Natural gold nanoparticles

Ore Geology Reviews 42 (2011) 55–61

Contents lists available at ScienceDirect

Ore Geology Reviews j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o r e g e o r ev

Review

Natural gold nanoparticles R.M. Hough a,⁎, R.R.P. Noble a, M. Reich b, c a b c

CSIRO Earth Sciences and Resource Engineering, 26 Dick Perry Avenue, Kensington, Perth, WA 6151, Australia Department of Geology, Universidad de Chile, Santiago, Chile Andean Geothermal Center of Excellence (CEGA), Universidad de Chile, Santiago, Chile

a r t i c l e

i n f o

Article history: Received 28 March 2011 Received in revised form 2 July 2011 Accepted 2 July 2011 Available online 26 July 2011 Keywords: Gold Nanoparticles Colloid High grade Exploration

a b s t r a c t The nanoparticulate gold component in ore deposits and exploration samples is yet to be fully realised but new methods of characterisation and analysis, access to high-grade gold samples and a strong focus on Au nanoparticles in the manufacturing sector provide a new impetus to quantify its significance. In geology, nanoparticles are increasingly recognised as a fundamental step in geochemical reactions, a critical component in weathering processes, in biomineralisation and metal migration. In ore systems, the nanoparticulate population of Au is a significant factor in the formation of economic deposits as a refractory component locked up in sulphides, in secondary supergene enrichments, the formation of surface geochemical anomalies and in the formation of high-grade accumulations. A better understanding and characterisation of this nanoparticulate gold could unlock hidden resources in known deposits through easier beneficiation and processing but also assist in the discovery of new deposits through advanced exploration methods. © 2011 Published by Elsevier B.V.

Contents 1. Introduction . . . . . . . . . . . . 2. Detecting nanoparticulate Au . . . . 3. Gold nanoparticles in hypogene ores 4. Pure gold nanoparticles as secondary 5. Conclusions . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . References . . . . . . . . . . . . . . .

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1. Introduction Colloidal gold (Au) and the resultant nanoparticles that form from colloidal deposition are not new to science, (Ciobanu et al., 2011; Deditius et al., 2011; Faraday 1857) and other early workers recognised their importance and some of their properties. Colloids with different sizes and shapes of Au nanoparticles in suspension lead to different colours of the Au suspensions because of the effect on light scattering in visible wavelengths, this property found early application in manufacturing in the production of stained glass. Today the Au nanoplates and nanoparticles are increasingly being manufactured from gold chloride solutions using reductive reagents, often biological, to form colloidal suspensions and subsequent deposits of Au and even

⁎ Corresponding author. Tel.: + 61 8 6436 8763; fax: +61 8 6436 8555. E-mail address: [email protected] (R.M. Hough). 0169-1368/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.oregeorev.2011.07.003

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Au/Ag nanoparticles (bimetallic nanoparticles). The different shapes of Au nanoplates that can be formed by these processes have been found to vary from triangles, hexagons, octahedra, squares, boxes and spheres to rods. These shapes of particles display unique properties compared to the bulk. The properties, high surface area and reactivity are important parameters in their application to medicine for drug delivery but also in processes such as catalysis in manufacturing. Similarly these are highly significant properties in considering the mechanisms of transport and deposition of Au within both the hydrothermal and supergene (low temperature) environments. Colloidal processes have been suggested as important in the formation of mesothermal lode gold accumulations (Herrington and Wilkinson, 1993), in bonanza style deposits (Saunders, 1990; 1994) and in secondary enrichments in the regolith profiles (Williams-Jones et al., 2009). Few, if any of these studies were able to directly image the nanoparticulate Au components but they correctly recognised this component and process as one that has been little studied. New

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analytical methodologies such as e.g. high-resolution electron (HR-TEM, HR-SEM) and scanning probe (AFM, STM) microscopies, and ion and laser-based mass spectrometry (SIMS, LA-ICP-MS) however are finally providing the imaging and detection capabilities that permit us to actually observe the residence and nature of such trace and nanoparticulate Au components in ore deposits and potentially in exploration samples. Recent successes in direct imaging of Au nanoparticles include within refractory gold ores, where the Au is locked in sulphides such as arsenian pyrite but also in supergene environments where Au nanoparticles reside freely on open surfaces as a result of critical zone metal migration processes. They have even been observed on the surface of alluvial gold grains where they appear to co-exist with organic biofilms that are playing an important role in solubilisation but also reprecipitation of the Au (Reith et al., 2010). We summarise some of the key advances made in the field of Au nanoparticle characterisation and its potential role in the formation of gold deposits and gold anomalism in exploration sample media.

