Gold can grow on trees

When Mel Lintern says gold grows on trees, he isn’t kidding. Lintern is a geochemist with the Commonwealth Scientific and Industrial Research Organisation, or CSIRO, in Kensington, Western Australia. A team he headed has just announced finding tiny grains of the precious metal in the leaves of eucalyptus trees.

If you’re picturing gold leaves glittering in the sun, forget it. The specks of leaf-bound gold are only one-fifth the width of a human hair and just about as long, Lintern points out. In fact, to find these nano-nuggets his group had to team up with specialists at a major scientific facility called the Australian synchrotron. It’s one of the world’s most powerful sets of X-ray “eyes.” This tool doesn’t look through something (as Superman would) but peers into samples to find incredibly small features. Like specks of gold.

The leaves are not worth mining. Still, the greenery can lead to real riches, Lintern’s group reported October 22 in the journal Nature Communications. How? The leaves can point to where mining teams might want to drill in search of a potentially rich seam of gold. Or of some other mineral — because sources of any rare mineral spotted in tree leaves may highlight ore hiding deep below the surface.

Geologists have actually known for years about the value of using plant or animal material to explore for buried minerals. The process is called biogeochemical prospecting, explains Lisa Worrall. A geologist, she works for Protean Geoscience in Lyneham, Australia. Biogeochemistry involves the movement of materials — including minerals — between living and nonliving parts of a natural ecosystem. “Lintern’s work builds on 40 years of biogeochemical prospecting,” Worrall points out.

Lintern wasn’t actually looking for new gold, however. He already knew a deposit lay 30 meters (98 feet) beneath some eucalyptus trees. So his study focused on imaging nanoparticles of gold within tree leaves. His team is also now probing how trees move and concentrate such a metal. “It was quite a surprise that the trees could bring it up from such a depth,” he observes. “That’s as high as a 10-story building.”

The company that Worrall works for helps mining companies use biogeochemical prospecting. Her research has focused on finding minerals hidden deep beneath regolith. That’s a layer of sand, soil and loose rock. This bio-prospecting is especially important in Western Australia, she explains. That’s because thick regolith blankets so much of a remote and largely desert region there known regionally as outback. Its thirsty plants tap deep through regolith in search of water. Sometimes those plants will bring up — and store — bits of gold or other telltale minerals with that water.

But plants aren’t the geologists’ only little helpers, Worrall notes. Termites need moist material to hold their big mounds together. In desert regions those insects have been known to bore 40 meters (131 feet) down, for example in Botswana. And occasionally they drag gold back up along with the mud they were seeking. Geologists may suffer the occasional termite bite while collecting samples from the insects’ mounds. Still, it’s worth it if they find a whiff of gold, said geologist Anna Petts. A specialist in using termite mounds for prospecting, she has plunged her hands into quite a few.

Non-digging animals can help too. Kangaroos, for example, eat plants that may have taken up gold. So resourceful Aussie geologists sample the kangaroos’ droppings — better known as “roo poo” — to get a jump on the location of buried gold, Worrall told Science News for Students.

Bringing gold to light is just accidental for the plants, insects and kangaroos. It can prove a huge stroke of luck for geologists, however After all, why dig and drill to look for gold if the local flora and fauna can do the dirty work for you? And biogeochemical prospecting really works, says Worrall.

She points to a major mineral discovery made in 2005. That’s when geologist Karen Hulme of the University of Adelaide found unusually high levels of gold, silver and other metals in the leaves of red river gum trees. They were growing near the mines west of Broken Hill, Australia. This remote mining town in New South Wales, Australia, is about 500 km (311 mi) northeast of Adelaide. “Those leaves pointed to the buried Perseverance Lode, a resource with an estimated 6 million to 12 million tons of ore,” notes Worrall.

That showed just how far a plant could go in helping prospectors, and turned a lot of heads in the mining industry. “Biogeochemical prospecting has huge potential,” says Worrall. With geologists already using plants, insects and kangaroos, what’s next? “Bacteria,” she says. “It’s the cutting edge.”

LEAVES OF GOLD  CSIRO geochemist Mel Lintern explains how and why his team is studying ways that plants concentrate natural gold from underground. Credit: CSIRO

Power Words

bacteria  (singular bacterium)  A single-celled organism forming one of the three domains of life. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.

biogeochemistry  A term for the movement or transfer (even depositing) of pure elements or chemical compounds (including minerals) between living species and nonliving materials (such as rock or soil or water) within an ecosystem.

biogeochemical prospecting  Using biological material to help locate mineral deposits.

fauna  The animal species that live in a particular region or at a particular period of time.

flora  The plant species that live in a particular region or at a particular period of time.

geochemistry  A science that deals with the chemical composition of and chemical changes in the solid material of Earth or of another celestial body (such as the moon or Mars).

geology  The study of Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists.

mineral  A chemical compound that is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in certain regular three-dimensional patterns).

mineral deposit A natural concentration of a specific mineral or metal.

nano  A prefix indicating a billionth. It’s often used as an abbreviation to refer to objects that are a billionth of a meter long or in diameter.

ore Rock or soil that is mined for some valuable matter it contains.

prospect  (in geology) To hunt for a buried natural resource, such as oil, gems, precious metals or other valued minerals.

regolith A thick layer of soil and weathered rock.

synchrotron A large, doughnut-shaped facility that uses magnets to speed up particles to nearly the speed of light. At these speeds, the particles and magnets interact to emit radiation — an extremely powerful beam of light — that can be used for many types of scientific tests and applications.

termite  An antlike insect that lives in colonies, building nests underground, in trees or in human structures (like houses and apartment buildings). Most feed on wood.