The lesser-known sciences of gold exploration

This is the age of technological innovation, and boy are Australia’s gold explorers glad about it.

For years — decades, even — the prevailing opinion has been that we’re not finding the same major gold systems that once established Australia as one of the world’s premier hotspots. That’s not to say economically viable gold deposits no longer exist, only that they’re somewhat harder to find.

It’s for this reason that the science and academia side of things has assumed an increasingly prominent role in exploration.

Let’s take a look at some of the most prominent recent scientific developments in the early-stage exploration space.

UltraFine+ soil sampling

Early stages of exploration often include soil sampling as a basic, first-pass method for assessing a new area. Traditional techniques had reasonable success in mineral discovery but were hamstrung by their lack of effectiveness in detecting, understanding, and evaluating near-surface geochemical anomalies in regions with transported cover.

In recognition of these weaknesses, the brainiacs at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) — in collaboration with LabWest and a number of sponsoring companies — developed, between 2015 and 2018, the now-commercialised UltraFine+ soil sampling technology.

Specifically designed for investigating areas of deeper cover, the method is based on separating and analysing only the ultra-fine particles — up to 2 microns, or 0.002 millimetres, in diameter — to which metals in transported cover tend to be adsorbed. By eliminating the dilutive effects of much larger sand particles, the enhanced sensitivity can deliver up to a 250% increase in concentrations of metals, particularly gold, copper, and zinc, thereby reducing both the sample size required and the labour involved in collecting it.

Notably, a 2019 study published by researchers from the CSIRO and the Geological Society of Western Australia revealed a marked decrease in censored results for gold, from 63% to 10% below the detection limit.

Such improvements make the UltraFine+ method amenable not only for use in previously unexplored areas but also for the reinterpretation of existing sample inventories.

“The application of the <2 µm particle-size separation and the UltraFine+ workflow importantly demonstrate the additional value from (re-)assaying regional soil and sediment samples to generate new targets and improve regional geochemical maps,” the study said.

“This is an exercise that can be applied to new greenfields surveys and, when exploration budgets are lean, to archived samples.”

“This is an exercise that can be applied to new greenfields surveys and, when exploration budgets are lean, to archived samples”

The uptake of the method is perhaps a testament to its effectiveness. In relatively short order, the UltraFine+ technology has spread to projects across Australia and the world, with the results to back things up.

Summit Minerals (ASX:SUM), for example, announced in October the results of an UltraFine+ soil sampling program at its Stallion REE Project in south-central Western Australia. Despite the almost 40m of cover, the 113 samples collected held a strong correlation with previous drilling, which Summit viewed as proof of a significant zone of rare earths mineralisation.

“The knowledge gleaned from this campaign gives the company confidence as to the size and scale of the potential resource,” Exploration Manager Jonathan King said at the time.

“It also helps us best target future drilling, which should significantly reduce future discovery and exploration costs across the tenement package.”

Likewise, Hamelin Gold (ASX:HMG) deployed the UltraFine+ technology this year to unlock new potential at its West Tanami Project in northeast WA.

“Following a series of trials and orientation geochemical surveys across the West Tanami, Hamelin determined surface soil sampling and analysis via the [UltraFine+] technology is a potentially powerful new tool to identify gold anomalism under thin transported cover,” Managing Director Peter Bewick said in a September announcement.

“These covered terrains represent a new exploration search space, and the results from our first regional survey are enormously encouraging.”

“the [UltraFine+] technology is a potentially powerful new tool to identify gold anomalism under thin transported cover”

Indeed, the UltraFine+ soil sampling method is a good example of the kind of scientific knowledge that can underpin commercial exploration. 

Gold in eucalyptus trees

If money doesn’t grow on trees, that it might grow in them is worth consideration. 

So thought Dr Melvyn Lintern, who published a study — ‘Natural gold particles in Eucalyptus leaves and their relevance to exploration for buried gold deposits’ — in the scientific journal Nature Communications in 2013.

