From gum leaves and termite mounds to CSIRO's latest breakthrough in ultra fine soil sampling – explorers are successfully using innovative tools to find new gold targets in Australia's covered landscape. LOUISE POBJOY reports

Over the past 30 years, CSIRO has worked with industry to develop innovative sampling techniques for gold exploration through covered terrain. With companies long considering cover – deep layers of regolith, rock and sedimentary material – a hindrance to mineral exploration, CSIRO has shown that understanding the geochemistry of the landscape and how metals move through it can lead to significant discoveries.

“We have shown that cover can work to your advantage,” CSIRO chief research scientist, Ravi Anand, says.

“You can discover orebodies by using this material, because it can give you a geochemical signature at, or near, the surface.”

Decades of regolith geoscience leads to new innovative exploration tools

CSIRO’s research in this field was shaped by two major leaps in understanding regolith geoscience and its use in mineral exploration.

The first, in the 1980s, involved developing landscape-based geochemical dispersion models, and mapping, sampling and analytical protocols for cost-effective exploration of in situ and shallow transported cover. This work led to laterite and calcrete sampling, which have helped locate gold deposits worth more than $12 billion.

The second, in the 2000s, involved identifying how metal migrates through cover to form geochemical signatures of orebodies.

More recently, CSIRO has been exploring biological mechanisms of metal migration, with industry quickly adopting vegetation and termite mound sampling techniques. Marmota, for example, recently identified new gold targets after finding a gold anomaly in leaves.

“Tap roots of vegetation like eucalyptus and acacias can go quite deep looking for water during drought. In taking up deep water, they bring metals with them,” Dr Anand says.

“Plants send that metal to their leaves and branches, which we sample to find out what is happening under the Earth’s surface.”

Termites also bring up small particles containing gold and stockpile it in their mounds. Like vegetation sampling, termite mound sampling is easier, cheaper and more environmentally friendly than drilling.

Other biological resources, like fungi and gas, could help identify target deposits as well. CSIRO is developing detection technologies, such as microbial DNA and passive gas sensors, to harness these resources.

Tools are also being developed to explore very deep or challenging cover, such as the interface between in situ and transported cover; iron oxide-rich gravel under sand dunes and Permian sediments.

Another important benefit is that these techniques reduce exploration impacts to the environment, because they are less invasive and aim to reduce the number or drill holes needed.

“Companies still need to drill at some stage, but they don’t have to start right from the beginning,” Dr Anand says.

A breakthrough in ultra fine soil sampling

While the exploration industry has already adopted some of these methods, CSIRO’s recent breakthrough in ultra fine soil sampling could become the new global standard for gold exploration.

Soil sampling has been used for around 60 years and involves sieving soil particles and analysing these for traces of metals.

Traditional soil sampling is effective for particles around a quarter of a milimetre in size (250 micron fractions), whereas the CSIRO-developed UltraFine+ can separate much finer particles – less than two microns. That’s about one-50th the size of a grain of salt.

“Really small soil particles, including clays and iron oxides, have more surface area which attracts gold,” CSIRO lead researcher, Ryan Noble, says.

“So they do most of the scavenging work, binding up little particles of metals that move through the environment to form geochemical signatures of orebodies laying 10 to 20 metres below soil or sand.”

Designed to detect gold, as well as copper and zinc

UltraFine+ works well for copper, zinc and other commodities. But it’s mainly designed to detect gold, offering better reproducibility and consistency in analyses.

“Its biggest advantage is that it works better than traditional techniques for gold in terms of geochemistry, because it reduces the nugget effect,” Dr Noble says.

Because no large particles or nuggets are collected, the method offers a more consistent gold analysis. It’s also more detailed and more accurate than other methods.

“Traditionally, analyses would show about 50 elements, such as the standard gold, silver, tin, calcium,” Dr Noble says.

“Now, you can get additional measurements like pH, conductivity, spectral mineralogy proxies, and particle size distribution.”

This detail and accuracy is inspiring companies to revisit old samples in the hope of finding new exploration sites.

“Companies who did surveys 10 or 15 years ago and didn’t find anything, can use UltraFine+ to re-analyse those samples, get much better quality data and potentially identify new targets,” Dr Anand says.

UltraFine+ grew from a partnership between CSIRO, the Minerals Research Institute of Western Australia, Geological Survey of Western Australia, several exploration companies and an analytical laboratory.

Encounter Resources reported a gold find after using UltraFine+

Encounter Resources, one of the sponsor companies, is trialing UltraFine+ at its Nazare gold project in Western Australia – with promising results.

“We had an area where we tried traditional geochemistry and had interesting results, but overall it was a bit of a null test because 75 per cent of the samples came back below detection,” Encounter Resources’ managing director, Will Robinson, says.

“We thought it could be a perfect location to try UltraFine+ to see if we could get better contrast to background and, sure enough, about 95 per cent of the samples came back above detection.”

In February 2019, Encounter Resources reported a gold find under shallow sand using the UltraFine+ technique. The explorer is also planning to use UltraFine+ in upcoming sampling of frontier land.

“If UltraFine+ makes a step change in the ability to use geochemistry in areas with proven fertility, then you can see it being integral to a number of new discoveries,” Mr Robinson says.

The probability of discovery is greater with UltraFine+ than other methods, because companies can cover larger areas or take more samples from an area relatively cheaply, easily and quickly. Once anomalies have been identified, companies can target drilling in those areas, reducing the number of drill sites and associated costs.

“The ability to screen and prioritise targets using surface geochemical sampling, like with UltraFine+, as opposed to a drill rig, could make a massive difference in reducing our drilling dollars,” Mr Robinson says.

Enabling easier exploration in remote areas and new measurement data

Dr Noble believes UltraFine+ also enables easier exploration of hard to reach areas.

“UltraFine+ only requires 100 grams of the sample, not one to two kilograms, so logistically it’s better when going into remote areas,” he says.

Geochemical techniques have changed the way companies operate around the world.

While some companies still use traditional sampling and analysis for single metals, most have adopted multi element data interpretation. For some, the number of elements they’re analysing for has increased from one or two – such as gold and silver – in the early 2000s, to up to 50 elements today.

UltraFine+ is the next logical step, adding another 30 measurements to help companies understand an area’s mineral prospectivity.

“I think in the next 10 years, companies will become more comfortable with getting all that additional data,” Dr Noble says.

“If we’re going to make new discoveries that are becoming increasingly harder to find, we’re going to have to shift the way we go about exploration. Getting more data and using it better through sophisticated analytics is central to that shift,” Dr Noble says.

This new data, coupled with its value for Australian conditions and successful overseas trials, suggests UltraFine+ may become the new global standard in geochemical sampling.

Ultrafine+ is available to the local industry through LabWest in Western Australia.

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