Turning over a gold leaf

Underexplored and highly prospective, South Australia’s Gawler Craton has long drawn interest from those interested in tapping its mineral potential.

It’s unlikely, however, that many have turned to the sprawling mulga trees which span the area as an indicator of what lies beneath; until now.

Gold explorer Marmota recently embarked on an innovative exploration campaign to sample the leaves of the mulga and senna trees on its Aurora Tank prospect in the Woomera Prohibited Area, with promising results.

When processed and tested, the tree leaves had absorbed and contained higher levels of gold over better mineralised areas, and low gold content over poorly mineralised zones.

It’s a process which has been around for some time abroad, but not one that’s particularly well-known or used, according to Marmota Executive Director of Exploration Kevin Wills.

“We decided we wanted to try something new because there’s a chance that, because the anomalies in the leaves are generated by tree roots which sink quite deep, they may be able to see mineralisation deeper than we would see by conventional sampling,” he said.

“The idea is to try and collect leaf samples over the area and look for anomalies, and we’re actually about to begin a program to test some of those anomalies we’ve found.

“That’s going to be interesting.”

A form of biogeochemical exploration, Dr Wills said testing leaves was a low-impact exploration method, requiring just two people on foot or quad bike and allowed for the collection of 60-90 samples per day.

Originally out of Soviet Russia, the technique is one which has been popular in Canadian exploration for a number of years but is yet to really penetrate the Australian exploration market, according to CSIRO Senior Geochemist Nathan Reid.

“Because Australia has a greater species diversity than in the northern hemisphere, there’s been a lot of scepticism around how you use plants to search for mineral deposits,” he said.

“There’s been a lot of research by CSIRO that’s also been part of some larger AMIRA projects which Ravi Anand has run. Collaboration with universities and Geoscience Australia has looked at different plants and environments where this might work, what sort of elements you look for and how you target different styles of mineralisation – not just gold, but all sorts of things like volcanogenic massive sulfide, copper and lead.”

Science is golden

Explorers are typically focused on the ground, so the idea that gold could make its way into the leaves of local trees is one which may seem counter-intuitive.

The key to this is in the plant roots, which often stretch further beneath the surface than auger drilling or soil sampling are likely to reach.

“If you have gold in an accessible part of the subsurface where roots are sitting, and the plants can access it, they’ll inadvertently take up the gold,” Dr Reid said.

“They don’t actually need gold to grow; what they do need is copper. In their absorption of copper, gold being of a similar size and charge will inadvertently pass up the same uptake channels and plant roots, and if there is a larger concentration in that zone you’ll see it transferred into the plant material.

“If you’ve got a gold system you’ll quite often look for arsenic in the plant as well, because a lot of the gold systems have arsenic pyrite in the system. Things like tungsten can be around, and silver sometimes – single elements are good but it’s better if you’ve got a multi-element halo.”

To test consistency, explorers are encouraged to sample at consistent heights and multiple trees within an area.

Dr Reid said uptake of the technology had been mixed in recent years, but the potential was there for further use in the mining space.

“The benefit really is that we’re moving into areas where pretty much everything that’s sticking out of the ground has been found,” he said.

“There’s not much scope left there, so you start looking at areas of transported cover, and the minute that cover gets any more than a metre depth then a lot of the usual techniques don’t work anymore – you either spend a lot of money to send out a drill rig or you use a different technique.

“A lot of people use soil sampling or other ways of testing, but in many cases soil sampling doesn’t work either. If you’ve got too much erosion and nothing’s building up on surface then soil sampling won’t work – but something with a little extra depth could.”

Back in South Australia, Dr Wills believes the exploration technique will bloom the moment someone makes a significant discovery with it.

“It needs more successful usage,” he said.

“As with any new technique, as soon as you find something everyone jumps on the bandwagon. We didn’t originate this technique, it’s not widely used. It is well known amongst the exploration community, but it hasn’t been able to be shown to be successful at finding things as yet.

“If we do find something with it it’ll certainly get a kick- along in terms of future use I think.”

The secrets of the Woomera mulga trees may soon be unlocked.

Image: Taking tree samples at Aurora Tank.

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