Money may not grow on trees, but gold does—or at least it accumulates inside of them. Scientists have found that trees growing over deeply buried deposits of gold ore sport leaves with higher-than-normal concentrations of the glittering element. The finding provides an inexpensive, excavation-free way to narrow the search for ore deposits.
Scientists have long had clues that trees and other vegetation pulled gold from the soil and transported it to their leaves, but the evidence wasn’t clear. The gold particles could have stuck to the leaves after being blown there as dust, for example. To bolster the case that the gold came from soil beneath the trees, researchers conducted a series of field studies and lab tests.
At one site in Western Australia, the scientists gathered leaves, twigs, and bark from eucalyptus trees growing above a known gold deposit. The deposit is about the size of a football field and lies 30 meters or more below ground, but at today’s gold prices it’s too small and sparse to be worth excavating. The team gathered the same parts from trees growing 200 meters away from the ore. Although background concentrations of gold in vegetation are typically less than 2 parts per billion (ppb), dried leaves from the trees growing above the ore deposit—but not those 200 meters away—had gold levels up to 80 ppb, says team member Mel Lintern, a geochemist at CSIRO’s Earth Science and Resource Engineering division in Kensington, Australia. (CSIRO, the Commonwealth Scientific and Industrial Research Organisation, is Australia’s national science agency.)
Likewise, field tests by Lintern’s group at a site in southern Australia showed that eucalyptus trees growing above a deposit lying 35 meters underground had 20 times more gold in the gummy substances coating their leaves than did trees that grew 800 meters away. Previous studies had noted anomalous concentrations of gold in the leaf-coating substances, Lintern says, but researchers couldn’t discount the possibility that the tiny particles of the metal had stuck to the leaves after being carried there by winds.
That’s where the team’s new study gleams. By growing seedlings in greenhouses insulated from airborne dust and watering them with gold-laced solutions, the researchers demonstrated that trees actually pick up the metal from soil and deposit it within their leaves. The scientists report their findings today in Nature Communications.
The new research provides “a conclusive set of evidence … from a very nicely constructed set of experiments,” says Clifford Stanley, a geochemist at Acadia University in Wolfville, Canada. “The tree is a conveyor belt bringing gold to the surface,” he notes. Like other such elements in the earth, gold gets sucked up by the plant as it absorbs nutrients in the soil. Then, as a dissolved mineral, it gets transported throughout the tree, although the highest concentrations are typically found in leaves. “When you see the particles of gold inside the plants,” Stanley says, “all doubt goes away.”
Don’t think about mining trees, however. Average concentrations of gold in the leaves are much higher than normal, but individual particles of the metal are still very small, few, and far between. Even the largest particles—which Lintern and his team have jokingly dubbed “phytonuggets”—were no more than 8 micrometers across, about half the diameter of the finest human hair. The trees don’t have a biological need for the element, Lintern says; indeed, it may be toxic to them. “To the trees, gold may be just another heavy metal to be got rid of.”
Though the phytonuggets are too small to be collected and mined, they can serve as a sign that gold deposits may lie within the reach of a tree’s roots. Eucalyptus trees, which can grow lengthy taproots to reach deep ground water in arid areas, may stretch down 40 meters.
Developing and using new techniques to find gold is becoming increasingly important, Lintern says. Worldwide, new discoveries of the metal are down 45% over the past decade. “All the easy gold has been found already,” he notes. By analyzing leaves and twigs, prospectors would waste no money on digging and cause no environmental damage. All that’s required is a field trip to gather large samples of leaves and then some chemical and x-ray analyses of the material back in the lab. “It’s a relatively inexpensive first pass at prospecting,” he says. “The trees are doing the work for you.”
ScienceNOW, the daily online news service of the journal Science.