- Scritto da Matteo Oberto
The biogenic cycle of gold
The combined action of different species of bacteria in some types of surface and deep environments can allow the formation of gold granules or the increase in size of granules already present.
Some microbial communities can mediate the gold cycle, which implies that the gold granules can increase in size and weight in different climatic contexts, ranging from sub-tropical, semi-arid, temperate to subarctic. The majority of the identified species live in very thin colonies called "biofilms", preferentially located in the depressions of the golden granules themselves. These infinitesimal thickness levels are often populated by β-Proteobacteria.
The biogenic cycle of gold - from primary mineralization to the surface
Gold is thought to be a stable noble metal in contact with atmospheric agents but its stability, as we will see, is influenced above all by the geochemical conditions of the site in which it is located, which in time and space are very variable (Fig. 1). The exposure of a rocky body to atmospheric weather and chemical processes at the rock-atmosphere interface and the hydrosphere generates over time a soil, also called soil if biological processes occur. The soil is divided, from above towards the rocky substratum on which it rests, in horizons:
O: horizon rich in organic matter (presence of organic complexes);
A: horizon in which the infiltration of surface fluids generates an impoverishment in some elements (leaching);
B: a horizon in which the infiltration of fluids enriched by the leaching processes generates an enrichment in some elements (illuviation); it may contain naturally enriched portions of gold both infiltrating from more superficial positions and rising from capillary portions of the underlying layer. An enrichment plume can be observed, composed of gold nanoparticulate, granules up to nuggets or more generally from gold in solution in gold complexes (thiocomplexes or organic complexes).
C: horizon close to the rocky substratum in which fragments of the same are denoted in chemical alteration and mechanical disintegration; the detectable fragments can reflect the presence of a primary mineralization in the substrate; it may contain naturally enriched portions of gold both infiltrating from more superficial positions and dispersing from the surrounding primary mineralizations. An enrichment plume can be observed, composed of gold nanoparticulate, granules up to nuggets or more generally from gold in solution in gold complexes (thiocomplexes or organic complexes).
R: substrate, a portion in which primary mineralization may be present; typically, if not fractured, it acts as an impermeable barrier, preventing the more superficial fluids from penetrating deeper.
The aqueous fluids infiltrating tend to descend into the porous soil crossing O, A, B, C until stopping over R. The height of the accumulated water column is variable in time and space and depends on many factors, including the climate, rainfall, groundwater management, etc.
It is also important to clarify that in the soil not all the portions are rich in oxygen and there could instead be a very low contribution, making such portions subject to anoxia (lack of oxygen). The presence of biofilms in these contexts must exploit other metabolic processes that do not involve oxygen (anaerobic processes).
Figure 1. The biogenic cycle of gold, ie solubilization and transport from primary mineralization, bioaccumulation through plant activity, reductive biomineralization and formation of secondary gold in the B and C horizon (Figure 2). These theoretical aspects have a practical implication, in fact they help the mineral prospectors to find new gold deposits and to provide new approaches to mineral processing (Zammit et alii., 2012). Note that gold is not only found in the environment in solid form (straws, granules, nuggets, etc.) but also in aqueous solutions, in the form of complexes. In such liquid substances it is impossible to see gold with the naked eye. Gold is typically in solution in the surrounding portions of the root system (which draws moisture from the primary dispersion plume) and goes up through the capillary roots along the stem, up to the crown. The fall of the leaves and branches containing gold in ultra-traces and their decomposition may over time generate an additional plume of dispersion called secondary.
As denoted in Figure 1, gold is not only dispersed in the portions surrounding primary mineralization but can also be concentrated and set in motion by the presence and activity of bacterial colonies (Figure 3). Some of them tend to bring it into solution, since in the surrounding environment they release sulfur or organic compounds, which could also already be present without direct bacterial activity. Solid gold is very susceptible to contact with organic fluids or rich in nitrogen or sulfur and passes into solution becoming part of the liquid solution.
