Supergene gold transformation: Secondary and nano-particulate gold from southern New Zealand

Chemical Geology 320-321 (2012) 32–45

Contents lists available at SciVerse ScienceDirect

Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo

Research paper

Supergene gold transformation: Secondary and nano-particulate gold from southern New Zealand Frank Reith a, b,⁎, Lachlan Stewart a, Steven A. Wakelin c a b c

School of Earth and Environmental Sciences, Centre of Tectonics, Resources and Exploration (TRaX), The University of Adelaide, North Terrace, SA 5005, Australia CSIRO Land and Water, Environmental Biogeochemistry, PMB2, Glen Osmond, SA5064, Australia AgResearch Ltd, Lincoln Science Centre, Cnr Springs Road and Gerald Street, Private Bag 4749, Christchurch 8140, New Zealand

a r t i c l e

i n f o

Article history: Received 12 March 2012 Received in revised form 22 May 2012 Accepted 24 May 2012 Available online 2 June 2012 Editor: J. Fein Keywords: Gold grains Gold nano-particles Gold mobility New Zealand Supergene

a b s t r a c t Biogeochemical processes drive the transformation of gold (Au) in surface environments. In this study we assess the link between surface morphologies of Au grains and supergene transformation processes, with a focus on the formation of nano-particulate Au in temperate settings. Gold grains were collected from six localities across the South Island of New Zealand. Deposit styles vary from eluvial-, alluvial-, and beach placer deposits in areas of moderate to very high levels of precipitation. Gold grains were assessed using optical microscopy (OM), field emission scanning electron microscopy (FEG-SEM), focused ion beam-scanning electron microscopy (FIB-SEM) coupled with X-ray dispersive analysis (EDXA) and electron microprobe analyses (EPMA). Morphologies indicative of Au- and Ag dissolution, e.g., grain boundary dissolution, as well as abundant Au neoformation- and aggregation morphologies were observed on all grains. The latter include a variety of secondary Au morphotypes, in particular nano-particulate- and μ-crystalline forms as well as bacteriomorphic Au, sheet-Au and porous, branched Au networks. Pervasive dissolution features on grains from an outcropping quartz-vein system as well as extensive nano-particle formation on weathered quartz-vein- and placer grains from the west coast of New Zealand's South Island, which is subject to very heavy orographic precipitation, suggest that these climatic conditions enhance the transformation of Au grains. At these sites, Au nano-particles are most abundant in the polymorphic layer (i.e., a coating of biofilms, secondary Au, siliceous and carbonaceous materials on the surface of transforming Au grains), and in soil materials associated with the grains. Nano-particulate Au is also highly abundant in carbonaceous, likely exopolymeric, coatings on Au grains from Orepuki Beach, suggesting that Au dissolution in seawater and microbial biomineralization are important contributors to Au alteration in beach placer deposits. In conclusion, surface morphologies of Au grains from New Zealand are the result of supergene transformations occurring in current environments. The formation of nano-particulate Au, which was previously thought to be evaporation-driven, is in these high-rainfall environments likely due to other mechanisms, such as biomineralization. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction Grains and nuggets between 0.1 and 4 mm in diameter are the most abundant sources of eluvial and alluvial Au (Mossman et al., 1999). They constitute economically important secondary Au deposits such as the quartz pebble conglomerate (QPC) systems in the Witwatersrand Basin (South Africa) and the Waimumu district (New Zealand; Mossman et al., 1999; Falconer et al., 2006). A number of fundamental questions surround our understanding of Au grain formation and Au mobility in supergene environments. These include: (i) Do Au grains in soils and placers form via detrital or accretionary processes; (ii) which processes are responsible for the dispersion of Au around primary mineralization;

⁎ Corresponding author at: Centre for Tectonics, Resources and Mineral Exploration (TRaX) School for Earth and Environmental Sciences, The University of Adelaide, CSIRO Land and Water, PMB 2, Glen Osmond, 5064, South Australia, Australia. Tel.: + 61 8 8303 8469; fax: + 61 8 8303 8550. E-mail address: [email protected] (F. Reith).