Sample preparation poses significant challenges due to the fact that TEM observations require electron-transparent samples with specimen thicknesses less than 200 nm. Problems with sample preparation for TEM have been successively overcome in recent years by using the tripod polishing method (Palenik et al., 2004; Reich et al., 2005, 2006) and focused ion-beam (FIB) sample preparation techniques (Wirth, 2004). Access to high-grade gold accumulations is key initially in these studies as it is this that allows new developments in imaging the gold. Supergene gold at the Golden Virgin deposit in Western Australia was readily observed by eye (Fig. 1) on fracture surfaces in quartz veins, direct imaging of that gold component revealed the existence of a population of Au nanoparticles, this imaging was only directed there because of the gold accumulation (Hough et al., 2008). This highlights the potential in both hypogene and supergene areas of using large accumulations of gold and its environs in order to test for the presence of various processes and mechanisms that are important in its formation. 3. Gold nanoparticles in hypogene ores

2. Detecting nanoparticulate Au Geochemical analyses by in-situ methods can now detect parts per billion levels of Au in a sample (LA-ICP-MS and SIMS spot) and element mapping developments (LA-ICP-MS map, Synchrotron-XRF, PIXE, nanoSIMS) provide the method to image the Au distribution across a sample (Barker et al., 2009; Ciobanu et al., 2009; Large et al., 2009; Ryan et al., 2010) and in the case of SXRF this can be achieved over large areas at submicron resolution. Having the ability to image the Au distribution and to detect local high concentrations of Au in complex samples, albeit at low concentration, then permits us to focus attention on direct imaging of such trace Au. A major challenge in the past has therefore been addressed by these new developments, previously it would have been a serious challenge to even define where to search for Au accumulations and assumptions of Au residence would have the potential to be highly misleading and time consuming with little chance of success. Once Au accumulations are defined they can now be imaged by field emission gun scanning electron microscopy (HR-SEM) (Hough et al., 2008) and in even more detail by transmission electron microscopy (TEM). Apart from its highresolution imaging capabilities, TEM techniques allow the in-situ determination of crystal structure by using selected area electron diffraction (SAED) and/or by Fourier transformation of high-resolution images, and a qualitative determination of major elemental distribution by means of energy dispersive X-ray spectrometry (EDS).