In the case where there is a high trunk vegetation, which has a vast and deep root system (some Australian eucalyptus species reach 30 meters deep with roots!), It is possible that during its metabolic cycle it absorbs infinitesimal particles of gold, contained in aqueous fluids. The roots absorb the moisture in the surrounding sediments and with it the gold contained in the complexes, so the plant and the crown (the ultimate destination of the sap) are enriched, over time, of the noble metal if it were present in the surrounding sediments.
The death of the plant or the annual change of the foliage can over time locally enrich the horizon O adjacent to gold, which is remembered that it has a nanometric size and is present in ultra traces. This horizon is very rich in organic compounds and the bacterial activity is strong and the gold present passes quickly in solution or precipitates locally only for short intervals before being recirculated. Thanks to the infiltration of rainwater, the fluids that contain the auriferous complexes tend to infiltrate, passing through the horizon A and B. It generates a plume of dispersion that has as its fulcrum the portions adjacent the tree and plunges towards the substrate following the flow direction of infiltrating precipitations.
The biogenic cycle of gold - from the surface to the horizon B and C: the genesis of the nuggets
In the B and C horizon, the fluids containing gold can come into contact with bacterial colonies, which thanks to their sensitivity towards the presence of some metals in the surrounding environments try to take advantage of them (metabolic use) or to reclaim their presence (toxicity) of the complexes for their metabolism). Some known bacteria are Cupriavidus metallidurans, Delftia acidovorans and Salmonella typhimurium; they have developed biochemical responses suitable for the fixation of gold particles by highly toxic complexes. These bacterial species are sensitive to the presence of gold in the surrounding fluids, in fact it is typically very harmful for their metabolic activity and therefore in response they generate effective resistance mechanisms such as the excretion of siderophores, which reduce the gold complex, allowing precipitation in the immediate vicinity (portions marked in red in image 2). The siderophoric substances catalyze the biomineralization of nano-particulate gold with a typically spheroidal morphology. Note that this is the size of the fixed gold that is invisible to the human eye and only the sum of these processes over thousands, if not millions of years, can give a remarkable result, with the production of nuggets both for summation on granules pre-existing that without this precaution. The passive growth of gold over time can occur both in the immediate vicinity of primary mineralization (a few meters) and in distant portions (50-100 meters).
Figure 2. Model of processes responsible for (trans) formation of gold grains in supergenic environments:
Along the longitudinal profile at the maximum slope of the slope it is noted that there are two main plumes of dispersion of the gold, the primary and most important has its fulcrum where the primary mineralization emerges and continues along the regolith. Here we also find physical fragments of primary mineralization and potentially also gold in the solid and crystallized state coming from the mineralization itself. The secondary plume has its own fulcrum in the space surrounding the tree, in fact, its metabolic cycle indirectly leads to the gold surface in ultratraces, then dispersing it in the environment, it tends to infiltrate with the percolating fluids as a typically organic complex.
The portions highlighted in yellow, present in the soil, highlight the presence of gold even in complex fluids, while the portions highlighted in red, show spatial positions in which the bacteria accumulate the gold from the surrounding auriferous complexes and precipitate it form occasional enrichment zones, with the genesis of granules and nuggets.
Applications in the mining sector
The toxicity of the gold complexes favors the development of specialized biofilms on the gold grains present, and therefore the gold cycle in surface environments, including soils (horizons O, A, B, C). The discovery of specific microbial responses to the presence of gold can guide the development of geobiological exploration tools (eg gold bioindicators and biosensors). Bioindicators would employ genetic markers from soils and groundwater to provide information on gold mineralization processes, while biosensors would allow field analyzes of gold concentrations in sampling media.
Returning to a more general picture, bacteria, archaea, fungi and algae play a fundamental role in driving cycles of carbon, nitrogen, sulfur and phosphorus as well as many cycles of other metals (Ehrlich et al., 1998 & 2008).
