Day 82, Gold Grows on Trees, it is not a myth is true, Part I

Researchers in Australia found that the roots absorb mineral and as they grow, they take up their leaves

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According to a study published in Nature , in Australia there are trees whose leaves grow literally gold particles.

Scientists conducted an expedition in the Goldfields – Esperance region , one of the , famous for hosting large deposits of gold secrets most arid and unpopulated areas of Australia .

The aim was to see if the trees growing in the region could act as indicators of land with gold deposits. The result was surprising , is that eucalypts growing in these places microscopic gold particles present in their leaves.

“Gold is probably toxic to plants , and so they moved to their ends (toward the leaves) to reduce the effect of possible biochemical reactions. ( This is ) the first proof of the existence of gold particles within the tissue of a living species , “explains the study of Nature.

The gold content in these eucalyptus leaves is only 46 parts of gold per 1000 million, however there lies no importance.

Mining companies could reduce their costs of exploration, because the trees would be a clear indicator of the existence of gold deposits.

Let´s see now the study published in Nature:

Natural gold particles in Eucalyptus leaves and their relevance to exploration for buried gold deposits

Melvyn Lintern, Ravi Anand, Chris Ryan & David Paterson

Eucalyptus trees may translocate Au from mineral deposits and support the use of vegetation (biogeochemical) sampling in mineral exploration, particularly where thick sediments dominate. However, biogeochemistry has not been routinely adopted partly because biotic mechanisms of Au migration are poorly understood. For example, although Au has been previously measured in plant samples, there has been doubt as to whether it was truly absorbed rather than merely adsorbed on the plant surface as aeolian contamination. Here we show the first evidence of particulate Au within natural specimens of living biological tissue (not from laboratory experimentation). This observation conclusively demonstrates active biogeochemical adsorption of Au and provides insight into its behaviour in natural samples. The confirmation of biogeochemical adsorption of Au, and of a link with abiotic processes, promotes confidence in an emerging technique that may lead to future exploration success and maintain continuity of supply.

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Mineral explorers need to find new ore deposits. New Au discoveries are down by 45% over the last 10 years1. Novel exploration techniques are required to find the more difficult deposits hidden beneath sediments2. Furthermore, these new techniques need to be underpinned by an improved understanding of mechanisms of metal mobilization that are currently poorly understood3. Biogeochemistry for mineral exploration is one such relatively new technique, but defining the boundaries as to when and where it may or may not be effective has been difficult4. One of the major technical problems facing research involving natural samples is that Au concentrations in vegetation are commonly very low (<1–2 p.p.b.)56. There is no unequivocal evidence that Au is actually absorbed by plants over mineral deposits and that currently measured Au is not merely a result of dust contamination of samples78. Thus explorers are reticent to embrace an important emerging technique in which they lack confidence in its applicability, data quality and interpretation of results.

The Freddo Gold Prospect is an ideal location to investigate potential mechanisms for the chemical transport of Au. It is located 40 km north of Kalgoorlie (Western Australia) in the Yilgarn Craton and lies beneath a gently sloped drainage catchment with no known sources of surface contamination: It is undisturbed by mining activity, there is no outcropping mineral deposit and the Au itself is concealed by ~30 m of barren, Cainozoic alluvial and colluvial deposits infilling a palaeovalley (Fig. 1)9. The mineral deposit at Freddo is a sub-economic supergene type measuring 100 m by 200 m at 35 m depth, discovered by exploratory drilling. The simplified regolith profile at Freddo consists (from top to bottom) of a red calcareous kaolinitic clay soil (2 m thick), grading to a mottled red and pale grey kaolinitic clay (12 m), grey smectitic clay (10 m), quartzose fluvial sand (8 m) overlying a saprolite of greenish brown smectitic clay containing Au (Fig. 2). Fresh rock occurs at about 40 m depth. Large Eucalyptus trees (some >10 m in height) grow over the deposit in an open woodland setting and make it an ideal study site to examine the uptake of Au.

