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

The demonstration of absorbed Au in vegetation in Eucalyptus foliage from Freddo and Barns is a crucial result in our understanding of Au mobilization. Identification of such naturally low concentrations of Au has not been attempted before but by carefully selecting samples (using the sequential procedure) with the highest concentration and using XFM–Maia, we maximized the probability of being able to achieve the observations. The larger natural Au particles appear to occur in an irregular shape similar to experimentally produced Au and, furthermore, the association of Au with Ca oxalate crystals was similar for natural and artificially produced specimens. Although Ca oxalate is principally involved with the regulation of Ca in the plant16, it has been implicated in toxic metal compartmentalization17. Gold is probably toxic to plants and is moved to its extremities18 (such as leaves) or in preferential zones19 within cells in order to reduce deleterious biochemical reactions. Gold toxicity is not confined to plants: recently, a biochemical mechanism for extracellular precipitation of Au by a bacterium was described20,21.

The findings from these field sites and laboratory experiments enable us to propose a tentative model for Au mobilization and precipitation for this biotic–abiotic system. We assert that climatic and biological factors at Freddo, Barns and elsewhere are crucial for the transportation of Au to the surface from great depth. The Eucalyptus trees are part of woodland at Freddo and at Barns stretching over several thousand square kilometres. Both prospects are in semi-arid climates with annual average rainfall of 260–290 mm, evapotranspiration rates in excess of 2,600 mm per annum and average maximum January temperatures of 34 °C. Despite these harsh climatic conditions, it is significant that the large trees at Freddo, in particular, are able to thrive. The Eucalyptus root system is unusually deep and extensive22 with sinker roots in one species (Eucalyptus marginata) documented at 40 m depth23 (Fig. 2) which, along with other physiological and anatomical adaptations, enables some species to flourish in an arid environment and be drought tolerant. For example, Eucalyptus species commonly have small, tough, vertically hanging leathery leaves comprised of hard sclerochyma tissue enabling them to resist wilting24.

The link between abiotic and biotic geochemical processes at the earth surface is becoming demonstrably and irresistibly stronger. Our combined data indicate that there is a significant but irregularly distributed mass of Au within the tree derived from 35 m below the surface, which is released and accumulates in the soil after leaf fall and when the tree dies. Calcrete (pedogenic carbonate or caliche) is a common component of the soil in southern Australia and provides an inorganic testimony for this organic activity25. The phenomenon of Au accumulating in calcrete has been well documented, primarily in Australia10,26,27. Calcrete and Eucalyptus woodland growing in semi-arid areas of southern Australia are commonly associated28. Samples taken from the soil profile near Freddo illustrate the typical strong relationship between Au and calcrete in Eucalyptus woodland found elsewhere (Fig. 5). The correlation between these two fundamentally different elements (Ca and Au) in calcrete is due to their similar physical response to rainfall and evapotranspiration (rather than a chemical response) averaged over thousands of years10. Calcrete is an evapotranspiration product, in part, of the trees rather than a specific adaptation by the trees to grow in alkaline soils29 and is commonly intimately associated with plant roots, for example, as calcareous rhizoliths30. Calcrete’s calcite principally originates from inputs of meteoric (Ca, marine31) and respiration (CO 2 , microbes and plant roots32) origins. Soil moisture is required for Au and Ca to be mobilized and water-soluble (ionic) Au itself is nearly twice as high in calcrete (15%) compared with other near-surface regolith materials (8% (ref. 33)). The Au association with calcrete has been widely exploited by the mineral exploration industry and many operating mines and prospects owe their existence to exploration targeting the Au in calcrete correlation34. Interestingly, most of these have been in Australia.

Our preliminary model for the biomediated formation of abiotic and biotic Au anomalies in Eucalyptus woodland is tempered by climatic processes. Gold at the woodland surface (in plants and soil) is supplemented through deep-rooted trees that absorb and transport Au from the buried deposit. It is translocated to the trees from the deposit during times of high water demand when sinker roots access deeper regolith moisture22. Then, ionic Au is transported in water through the vascular system of the trees at sub-toxic concentrations then reduced (to Au0) and precipitated within the cells of the tree. Gold occurs throughout the trees but the highest concentrations are found in the foliage where hydrostatic pressures fluctuate greatly. Gold crystals may grow as a result of the propensity of ionic Au to reduce and autocatalytically precipitate35. Gold is released from trees through leaf fall, leaf exudates, bark shedding, limb/branch loss and plant death and is transferred to soil. It may be dissolved in the soil and adsorbed through the lateral roots of understorey plants and trees. Plant debris (from trees and shrubs) contributes to degrading organic matter and creates subtle Au anomalies in the soil. Finally, Au is transported laterally and vertically in the soil (including those dominated by calcrete) through processes of physical erosion (litter and soil) and chemical dispersion36. Gold (and calcite) is precipitated with calcrete when moisture is removed by evapotranspiration but some is re-absorbed and precipitated in the plants. Micro-organisms37 and fungi38 are implicated in the mobilization and intra- and extra-cellular precipitation of Au in the rhizosphere. A dynamic equilibrium in the Au flux occurs for the soil-vegetation-deposit system although absolute concentrations in soil and plants are determined by other factors. For example, random dispersive environmental events such as wildfire, flood and strong winds serve to lower the flux so that Au brought to the surface by deep-rooted trees will no longer be able to sustain the soil Au concentrations at a sufficient rate to counteract erosional losses.

We have considered alternative mechanisms of Au transport operating at depth, beneath cover but have found these to be unconvincing in these settings2. For example, the water tables at our study sites occur at about 30 m which reduces the possibility of transport of Au nanoparticles via gas bubbles39. The deposits are located within the tectonically stable Archean cratons and the paucity of recent tectonic activity to create faults suggests that seismic pumping of groundwater or barometric pumping of gases is unlikely to bring Au up to the trees40. Hydraulic lift by deep-rooted vegetation is a compelling mechanism of transport41. It not only provides an explanation for the Au adsorbed by the trees but also the existence of the Au in calcrete anomaly beneath them.

Despite the decline in discoveries1, falling ore grades and increasing demand for Au42, new exploration technologies for Au deposits, incorporating the deep penetrating ability of certain trees, have been seldom reported43. Analytical problems, extremely low concentrations and variable data in comparison with traditional soil sampling have held back the use of vegetation for Au exploration4. Our results add to the mounting evidence that demonstrate enrichment of Au in biota and soils are inextricably linked and that disparity between biotic and abiotic processes at the Earth’s surface is, at best, indistinct44. Mineral exploration will benefit by embracing and understanding mechanisms of the biota-mineral-climate continuum and will be important in finding new Au deposits undercover into the future.