The gold that grows on trees, a new way for mining29/01/2020
Trees such as eucalyptus, with deep and deep roots, can transport gold from the subsoil to its leaves, thus pointing the location of mineral deposits. Credit: Oat Phawat
DORY GASCUEÑA LÓPEZ | Tugsteno
Mining, an industry with thousands of years of history and fundamental for the economic development of many regions of the planet, has discovered a new language to understand what happens in the subsoil without having to drill. The key lies underground, but also in the surface vegetation or even in the snow, which are capable of reacting to the minerals located below the surface and thus reveal the presence of deposits of precious minerals such as gold.
In fact, the roots of trees and shrubs are the best connection between what happens in the soil and what we see on the surface. This has been demonstrated by a team of researchers from the Australian scientific agency CSRIO led by Melbyn Lintern, who based their study on eucalyptus leaves from various areas of the Kalgoorlie region (Australia) and others grown in greenhouses. Eucalyptus grow in very diverse landscapes and their roots can extend up to 40 metres deep. In 2013, Nature Magazine published confirmation of the origin of the tiny particles of gold found in the leaves of these trees. A miniscule part of this precious metal dissolves as ions in the water that the roots absorb from the soil. Being a toxic mineral for plants, they trap it in small calcium oxalate crystals, similar to kidney stones in humans and mammals, to prevent them from interfering with normal cell function.
The roots of trees are able to absorb the gold particles dissolved in the subsoil water, transferred there from the oldest to the youngest sediments. Credit: CSIRO.
Acacias and termites
Following the premise of "reading the landscape" to understand the chemical processes in the subsoil, Ravi Anand and other CSIRO researchers collected samples of sediments, soil and acacia leaves at the Australian Moolart Well mine with the intention of reconstructing the biochemical process of gold formation, and thus explain how it led to the appearance of this mineral in the oldest sediments, originating millions of years ago in a more humid climate.
According to their findings, gold would have been transferred from the underlying gold-bearing rock to younger sediments thanks to phenomena such as landscape erosion, floods or the intervention of animals and humans. Samples from termite mounds and acacia leaves have revealed that gold is preferentially absorbed by living material, making it more likely to be found in areas rich in organic carbon.
Samples of termite mounds also show the presence of mineral residues, indicative that digging under them could lead to veins of gold. Credit: Sssnole
Snow also leaves traces
At first glance, finding clues about what happens underground in a snowy landscape may seem more complicated, but it turns out that the snow also leaves chemical clues about the composition of the subsoil. In British Columbia a province in western Canada full of mountain ranges, the non-profit society Geoscienece BC is investigating new analytical methods to discover mineral resources that also follow the sustainable line of the Australian experiments.
The decomposition of soil through microbial processes and the breakdown of rocks causes different chemical elements to move to the earth's surface, as the earth degasses in a constant process by which certain gases are released to the surface. In this upward flow, chemical elements such as halogens (fluorine, chlorine, bromine...) are released and remain trapped in the vegetation or the soil, and also in snow water.
To capture these halogens artificially, the researchers placed plastic bags on branches and other parts of the foliage and soil for months. Catching the "sweat" from the trees revealed that these halogen elements were passively absorbing the gold ions. Analysing samples of melted snow confirmed the process, as water can intercept and store certain halogen elements, particularly bromine and iodine.
Although several recent experiments in very diverse regions of the world have finally confirmed this fascinating mechanism by which trees 'store' gold, it has taken science a long time to reach a consensus. This idea of mineral transmission from soil to trees has existed since the 1940s, but evidence was lacking to confirm that the particles of gold or other minerals were indeed coming from below the surface and had not merely been blown onto the leaves, as some researchers had thought.
While this new way of analysing the subsoil does open the door to mining, it is important to understand that the trees themselves are not the "goose that lays the golden eggs." The level of metal contained in the leaves of the trees is so insignificant that Melvyn Lintern, head of the experiment published in Nature, says that it would take about 500 trees to make a wedding ring. However, it is a promising technique for detecting deposits and, according to The Economist, by spring 2019 it had already been tested commercially in Australia. In addition, scientists believe that it can also be used to find metals such as cooper or zinc.
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