Volcanic fumaroles produce deposits of nanoparticulate gold (Larocque et al., 2008; Meeker et al., 1991; Taran et al., 2000) and silver on the vent walls and these are inferred to be products of colloidal transport and rapid deposition directly from magmatic vapours. In present day geothermally-active systems such as from New Zealand, vapour deposition of Au has been noted and insertion devices to monitor metal deposition from the gases reported Au being deposited on the walls (Simmons and Brown, 2007). Furthermore, deep geothermal brines of magmatic origin can host up to 15 parts per billion Au, and with an estimated current Au flux of 24 kg/year (e.g., Landolam deposit in Papua New Guinea, Simmons and Brown, 2006), efficient metal precipitation can lead to the formation of a giant hydrothermal gold deposit in a short period (b60,000 years). In the hydrothermal system destabilization of the colloid may occur through boiling of shallower meteoric components by fluid mixing, change in temperature, pressure, pH or physical interaction with mineral surfaces (Saunders, 1990; Williams-Jones and Heinrich, 2005). During transport in some environments, colloidal silica and gold may travel together (Frondel, 1938a,b,) and are stable as a combined colloid up to 350 °C, where the gold is shielded by the silica particles. In such a mixed colloid both silica and gold occur as negatively charged particles and supersaturation leads to the precipitation of amorphous silica when quartz might be the thermodynamically preferred phase, in epithermal and porphyry environments the gold may then co-precipitate with this silica to form gold rich veins. In refractory ores, arsenian pyrite (FeS,As)2 is the most important and in many cases the only Au-bearing mineral in Carlin-type and some epithermal Au deposits (Bakken et al., 1989; Muntean et al., 2011). Because it incorporates Au from solutions that are undersaturated with respect to native Au (Au0), stabilization of arsenian pyrite is the key to formation of many Au deposits (Fleet and Mumin, 1997; Reich et al., 2005). “Invisible” Au can be incorporated in arsenian pyrite in two main forms: (a) as structurally bound in the host sulfide structure (solid solution, Au + 1) (Reich et al., 2005; Simon et al., 1999), and (b) as submicrometer-sized mineral inclusions or nanoparticles (Palenik et al., 2004; Reich et al., 2005). It has been shown that the occurrence of solid solution vs. nanoparticulate Au in arsenian pyrite is closely related to the empirical solubility limit in log(Au)–log(As) space (Fig. 2), CAu = 0:02⋅CAs + 4⋅10

Fig. 1. Optical microscope image of pure triangular and hexagonal secondary Au on a weathered quartz vein fracture surface. Arrows point to the speckled surface texture of the pure Au plates in places.

−5

;

where CAu and CAs are Au and As concentrations in mol%, respectively. In Fig. 2C, the occurrence of nanoparticles of native Au (TEM images, Fig. 2A, B) is restricted to samples with high Au/As (above the line), while samples plotting below the solubility limit (and hence with

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Fig. 2. A, B) TEM images of Au nanoparticles in arsenian pyrite, C) an Au–As plot of analyses of arsenian pyrite from Carlin and Epithermal deposits, samples above the solubility line indicate the presence of nanoparticulate populations. D, E) TEM images of low Au/As ratio areas showing the absence of Au nanoparticles. All images from Reich et al. (2005)

lower Au/As ratios) contain most of the Au in solid solution, and no nanoparticles are observed (TEM images, Fig. 2D, E). Gold nanoparticles in arsenian pyrite generally range in size between ~ 5 and ~10 nm, and are characterized by rounded shapes and well-defined boundaries (Fig. 2A, B) (Palenik et al., 2004; Reich et al., 2005; 2006). Fig. 2B shows a high magnification (4 × 10 6 times) TEM image of an individual nanoparticle, identified as native Au (Au 0) by EDS and Fast Fourier Transformation (FFT) of HR-TEM images (Reich et al., 2006). Detailed TEM characterization indicates that the matrix surrounding the Au nanoparticles is a polycrystalline mixture of randomly oriented pyrite and arsenopyrite domains of ~20 nm 2 (Palenik et al., 2004), consistent with nanoscale exsolution resulting

from high (N6 wt.%) arsenic contents of the matrix (Reich and Becker, 2006). Results from these studies have shown that Au nanoparticles (5–10 nm), identified at concentrations of up to 4 wt.% in arsenian pyrite, are abundant enough to account for the majority of Au present in high Au/As molar ratio samples. Based on Au–As data (Fig. 2C), it can be estimated that up to ~8% of the Au budget in Carlin-type deposits occurs as nanoparticles of native Au. Two main mechanisms have been proposed to explain the occurrence of Au nanoparticles hosted by arsenian pyrite in high temperature (N100 °C) hydrothermal systems: (a) that Au nanoparticles formed by direct precipitation from a hydrothermal fluid into