With regard to metals, cycles can be driven directly by micro-organisms because:
- They can require metals, as micronutrients, for their cellular growth;
- They may be able to obtain metabolic energy from breathing or from oxidation and reduction of metals;
- They can offer extensive capacity for metal detoxification from liquid solutions (Ehrlich et al., 1998; Southam et al., 2005; Reith et al., 2008; Gadd et al., 2010). The microorganisms also influence the metal cycles in an indirect way, in fact because of their metabolic rates they can control the geochemical parameters (pH and redox conditions). Their activity concerns the formation, secretion and decomposition of complexing ligands (high molecular weight organic acids, siderophores, hexopolymers, cyanides and sulfur compounds) in soils with for example regulatory materials, unconsolidated sediments even in the presence of surface waters and underground (Ehrlich et alii., 1998; Reith et alii., 2008).
The biogenic cycle of gold in surface environments - Silver extraction and nanoparticulate gold precipitation
An example of a biogeochemical cycle of a metal, which until recently was considered inert, immobile and not biologically active in terrestrial surface conditions is that of gold (Figure 1; Reith et al., 2007; Southam et al. , 2009). In the past, the formation of secondary gold in surface environments was also considered mediated exclusively by abiogenic processes (eg Hough et alii., 2007). Instead, according to Hough et al. (2007) the gold nuggets found in surface environments are of bacterial origin (Figures 1, 2, 3 and 4).
The most accredited and disseminated theory so far sees the presence of gold in surface environments as influenced by the action of atmospheric agents on the rocks that host primary mineralization (mechanical disruption and chemical alteration) and the distribution of the noble metal is linked to physical reconcentration and mechanical accumulation (eg Hough et alii., 2007). The weak forces of attraction act when the various gold granules are very close to each other forming new typically larger sizes. According to this theory it is mainly the combined action of the transport and the very close positioning of the gold granules to provide the possibility for them to form a smaller but larger number. To better imagine the genesis of nuggets with this theory imagine a ball of snow that slipping along a snowy slope becomes a ball of ever larger size.
Furthermore, on the most external levels, which are in contact with the external environment, we can see very pure gold coatings (up to 99.9% in content). They can be interpreted as the result of the chemical mobilization of silver from the gold-silver primary alloys (Hough et al., 2007) due to the greater susceptibility of silver to pass into solution. In this case, gold increases its passive content (figure 3a). Obviously, the levels considered are those closest to the surrounding environment (the most external ones) since chemical reactions occur at such sites, which act by extracting the silver present in the alloy.
Gold can also pass into solution (Au + 1, Au + 3, against solid gold Au0) in the form of complexes (figure 3b). This tends to take place more in wet seasons and in any case in the presence of the components necessary to form the complex (organic matter: organic complexes, sulfur, thiocomplexes, etc). In the arid seasons the same complexes tend to shrink due to the decrease in water in the subsoil in the sites where they are present (it always speaks of superficial portions of the soil), this causes the precipitation of the gold in nano-phases (nano-particles and slabs gold nano-particulate matter of about 200 nm). These reactions are accompanied by the formation of evaporative minerals, for example, barite and halloysite (Hough et alii., 2008 & 2011).
Figure 3 (a, b, c). Different situations show information about the processes that have taken place:
- Biofilms tend to reduce the size of the gold granule, using the noble metal for their metabolic processes and disperse it in the surrounding environment as waste bound to organic complexes.
- Biofilms are not present, the silver present in the alloy tends to be removed over time by alteration. The outer levels tend to passively enrich themselves with gold.
- Biofilms present in the depressions generate polymorphic layers and gold particles over time on the surfaces of natural secondary gold grains.
The biogenic cycle of gold in non-superficial environments
While abiogenic processes play an important role in the surface gold cycle, recent research has shown that microbiotics can also be involved in every phase of the biogeochemical cycle of gold, from the formation of the primary mineralization in the deep subsoil to its solubilization, dispersion and reconcentration as secondary gold in surface environments (Figure 1; Reith et alii., 2007; Southam et al., 2009).
