How trees drinking gold can help the mining industry

A paper published in Nature Communications reports that trees sip up gold deposits below, a find that could help prospectors pinpoint the location of buried reserves.

Leila Navidi/Las Vegas Sun/AP
A prospector shows off some of the gold he has found over many hours of panning during a Gold Searchers of Southern Nevada outing at a claim near Meadview, Ariz. in September. Researchers report that buried gold reserves can be found based on gilt found in leaves of the trees growing above the deposits.

It’s not just the gods of antiquity who sloshed back cups of liquid gold. Trees drink gold, too.

A paper published this week in Nature Communications reports that gold crystals can be found in Eucalyptus trees growing above buried deposits of the mineral.

The report offers a tentative solution to a worldwide slump in new gold deposit discoveries, suggesting that, somewhat counterintuitively, an effective means of peering into the Earth is to look up – not to the gods, but to the trees.

Trees leaves filled with gold sapped up from below ground can help gold explorers “see what is buried beneath,” says Mel Lintern, a researcher at the Commonwealth Scientific and Industrial Research Organization in Australia and the lead author on the paper.

“The work will enable mineral explorers to cover more ground with a limited amount of funds,” he said.

Prospectors have known for decades that aboveground brush and trees are at times proverbial X's marking the spots of brimming mineral deposits below – not just gold, but copper and silver, too. At times, mineral explorers had used this treasure-laced foliage to uncover buried reserves.

Still, the method, called biogeochemical analysis, had failed to become mainstream, in large part because it had been unclear if the auric leaves were reliable indicators of gold below or just red (or, rather, gold) herrings: sure, it was possible that the gold been sipped up from buried troves. But it was just as possible that it had been ferried there from elsewhere, settling in the trees’ leaves as glittering dupery.

Even over the last decade, as scientists had begun to proffer proof that trees do, in fact, drink gold, the exploration technique had not caught on in the prospecting world, says Dr. Lintern. Since most greenhouse and lab studies used higher quantities of gold than occur in the natural process, prospectors had lacked confidence that the method was applicable to the field, he said. And no previous research had identified gold crystals collecting in free-growing trees out in the field, he said.

“We need to build the technique up into a robust method so that explorers can use it confidently,” says Lintern.

Drinking gold for trees

In the latest research, the scientists directly sampled Eucalyptus and Acacia trees (also called thorntrees) in the Freddo Gold Prospect, in western Australia, and the Barns Gold Prospect, in southern Australia. At both sites, the gold is buried around 100 feet underground. The researchers also conducted supplemental greenhouse work using potted plants from the two sites.

The gold in the leaves is not visible to the eye (no, Australia is not bejeweled with giant gilded Eucalyptus trees). Instead, detecting it requires the help of a high-powered X-ray machine – a super, super high-powered machine.

If all the dimensions are scaled up by factor of 1,000, finding a gold crystal in a Eucalyptus leaf is like “finding a single minute particle of gold the size of a grain of sand in the area of a large football field searching the soil to a depth of 1 metre [3 feet],” said David Paterson, principle scientist at Australia Synchrotron, where the leaves were imaged, and an author on the paper, in an email.

After finding gold in the leaves, the researchers then ran tests for its origins, confirming that it was drawn up from the rock below. The team proposes that ionic gold is absorbed through the tree’s deep roots during periods of drought, when the trees roots reach down deeper and deeper for a drink. The ions are then distributed throughout the tree’s cells and precipitate into crystals.

Gold crystals are found in highest concentrations in the tree’s leaves, possibly because large quantities of gold are toxic to a plant, the researchers say. In order to keep concentrations to a minimum, trees must ferry the ions out to their furthest extremities, like King Midas’ daughter willing the spreading gold not to reach her heart.

Global Gold Slump

The research is part of an ongoing project to develop new, more cost-effective methods for identifying new gold deposits, addressing a troubling dip in gold reserve discoveries, even as exploration expenditures rise.

After a series of booms in the price of gold in the 1990s and early 2000s, and after subsequent surging investment in exploring for new deposits, most of the world’s large, high grade, and easily accessible gold deposits have now been discovered, says Mark Ferguson, a senior industry analyst at SNL Metals Economics Group who was not involved in the research.

“Most of the easy gold, so to speak, has already been found,” he says.

That has put prospectors in a tight spot: what gold is left is in ever more remote locations and at ever deeper deposits underground. Accessing it is an expensive proposition. In 2010, the mining industry spent some $5.3 billion on identifying new gold deposits, according to a report from MinEx, a mining consulting group. But, despite that investment, new gold discoveries have dipped some 45 percent over the last decade, according to a 2012 report from IntierraRMG, another consulting group.

“The exploration sector has been pouring a lot of money into discoveries, but we’re not seeing big discoveries like those that were made back in the 1990s,” says Mr. Ferguson.

In response, industry analysts have been in recent years pressing the need for new, more cost-effective exploration technologies. But some researchers have expressed doubt that the biogeochemical prospecting method will ease the mining industry’s troubles.

Biogeochemistry “can’t be considered to be a magic-bullet for solving the industry’s problem of finding sufficient new deposits to replace what it mines,” says Richard Schodde, managing director of MinEx Consulting, and who was not involved in the research.

That’s in part because the method will be little help in uncovering the untapped gold deposits at deep depths – perhaps hundreds of feet deep – since these reserves are far deeper than trees roots reach, he said. To find these deposits, ever more important as shallower deposits are used up, the industry has a pressing need for lower cost drilling technologies, he said.

Even in finding shallow deposits, the method is still useful just in places full of trees, ruling it out for use in the Arctic and in desert regions, he says. It is also applicable only in areas where the proverbial straw-length-to-glass-length ratio – the depth of the gold versus the depth of the roots – is just right, he said.

Still, in areas where all these conditions are satisfied, the method could be “an intriguing and potential useful tool for gold exploration,” said Mr. Schodde, noting that most exploration work takes a multi-layered approach, using various means to narrow the possible location of the buried reserves.

“Biogeochemistry can potentially be one of these layers – and so add to the picture,” he said, adding that, given slumping discoveries, any contribution to new mining technologies is welcome.

Stephen Kesler, a geology professor at the University of Michigan who was not involved in the research, agreed that prospectors could use the technique as a “second pass” over zones that have already been found to possibly contain gold, using more basic survey methods, as well as that even slight improvements in prospecting methods were valuable.

“Gold is a hard business,” he said. “We have to find new deposits, and that requires new methods and new theories.”

“Nobody needs gold but everybody wants it,” he said.

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