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At Barns Gold Prospect (25 km north of Wudinna, South Australia), previous studies have shown that vegetation samples, soil and calcrete have anomalously high Au contents10. Like Freddo, Barns is an undisturbed site well-suited to Au mobilization studies. The Barns Au deposit lies within Archean rocks of the Gawler Craton that have been weathered to 50 m depth11. The Barns regolith consists of saprolite overlain by up to 8 m high aeolian sand dunes that have developed over the last 20,000 years10. Gold is patchily distributed in the saprolite and there is a sub-economic Au supergene deposit (500 m by 200 m) 35 m below the dune. Calcrete within the top 2 m of the saprolite beneath the dune contains anomalous Au concentrations (35 p.p.b.) against a background of <1 p.p.b. Younger calcrete, intimately associated with plant roots (rhizoliths), occurs within the sand dune, and has an anomalous Au content (up to 9 p.p.b.)10. Small Eucalyptus trees (mostly <5 m in height) grow in the sand dunes over the deposit.

The precipitation of nanoparticulate Au from laboratory studies, using a variety of biological, physiological and chemical procedures, are widely reported in the literature12 and nanoparticulate Au has been documented in inorganic environments13. Greenhouse and laboratory experiments utilizing the uptake of Au by plants for nanoparticle production are frequently undertaken. These experiments typically use concentrations of Au much higher than that is found in the natural environment. However, there have been few reported greenhouse experiments undertaken to investigate Au uptake of plants commonly used in mineral exploration. Plants that have a natural barrier to Au uptake would negate their use in biogeochemical exploration4 as the Au concentration in a plant would be unrelated to the Au concentration in the regolith in which it was growing.

Here, native Australian Eucalyptus and Acacia seedlings were grown experimentally under greenhouse conditions in sand pots dosed with Au to investigate the location and nature of nanoparticles. We collect samples from two field sites (Freddo and Barns), conduct greenhouse experiments and complex analytical procedures to investigate the nature of Au variability and biotic–abiotic-linked mechanisms for Au mobilization and precipitation. A Eucalyptus tree investigated over the deeply buried Au deposit at Freddo has Au particles forming in its foliage with similar features to those grown in our laboratory experiments, whereas leaves at Barns demonstrate the release of Au-containing exudates. Collectively, 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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In order to investigate the possibility of particulate Au in the Eucalyptus foliage, a three-stage sequential experimental procedure was adopted. First, ten foliage samples, consisting of leaves and twigs, were each collected from around the canopies of two trees, one over the deposit (RD50) and one over background, 200 m distant (RD68) (see Fig. 3 for location). Leaves (100 g sub-samples), separated from each of the ten foliage samples over the deposit, were analysed and had highly variable Au contents ranging from 1 to 68 p.p.b. (RD50) and 1 to 16 p.p.b. in background sample RD68 (Supplementary Fig. S1 and Supplementary Table S2). Second, variable Au concentrations (5–359 p.p.b., mean 46 p.p.b. and standard deviation 64 p.p.b.) were noted again when 20 individual leaves were randomly selected from bulk sample RD56 (see Fig. 3 for location), divided into three leaf parts (n=60) and separately analysed (Supplementary Fig. S2 and Supplementary Table S3). Third, a further 20 leaves were randomly selected from bulk sample RD56. Sixty leaf discs (diameter of 6 mm) were punched out from the 20 leaves and analysed using the synchrotron X-ray Fluorescence Microprobe (XFM) equipped with a Maia detector14 to maximize the probability of finding Au particles and to map the distribution of other elements (Supplementary Figs S3 and S4). High-resolution (1 μm) mapping of the discs identified several Au particles up to 8 μm in length indicating that Au had precipitated naturally within the leaves (Fig. 4a,b). The XFM–Maia combination is particularly well-suited to penetrate organic matrices non-destructively, speedily map large areas at high resolution and detect small (1–10 μm) discrete heavy metal particles, for example, Au located within the matrix15. Some Au particles were associated with Ca oxalate crystals (Fig. 4b). This is the first time, to our knowledge, that naturally occurring (non-laboratory generated) Au particles have been imaged within the cells of biological tissue. Gold particles have been imaged adsorbed on the surface of tree bark but these probably have an aeolian origin4.

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