold grade gradually increases with depth, and then suddenly decreases. They wanted to know why this happened and, more importantly, if the pattern was likely to continue further south where they intended to explore for more gold.
“CSIRO have all the latest equipment and the ability to do the kind of research that we don’t get time to do,” Northern Star’s exploration manager, Jamie Rogers, says.
“Our projects with CSIRO are designed so they have a business outcome for us – so that we can use and implement the outcomes in our day-to-day work.
“It can be an idea or a process or a tool, anything that adds to our basic knowledge of how to go looking for gold.”
According to Mr Rogers, it could be argued that the seminal work CSIRO undertook in the 90s on the science, models and approaches to sampling in areas of regolith cover have led to some of the biggest recent gold discoveries in Australia (see Land of gold soils).
In the case of Jundee, the CSIRO researchers set about looking for answers in diamond drill cores that Northern Star had collected from a zone of high-grade gold.
In consultation with company geologists, the researchers selected about 10 different 10- to 20-metre lengths of core from a range of depths in parts of the deposit the geologists knew well. Company data could detail exactly where gold had been found in these cores.
Applying advanced analytical techniques at different scales
Initially, the researchers scanned each core with a Swedish-developed instrument, the Minalyzer. This uses X-ray fluorescence to provide an analysis of the proportions of chemical elements in successive 10-centimetre chunks of the core.
CSIRO is the first science agency in the world with in-house access to this technology. It’s based in its new drill core laboratory facility at the Australian Resources Research Centre in Perth, Western Australia.
“We were looking for changes in the concentration of elements, particularly those we know are associated with gold in this region, such as potassium and arsenic, which would pick out boundaries between different zones in the rock” Dr Pearce says.
“Then we can select areas of core of particular interest – where specific elements increase or are in high concentration – and see if there is any association with where the gold is.”
On the basis of this, they undertook further analysis of several 30-centimetre pieces of core in much greater detail using the CSIRO-developed Maia Mapper to map where particular elements are actually located on the core.
The data was then combined with results from CSIRO’s HyLogger, which uses reflected light to identify specific minerals, to build a picture of where the elements were sitting.
Using these maps to provide an understanding of variability of the samples the researchers then selected centimetre-sized pieces to make into thin sections to analyse using several scanning electron microscope-based techniques.
Techniques included automated mineralogy, which reveals the minerals associated with the gold; electron back-scatter diffraction, which provides information about the orientation of crystals and how rocks are deforming; and scanning electron microscope imaging down to less than a thousandth of a millimetre, which can be used to investigate relationships and interaction between minerals.
In this way, the CSIRO researchers built up a picture of the microscopic environment which led to the deposition of the gold.
Science to better target new mineral systems
With respect to Jundee, they could tell the miners that a really intensive addition of carbon dioxide produced zones rich in the calcium carbonate mineral calcite that were associated with the gold. They also demonstrated that these zones of interest could be picked out at the 10-centimetre to metre scale using the Minalyzer’s geochemical data.
In regards to the original puzzle of why the gold is concentrated into narrow bands at a particular depth, the researchers were able to hypothesise that the gold is carried in a fluid made up of about 10 per cent carbon dioxide dissolved in water. Due to the fracture pattern of the rock, this fluid becomes trapped. The carbon dioxide reacts with the rock and the gold precipitates as a consequence.
About a year ago, Northern Star announced that at Jundee it had diamond-drilled the deepest exploration core in Australian history to a depth of more than 2600 metres.
The CSIRO team has been invited to analyse that core with the Minalyzer, in the first big project underway at the drill core laboratory facility. They are looking for chemical indicators of the various rock types that are present.
“We now do a lot of these multi-scale studies, not just for gold but in a whole range of different commodities,” Dr Pearce says.
“We can apply the methodology quite broadly.”
The group is currently working on a project in Mt Isa, for instance, which is linking changes in mineralogy to changes in geophysical properties.
“We are hoping to be able to use big-scale geophysical surveys to predict changes in mineralogy and mineralisation.”
This will ultimately benefit the wider exploration industry, by improving targeting of new mineral systems.