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Fig. 3. TEM images of Ostwald ripening of Au nanoparticles in an arsenian pyrite host with increasing temperature. Yellow arrows show particles that become dissolved whereas red arrows point to those that coarsen. Scale bar is 100 nm. Images from Reich et al. (2006)

arsenian pyrite, (b) that the Au nanoparticles exsolved from a metastable arsenian pyrite matrix during later stages in the evolution of the deposit (Palenik et al., 2004; Reich et al., 2005). In both cases, the mineralogical form of Au is controlled by the solubility of Au in arsenian pyrite (Reich et al., 2005). The first mechanism requires that the empirical solubility limit of Au in arsenian pyrite is exceeded during crystallization of arsenian pyrite rims. Arsenic-coating in the Fe–sulfide mineral surface facilitates adsorption of Au and formation of clusters of Au atoms that further grow into nanoparticles (Becker et al., 2010 and references therein; Reich et al., 2005, 2006, 2010). In contrast, the second mechanism for the formation of Au nanoparticles involves the exsolution of native Au particles from an Au-saturated, metastable arsenian pyrite matrix (Palenik et al., 2004; Reich et al., 2006). An alternative mechanism involves the coagulation of a suspension of Au colloids (nanoparticles) due to cooling/boiling/catalysis (Saunders, 1990). In the Au Sleeper deposit (Nevada), textural evidence indicates that colloidal Au and silica were initially precipitated at depth, then transported upward and finally deposited due to cooling and/or boiling of the hydrothermal fluid (Saunders, 1990). As temperature plays an important role in many geological and environmental processes, the role of this intensive variable on model nanoparticulate systems needs to be addressed. In addition, special attention to nanoparticle–crystalline host interactions is of critical importance, as very limited information is available about the role of the interfacial energy on nanoparticle stability in natural systems (Becker et al., 2010). Dramatic changes in the thermodynamic behavior of native Au nanoparticles incorporated in arsenian pyrite from a Carlin-type deposit have been documented (Reich et al., 2006), confirming a relevant role for temperature-dependent nanoscale phenomena in geological systems. Using high-resolution transmission electron mi-

Fig. 4. The behaviour of Au nanoparticles in an arsenian pyrite host with temperature, lower curve shows temperature of complete dissolution. This shows how Au nanoparticles dissolve into the arsenian pyrite and do not melt. Figure from Reich et al. (2006)

croscopy techniques coupled to a heating stage, the authors showed that Au nanoparticles encapsulated in the sulfide host (Fig. 3) remained immobile during heating and no coalescence ripening (where two larger particles combine) was observed throughout the duration of the experiment (~45 min). At 550 °C, the upper temperature limit of this experiment, three nanoparticles (~30–35 nm) have survived in the area of observation, replacing the initial 115 particles of average size ~4 nm (Fig. 3). Upon cooling from 550°c back to room temperature, the particle-growth process is not reversed. These results showed that nanoparticulate metals, usually thought to be prevalent in low temperature (b100 °C) aqueous environments, can also occur and remain stable at higher temperature in well-defined temperature-particle size limits (Fig. 4) and provide new insight about metal nanoparticle formation and preservation in natural materials.

4. Pure gold nanoparticles as secondary gold The term “invisible gold” has not just been used historically in relation to hypogene ores but also to weathered environments where secondary gold deposition leads to the formation of supergene deposits. These are dominated by a mineralogy of clays, iron oxides and carbonates. These materials can contain significant gold grades but the gold is rarely observed optically. The gold is often inferred to either be adsorbed to the surfaces of the clays or iron oxides, in solid solution in carbonates or to have occurred as submicron “nuggets” or particles (Freyssinet et al., 2005). Assays of supergene deposit materials reveal that most of the Au is high fineness, i.e., little Ag is present and this indicates the invisible Au is a secondary precipitate and deposited from solution where the primary Ag is now dispersed and separated from the Au based on its higher solubility (Hough et al., 2009). The most common morphologies of the secondary gold particles when the groundwater conditions are saline are triangles and hexagons, with less common forms such as spheres observed in fresher vadose environments. Recent research has directly imaged a natural population of gold nanoparticles in a supergene zone at the Golden Virgin Pit in Western Australia (Hough et al., 2008). This study was initially focused on micron sized Au plates that occur in saline vadose zones but subsequently found smaller, nano sized populations (Figs. 1 and 5). Gold nanoparticles keep the same morphological features as their larger counterparts, being euhedral, face centered cubic, isometric crystals that are anisotropic and oriented with respect to the {111} plane of Au (Fig. 5). The gold nanoplates, nano because they are very thin plates of Au (approximately 10–20 nm thick), display strain textures in the single crystal lattice with bend contours/deformation and high concentrations of dislocations. There are signs that the Au nanoparticles develop growth structures with parallel crystal growth forming chains of gold nanotriangles into a belt and into sheets (Hough et al., 2008).