Bacteria and archaees are ubiquitous in deep subsoil up to several kilometers deep in preferentially fragile and permeable structures and generate colonies in porous sedimentary and metamorphic sedimentary rocks (very low metamorphic grade) and seem to contribute to the formation of mineral deposits (Gold et alii., 1992; Fredrickson et alii., 2006; Fry et alii., 2008; Reith et alii., 2011). They could play a fundamental role in the localized enrichment of the following elements: iron, fluorine, manganese, calcium, magnesium, potassium, sodium, trace metals and ultra-trace (silver, molybdenum, chromium, copper, nickel, palladium, selenium, tungsten, vanadium, uranium and gold). It is thought that also mercury, carbon and zinc in the earth's surface and in some crustal environments are controlled by microbial processes (Ehrlich et al., 1998; Gadd et al., 2010; LIoyd et alii., 2003; Reith et alii., 2007).
In a recent study, Tomkins (2013) suggested that microbial processes may have had a markedly greater influence on the formation of orogenic gold deposits as previously believed. His study indicated that interactions between tectonic processes and the biosphere may have led to changes in the global geochemistry that generated more suitable conditions for the absorption of gold into sedimentary pyrite. For example, suitable anaerobic and heterotrophic bacteria are active for 3.5 billion years to reduce the sulfate and thiosulphate into hydrogen sulphide (H2S) and release the latter as a by-product of the metabolism. Some bacteria, such as the species Desulfovibrio spp., Are able to reduce thiosulfate from mobile gold thiosulfate complexes; this destabilizes the gold in solution, which then tends to precipitate in an intracellular position or be incorporated in the newly formed sulfide minerals, for example the sedimentary pyrite (Lengke et alii., 2006 & 2017). This allows the formation of potential sedimentary sequences useful as mother rocks (host roks) from the metallogenic point of view, which are the ideal source rocks for low-medium temperature hydrothermal auriferous deposits. The catalyzed enzymatic precipitation of gold was also observed in thermophilic and hyperthermophilic bacteria (temperature up to 200 ° C) and archaea (for example, Thermotoga maritime, Pyrobaculum islandicum), and their activity led to the formation of gold alloy and silver in New Zealand thermal spring systems (Jones et alii., 2001).
Considerations on the species of bacteria involved
Iron and sulfur-oxidizing bacteria (e.g., Acidithiobacillus ferrooxidans, A. tiooxidans) are known to oxidize sulphide minerals which host gold in primary mineralization zones and therefore indirectly lead to the release of the associated gold in the process (Figures 1 and 2). These and other bacteria produce thiosulphate, which is known to contribute to the mobility of gold by forming water-soluble complexes stable with gold (Etschmann et al., 2011). Other microbial processes, for example the excretion of low molecular weight organic acids and cyanide, can guide the solubilization of the gold in the sediments in which it is dispersed (figure 3a). A characteristic of the IB group metals, such as gold, is their ability to bind strongly to organic matter, and gold has been shown to readily form complexes with organic ligands (Vlassopoulos et alii., 1990; Gray, 1998) (Figure 1). The interaction of gold and organic matter mainly involves electron donor elements, for example nitrogen, oxygen, and in particular sulfur-containing groups (Reith et alii., 2007). The cell walls of microorganisms contain large amounts of highly reactive thiol groups that mediate metal uptake (Reith et alii., 2007). This causes micro-organisms to be at the center of an accelerated precipitation of gold in environmental systems compared to less reactive mineral surfaces (Figure 3c) (Fairbrother et al., 2012). Therefore, a large number of studies using a number of environmentally relevant auriferous complexes demonstrated the ability of many groups of microorganisms to passively and rapidly accumulate gold complexes (eg (Reith et alii., 2007). certain numbers of bacteria and archaea are also able to actively catalyze the precipitation of toxic gold complexes (Reith et al., 2007). Precipitation due to reduction processes of these complexes, can improve the survival rate of bacterial populations that I'm able to:
- Obtain metabolic energy using gold complexing ligands (eg A. ferroossididans thiosulfate);
- Detoxify the immediate surrounding cellular environment by detecting, expelling and reducing the gold complexes (for example, Salmonella typhimurium, Plectonema boryanum and C. metallidurans (Checa et al., 2007; Lengke et al., 2006; Reith et alii., 2009).