A 38-year veteran of the CSIRO, Dr Lintern’s work on the gold content of plants began in 1985, at what was to become the Bounty gold mine near Mount Holland in WA.

“We were given access to the area that was going to be the open cut. We analysed the hell out of it, and one of the things that struck me at the time was all the vegetation and the role that this must play in moving metals around at the surface,” Dr Lintern tells Mining.com.au.

“One of the things we did, apart from analysing the soil in great detail, was analyse the plants. Not just the gum trees, but a whole series of plants. And we found gold in all the plants that were over the deposit.”

“And we found gold in all the plants that were over the deposit”

Though the phenomenon wasn’t entirely unheard of, there was scepticism at the time about whether these gold particles were really absorbed by the plants or simply swept onto their surface by ‘aeolian contamination’.

One of Dr Lintern’s breakthrough observations came during his time in South Australia, studying eucalyptus, acacia, spinifex and other species of flora growing in dunes over the Barns deposit near Wudinna. After excavating and dating the dunes to an age of just 10,000 years, he found that the gold wasn’t just forming in the soil, it was being brought up from a depth of about 10m by the roots of the plants.

“That showed to us that not only were the roots bringing it up to the surface, but when we came to analyse the leaves, we could actually see gold within the leaves themselves, not just on the surface, indicating that indeed they were being brought up by roots and not accumulating fortuitously as a result of that dust just only over the deposit.”

As an exploration method, however, the gold content of plants remains an unreliable one. Central to this is a lack of understanding about how the soil-root interface works, as well as the toxic nature of gold to plants.

Because of this toxicity, plants will often attempt to compartmentalise the low levels of gold they absorb, meaning that concentrations can vary not only from tree to tree, but within the tree itself.

“The thing with biogeochemistry, particularly for gold and the low levels that we see, is that it’s quite hard to develop into a robust geochemical exploration method,” Dr Lintern says.

“We would analyse trees over deposits, and some of them would be anomalous for gold, and some of them wouldn’t be very anomalous for gold at all.”

“The thing with biogeochemistry, particularly for gold and the low levels that we see, is that it’s quite hard to develop into a robust geochemical exploration method”

Indeed, the gold content of plants, as Dr. Lintern attests, is simply “another string to the bow, another tool in the box for the explorer”.

Metals in groundwater

Likewise, the concentration of metals in groundwater is a similar bowstring.

“We have all these wells, the classic windmills all over Australia. These are extracting water, and the CSIRO has been involved in a study on these wells,” Dr Lintern explains.

“They’ve come up with a whole bunch of different metals and created these indices which work for different metals, gold or nickel or base metals. And those are being used by companies to provide a first pass in an area to see if it’s productive or could be a productive area for mineral deposits.”

Another study published in 2023 — ‘Gold exploration using groundwater in Western Australia’ — identified challenges in using groundwater to identify deposits but presented the addition of a ‘preconcentration step’ to get around the low sensitivity nature of groundwater geochemistry.

Targeting the groundwater-rich Northern Yilgarn Craton, more than 5,000 samples were taken across an area of 315,000km-square, including several mine sites and buried ore deposits.

The study found that gold in groundwater can be a ‘powerful’ technique in finding areas of gold mineralisation under cover, particularly when detected in association with pathfinder elements. 

“Overall, the application of groundwater geochemistry to Au exploration in the Northern Yilgarn Craton is promising,” the study said, “as long as the data are put into a regional geo-environmental context and used for large-scale exploration such as tenement selection rather than deposit delineation.”

Of course, many of these fringe methods of exploration are just that — fringe methods. It’s these, however, that form the skeleton of geological understanding, and in some cases have blossomed into commercial applications with great success.

It will no doubt be interesting to see how exploration for gold and other commodities continues to evolve with the steady trudge of scientific and technological advancement.

Write to Oliver Gray at Mining.com.au

Images: CSIRO
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Written By Oliver Gray
Originally from Perth, Oliver has a keen interest long-form journalism. He has written for a number of publications and was most recently Contributing Editor of The Market Herald’s opinion section, Art of the Essay.