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Fig. 5. Closer examination of the surface of a weathered quartz fracture shows the supergene nanoparticulate gold population hosted on larger gold particles. A) light microscope image of black and shiny Au triangular and hexagonal crystals; B) FEGSEM image of Au nanoplates; C) a close up of the darker band arrowed in B with nanoparticulate heaxagonal Au (grey hexagons) and halloysite (rods); D) gold nanospheres; E) 20 nm nanospheres arrowed in the background next to coarser Au nanoparticles.

The Au nanoparticles occur with halloysite crystals (Fig. 5) but also intergrown with gypsum. The pattern of the nanoparticles on the surfaces of larger gold nanoplates are reminiscent of drying and suggests the deposition of co-occurring halloysite and gold nanoparticles extends beyond the nanoplate surface and is therefore present on the substrate as well. When viewed optically, the nanoplates hosting nanoparticles display a dappled surface texture (arrowed on Fig. 1), this is the same for a large number of the coarse nanoplates on the weathered fracture surface, suggesting the nanoparticulate gold population has an optical effects on the gold surface and the nanoparticulate population is indeed widespread. Gold nanoparticles are thus a significant fraction of the Au deportment in these samples that was hitherto unknown and certainly not observed. The nanoparticulate Au observed in Western Australian samples range from 60 to 600 nm in size. The ~60 nm particles are clearly

triangles and hexagons, but a smaller population of 10–20 nm particles, just visible in the background of Fig. 5E, appear to be nanoparticulate spheres. The smaller nanometre triangles and hexagons are estimated to be only 4–10 nm in thickness. In a different supergene environment but where groundwater is fresh, nano and micro-particulate Au spheres have recently been observed and are related to the presence of opaline silica (Fig. 5D) (Hough and Noble, 2010). At the tens of nanometre diameter some of these spheres are not just Au single crystals but may in fact be Au coatings of spherical silica particles. Additional populations of micro and nano (N 500 nm) spherules of gold were found on Fe oxyhydroxides in Mali (Hough et al., 2009). Gold bearing particles were found on the rim of kaolinite and illite clays with the particle size of 15–30 nm in the Shewushan supergene deposit in China, but the imaging resolution was 5 nm and the true morphology of these nanoparticles difficult to determine

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Fig. 6. Nanoparticulate Au in its different environs playing an important role in the formation of hydrothermal and supergene gold deposits as well as for surface geochemical anomalies.

(Hong et al., 1999). The bulk of the Au grains were estimated to be b20 nm in this deposit and associated with adsorption to the edge of clays and goethite (Hong and Tie, 2005). The present environment can also actively influence Au nanoparticles. In humid areas supergene Au is concentrated in the saprolite and ferruginous zones, whereas in more arid climes Au can be mobilised and precipitated with carbonates. The tree roots may play an active role in precipitating nanoparticulate Au, although some of the Au is in ionic form in these zones (Lintern et al., 2009). Other macroflora and fauna may also be influential in nanoparticulate Au formation, but this has not been studied. Another influence on the morphology of nanoparticulate Au and the chemical environment is bacteria. Bioprecipitation studies show bacteria are able to mobilize and precipitate Au and as such they can influence the transport and deposition or formation of supergene deposits (Lengke and Southam, 2005; 2006; Mossman et al., 1999). These studies show spherules of Au 5–10 nm in diameter that are proposed remnants of metabolic processes and larger Au nanoparticle spherules (with sub-octahedaral habits) and hexagons of N700 nm (Lengke and Southam, 2006). An Au nanoparticle has been observed possibly residing inside a single bacterium from laboratory experiments (Reith et al., 2009). Some micrometre sized bacterioform Au particles with a few smaller spherules of nanoparticulate size on the outside have been shown on placer gold (Southam et al., 2009). Biofilms living on the outside of gold particles change the morphology of the outer edges of the larger gold grains (influence up to 40 μm). These biofilms exhibit concentric crystallographic growth structures and contain nanocrystalline, neoformed Au that forms a coating or rim on the outer surface. This rim may grow further to totally encase the primary gold grain within a secondary Au film (Reith et al., 2010). The bacteria may be playing a key role in the local solubilisation and re-precipitation of Au. Some authors claim the morphology of the Au particles that ultimately result from laboratory based bacterial experiments mimics that seen in nature. Triangles and hexagons of Au are however very common from a range of Au reduction to precipitation mechanisms including straight evapora-

