Roots of the eucalyptus tree tap gold deposits forty meters deep, says S.Ananthanarayanan.
While demand for gold, as ornament, investment and in industry is rising, known gold mines are getting depleted and new deposits of gold ore are hard to find. Mining for gold has been directed mainly by historical record of deposits, or by chance discovery and systematic prospecting is costly and involves deep drill holes and environmental damage.
A comparatively simpler method of discovering underground deposits could be to detect traces of gold in vegetation above the place where gold is buried. But the method is often misleading and no mechanism by which gold particles could be harvested by plants has been proposed. Melvyn Lintern, Ravi Anand, Chris Ryan and David Paterson, working in Australia, report in Nature Communications, a clear demonstration of gold particles being transported by the eucalyptus tree, which could promote confidence in a new method of prospecting.
Deposits of heavy metals are typically located at great depths. This is because the earth, in its early geological history, was a molten mass, and heavier elements sank deepest. Most of the gold in the earth may hence lie in the earth’s core and any gold that is discovered on the surface is considered to have been come there via meteorites. But gold from deep underground is also raised nearer the surface by volcanic activity or geological disturbances. The Witwatersrand basin, in South Africa, which has the world’s richest deposits of gold, is believed to have been created by the asteroid that struck the earth 2000 million years ago and formed the 300 km Vredefort crater, distorting the rock strata and raising gold deposits.
Nearly 50% of all the gold mined on earth has come from the Witwatersrand rocks. All other mines are in the form of wide, open cast mining or through deep tunneling. Discovery has been historical or by chance, often by casual prospectors finding nuggets in the water with rocky material carried down by streams. Prospectors ‘pan’ the flowing water and any gold stays back while other rock is washed away. Once gold is found like this, tracing the stream to its source reveals where the gold came from, for mining.
But except for discovery like this, actually searching for gold deposits is neither practical nor economical. The methods used are painstaking inspection or laboratory examination of different kinds of rock that are at the earth’s surface or within reach, selected according to rock types that are known to have gold content, or by geophysical anomalies. When there are prospects of discovery, drill holes are made and rock samples from deep underground are analysed. But gold deposits are typically just a few hundred meters wide and a great many holes need to be drilled before deposits can be located.
The possibility of natural probes that go deep underground have been proposed and one agent proposed by Australian scientists was of termites. Termites burrow beneath the earth that may hide a vein of gold and ingest nano-particles of gold. The excreta of the termites would then have traces of gold and examining termite nests may be a non-invasive and hopefully economical way of detecting underground deposits. The other natural probe is of root structure of trees. Some traces of gold have been found, since long, in the leaves of the eucalyptus tree, which is common in Australia, but the traces are so minute that it has not been possible to say surely that they are not due to surface level contamination. This biogeochemical method of prospecting has thus not been seriously pursued.
The main features of the eucalyptus tree is that it has roots that go deep and that the root structure rapidly raises underground water to the surface. Planting eucalyptus has thus been a method of draining swampy land or for lowering the water table, which may be needed for construction. As eucalyptus can extract water from deep underground, the tree can survive arid conditions and has been planted in dry places to ‘promote greening’, only to find that the water table recedes and conditions become more arid than before.
But eucalyptus abound in Australia and the authors of the paper in Nature Communcations were able to study the trees that grew in the earth above two known deposits of gold, and also in the laboratory, to analyse the real source of traces of gold in eucalyptus leaves. The two locations were the Freddo Gold Prospect, in western Australia and the Barns Gold Prospect in southern Australia.
The deposits at Freddo are about 35 metres underground and were discovered by exploratory drilling. But the deposits are not worth mining and the area is undisturbed, and covered by large eucalyptus trees in an open, woodland setting. The picture shows the kind of strata and the depth of the root structure. At Barns, which lies over a sub-economic gold deposit, previous studies have shown that the fluids extracted from the trees that grow there, as well as the calcium rich mineral content of the soil, have unusually high gold content.
Freddo and Barns thus provided undisturbed samples of eucalyptus trees in a natural setting, for laboratory testing. That plants take up gold nanoparticles has been established, but earlier studies have used gold concentration that is higher than what is found in real locations. And the studies have not always used the plants that actually grow in the areas of interest. The present study hence used greenhouse plants of the same species found in Freddo and Barns and studied where and how much the gold accumulation was, for different concentrations in the soil.
The results showed that foliage of trees at Freddo, over deeply buried gold deposits, and in the case of Barns, the fluid extracted, had gold particles in the same way as in the laboratory. The detailed study also shows the role of other elements, particularly calcium, which participates in the uptake of gold and apparent sequestering of gold to the periphery of the plant, for the protection of the plant. And also the collection of gold in the soil as a result of leaves that fall from the tree. The study, in any case, shows that biogeochimistry is a viable and reliable prospecting tool. “Collectively,” the authors say “these results are important for mineral explorers to consider as they access more difficult terrains with deeper sediments and assess more subtle surficial geochemical anomalies.
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