Gold and Nickle 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.

Hyperaccumulators

Although most plants do not take up much metal, such as nickel, copper, cobalt or zinc, in their biomass, some rare plants have the unusual capacity to specifically absorb and concentrate these metals from the soil in their living parts. Such plants can contain concentrations of three per cent or more of nickel, for example, in their leaves. They are called ‘hyperaccumulator plants’ and have been identified from all around the world.

The majority of hyperaccumulator plants are restricted to naturally mineralised soils or ore outcrops that also form minerals extraction targets, and are hence under threat from mine development. The minerals industry, however, could capitalise on this unique biological resource by adopting a new technology called ‘phytomining’.
Phytomining

Phytomining is a method for literally ‘farming’ metals by growing hyperaccumulator plants and then harvesting the biomass rich in a particular metal. The potential for phytomining is greatest for nickel because of the occurrence of vast areas of ultramafic soils that are naturally enriched in nickel and occur around the world, and also because there are a number of potentially suitable hyperaccumulator species to be used as ‘metal crops’.

Rufus Chaney from the United States Agricultural Department first envisaged phytomining in the early 1980s. He and Robert Brooks, Alan Baker, Roger Reeves and colleagues later embarked on extensive laboratory and field trials in the United States and elsewhere. This work has demonstrated that phytomining is not only possible, but also highly efficient in extracting metals from the soil.

Using herbaceous plants in the genus Alyssum, the yields of nickel metal achieved in trials exceeded 100 kg per hectare per annual harvest. More recently, work undertake in Malaysia has led to the discovery of a whole range of hyperaccumulator plants, some with up to 16 per cent nickel in their sap. These tropical plants are mainly trees; a mature specimen of one particular species (Rinorea bengalensis) can contain up to 5 kg of nickel metal (Figure 1).