tion, as such morphology of Au particle cannot be taken as a direct indicator of a given process. Transport of gold during weathering can occur by a variety of mechanisms (Fig. 6; Noble et al., 2009a,b). The most likely transport mechanism is in the form of a solution or mobilised as a nanoparticulate colloidal suspension (Hough et al., 2008). Under different conditions, different Au complexes occur. Thiosulfate, organic, hydroxyl or halide complexes are common. In environments where Fe 2+ is common, Au 3+ can be reduced forming Fe oxides (ferrihydrites and goethite) with colloidal nanoparticulate Au and Au hydroxo-chloro complexes (Greffie et al., 1996). In supergene environments with extensive Fe–Au associations this is a viable transport mechanism. Pure spherules of Au in association with Fe oxides had been observed in Mali, Brazil and Ghana (Greffie et al., 1996). Spherules of nanoparticulate Au have also been observed cemented in silcrete indicating a colloidal transport mechanism is most likely in this silcrete forming environment that lacks halide complexes (Hough and Noble, 2010). Another mechanism of mobilising nanoparticulate gold to the supergene environment is via gas streaming or bubble migration. Carrier gases in the near surface are most likely CO2 and CH4 (Etiope and Martinelli, 2002) along with S gas species from weathering sulfide ore bodies (Aspandiar et al., 2006). Experimental evidence of ~10 nm rounded Au particles transported by gas into alluvial sediments (Cao et al., 2010) indicates small colloids could be mobilised into secondary deposits via this mechanism, and some evidence of this occurring near actual gold deposits exists using soil–gas collectors (Cao et al., 2009). 5. Conclusions Gold nanoparticles have received significant attention in recent years due to their unique electronic, photonic, and catalytic properties that have led to several technological and biomedical applications. Similarly, the nanoparticulate fraction of Au in ore systems is increasingly recognised as an important component of economic deposits as “invisible” (refractory) in sulphides, in secondary supergene enrichments, the formation of