C. metallidurans has been found on biofilms that form on gold grains from Australian sites located in moderate and humid tropical climate zones, indicating that bioaccumulation of gold can lead to the biomineralization of gold through the formation of "bacteriomorphic" gold. secondary (Figure 3c) (Reith et alii., 2006). Also the formation of secondary octahedral gold crystals from gold chloride solution was promoted by a cyanobacterium (P. boryanum) through an intermediate amorphous-sulfide gold (Lengke et al., 2006 a & b). Secondary and bacteriometric gold is common in deposits of quartz pebble conglomerates, such as Witwatersrand, which is one of the world's largest and most exploited gold districts (Mossman et al., 1985; Frimmel et al., 1993). In this deposit, gold is commonly associated with the organic bituminous matter of presumed microbial origin. Falconer et al. (2006) and Falconer & Craw (2009) provided further evidence that geobiological processes play an important role in the formation of primary gold deposits by showing that carbonate pebbles within a detrital sedimentary sequence in New Zealand contain granules of gold of detrital origin and gold of secondary origin showing leaf-like bacteriomorphic morphologies. Gold has a porous morphology similar to a sheet and is comparable to the gold associated with the carbonaceous material found in Witwatersrand. In addition, autogenic sulfides (sedimentary pyrite) in New Zealand are similar to other sulfides found in similar contexts and sulfur-measured isotopic ratios indicate biogenic origins (Falconer et al. (2006); Falconer & Craw (2009).
Figure 4. General morphology of biofilms, single cells and associated gold biominerals.
(a) polymorphic layer containing gold nanoparticles on the surface of a gold granule depression;
(b) biofilms showing nanowires in the gold granule depression;
(c) spheroidal secondary gold which is formed in the depression of the gold granule (Fairbrother et alii., 2013).
The bacterial activity therefore opens new substantial frontiers from the scientific and applicative point of view.
From the point of view of primary mineralization, some bacterial species are able not only to make the solution transport of gold possible, but also to make it fall on the spot, forming important concentrations over time. The presence of particular bacterial species also tends to maximize these processes and positively affect the system.
Once the primary mineralization remains exposed to the surrounding environment it is not only the mechanical disruption and therefore the dispersion of the minerals containing gold in the environment, but also the bacterial activity that can involve for example the pyrite, releasing the noble metal present.
The path of the gold dispersed in the environment is punctuated by a series of events that potentially put the bacteria in contact with the granules themselves or the fluid auriferous complexes, in these cases the bacterial species can:
- Identify the presence of gold in the surrounding complexes (sensory activity);
- Proceed to the excretion of siderophoric substances which allow gold to precipitate and become solid. In this situation the bacteria create a gold-free environment in complexes in order to decrease the environmental toxicity which is fundamental for their metabolic processes;
- Assimilate the complexes to gold in order to use the components for their metabolic activities, the gold would precipitate both inside the bacterium and outside following the metabolization of the complex that keeps it in solution;
- Assimilate the complexes to gold in order to use the metal itself for its metabolic activities
in this case, gold precipitates as reduced by bacterial activity or is subsequently managed as waste.
The illustrations shown in the article are from the repertory of the primary author Oberto Matteo.
- Reith F., Brugger J., Zammit C., Nies D., Southam G. Geobiological Cycling of Gold: From Fundamental Process Understanding to Exploration Solutions. Articolo opensource disponibile in formato originale su: http://www.mdpi.com/2075-163X/3/4/367/htm. Pubblicato il 4 November 2013 su Minerals 2013, 3(4), 367-394; doi:10.3390/min3040367.
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