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surface geochemical anomalies and in the formation of high-grade accumulations. Nanoparticulate gold behaves very differently from its coarser equivalent and only now are the properties of such gold becoming apparent. New high-resolution techniques such as HR-TEM/ SEM, LA-ICP-MS, nanoSIMS, SXRF and AFM/STM now allow imaging of trace and nanoparticulate Au, to maintain the context to the larger sample and therefore making it easier to detect in different sample media. The tools even permit the imaging and analyses of individual Au nanoparticles in different matrices; including minerals, soils and water samples. Considering the fact that gold nanoparticles have been increasingly documented in both high and low-temperature ore related environments further studies are needed to better understand the potential role and mechanisms involved with colloidal transport and deposition in ore systems. Colloids can contain very high concentrations of Au in suspension and this may be a very significant feature in the formation of high-grade gold accumulations in hypogene systems where gold transporting fluids are usually only considered to contain ppm levels of the metal in solution. A new knowledge of nanoparticulate Au will also improve our ability to recover gold from ores and in surface detection of buried gold deposits in geochemical exploration. Acknowledgements RMH and RRPN acknowledge funding support from the Minerals Down Under National Research Flagship and a CSIRO Julius award to RMH. We thank Peta Clode and Paula Smith for assistance with images. M.R. would like to acknowledge Fondecyt Grant #11070088, and thank Alejandro Zúñiga and Christian Nievas for their help with TEM observations. We thank the editor for the invitation to submit this article and two reviewers. References Aspandiar, M.F., Anand, R., Gray, D., 2006. Mechanisms of elements dispersion through transported cover: a review. CRC LEME Report 230, pp. 1–81. Perth, Australia. Bakken, B.M., Hochella Jr., M.F., Marshall, A.F., Turner, A.M., 1989. High-resolution microscopy of gold in unoxidized ore from the Carlin mine, Nevada. Econ. Geol. 84, 171–179. Barker, S., Hickey, K., Cline, J., Dipple, G., Kilburn, M., Vaughan, J., Longo, A., 2009. Uncloaking invisible gold: use of nanoSIMS to evaluate gold, trace elements and sulfur isotopes in pyrite from Carlin-type gold deposits. Econ. Geol. 104, 897–904. Becker, U., Reich, M., Biswas, S., 2010. Nanoparticle–host interactions in natural systems. In: Brenker, F., Jordan, G. (Eds.), Nanoscopic Approaches in Earth and Planetary Sciences: EMU Notes in Mineralogy, vol. 8, p. 1-52. Cao, J., Hu, R., Liang, Z., Peng, Z., 2009. TEM observations of geogas-carried particles from the Chankeng concealed gold deposit, Guangdong Province, South China. J. Geochem. Explor. 101, 247–253. Cao, J.J., Hu, X.Y., Jiang, Z.T., Li, H.W., Zou, X.Z., 2010. Simulation of adsorption of gold nanoparticles carried by gas ascending from the Earth's interior in alluvial cover of the middle-lower reaches of the Yangtze River. Geofluids 10, 438–446. Ciobanu, C.L., Cook, N.J., Pring, A., Brugger, J., Danyushevsky, L.V., Shimizu, M., 2009. “Invisible gold” in bismuth chalcogenides. Geochim. Cosmochim. Acta 73, 1970–1999. Ciobanu, C.L., Cook, N.J., Utsunomiya, S., Pring, A., Green, L., Shimizu, M., 2011. Focussed ion beam–transmission electron microscopy applications in ore mineralogy: Bridging micro- and nanoscale observations. Ore Geol. Rev. 42, 6–31. Deditius, A.P., Utsunomiya, S., Reich, M., Kesler, S.E., Ewing, R.C., Hough, R., Walshe, J., 2011. Trace metal nanoparticles in pyrite. Ore Geol. Rev. 42, 32–46. Etiope, G., Martinelli, G., 2002. Migration of carrier and trace gases in the geosphere: an overview. Phys. Earth Planet. Inter. 129, 185–204. Faraday, M., 1857. Experimental relations of gold (and other metals) to light. Philos. Trans. R. Soc. Lond. 147, 145. Fleet, M.E., Mumin, A.H., 1997. Gold-bearing arsenian pyrite and marcasite and arsenopyrite from Carlin Trend gold deposits and laboratory synthesis. Am. Mineral. 82, 182–193. Freyssinet, P., Butt, C.R.M., Morris, R.C., Piantone, P., 2005. Ore-forming processes related to lateritic weathering. Economic Geology 100th Anniversary, Volume, pp. 681–722. Frondel, C., 1938a. Stability of colloidal gold under hydrothermal conditions. Econ. Geol. 33, 1–20. Frondel, C., 1938b. Stability of colloidal gold under hydrothermal conditions. Econ. Geol. 33, 1–20. Greffie, C., Benedetti, M.F., Parron, C., Amouric, M., 1996. Gold and iron oxide associations under supergene conditions: an experimental approach. Geochim. Cosmochim. Acta 60, 1531–1542. Herrington, R.J., Wilkinson, J.J., 1993. Colloidal gold and silica in mesothermal vein systems. Geology 21, 539–542. Hong, H., Tie, L., 2005. Characteristics of the minerals associated with gold in the Shewushan supergene gold deposit, China. Clays Clay Miner. 53, 162–170.

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