and (iii) what is the role of microbiota in the mobility of Au and the formation of secondary Au? A recent study of Au grains from subtropical Australia showed that a model with a strong biogenic component describes the formation and landscape position of Au grains best (Reith et al., 2010). According to this model, Au grains originate as primary Au in hydrothermal vein quartz systems. Primary Au grains commonly occur as electrum, a Au-Ag alloy, and display large (10 to >100 μm), often twinned, internal crystals (Hough et al., 2007). The occurrence of Au grains in supergene environments, such as soils, sediments and placers, is due to weathering and erosion of the host materials, and the physical redistribution of Au grains in eluvial (i.e., in situ weathered), colluvial (i.e., deposited by sheet-flow) and alluvial (i.e., deposited by running water) systems (Hough et al., 2007; Reith et al., 2010; Hough et al., 2011). While physical processes control the distribution of primary Au in the landscape, (bio)geochemical transformations control the formation of secondary Au (Reith and McPhail, 2006; Reith et al., 2006, 2007; Southam et al., 2009). Secondary Au is very

0009-2541/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2012.05.021

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

pure (>99 wt.% Au), finely crystalline (0.01 to 5 μm) and occurs as nano-particulate, bacteriomorphic, sheet-like-and wire Au, as well as euhedral, hexagonal, octahedral and triangular crystals and secondary grains (Reith et al., 2007; Falconer and Craw, 2009). Processes that transform primary Au grains include de-alloying and electrorefining (removal Ag and Cu), (bio)solubilization, re-deposition of Au onto Au surfaces, sorption of Au onto soil minerals and organics and biomineralization (Mann, 1984; Reith et al., 2007; Southam et al., 2009). Aggregation of Au nano-particles can then lead to the formation of nano- and μ-plates, triangles and spheres that can amalgamate to more complex structures (Lengke and Southam, 2006, 2007). These processes lead to the formation of secondary Au grains, and as more commonly observed, a variably thick layer of secondary Au overlying the primary core (Reith et al., 2006; Hough et al., 2007; Falconer and Craw, 2009; Reith et al., 2010). Recent research has shown that a number of these processes are mediated by microorganisms forming biofilms and polymorphic layers on the surface of Au grains (Reith and McPhail, 2006; Fairbrother et al., 2009; Reith et al., 2010). A polymorphic layer commonly covers surfaces of Au grains in supergene environments. It consists of microbial biofilms, residual biofilm materials, secondary nano-particulate, spheroidal and bacteriomorphic Au, carbonaceous materials as well as clays, iron-oxides and silicate minerals that were formed by active or passive (bio)mineralization (Reith et al., 2010). In recent years the formation of nano-particulate Au has been recognized as a critical component in weathering and migration of Au, leading to the formation of supergene anomalies as well as highgrade accumulations in soils and sediments (Hough et al., 2011). While it is commonly understood that biogeochemical mechanisms lead to the formation of Au nano-particles (Reith et al., 2007), to date natural Au nano-particles in supergene environments were predominantly observed in arid environments, and their formation has been attributed to evaporative mechanisms (Hough et al., 2008). The formation of nano-particulate and secondary Au in New Zealand placers is likely linked to supergene dissolution and reprecipitation. However, the factors contributing to nano-particulate Au formation, as well as drivers of dispersion into surrounding soils or sediments are little understood. It is also not clear, if Au transformations occur under modern environmental conditions, or if secondary Au‐particles are just windows into the past. ‘Active’ geological and geomorphological processes, climatic conditions and soil properties are important determinants of geochemical conditions in surface environments, and hence directly influence the biogeochemical transformation of some metals (Aspandiar, 2004; Haferburg and Kothe, 2007). Active tectonic and weathering processes re-supply supergene environments with ‘new’ primary Au, and control its distribution in surface environments (Boyle, 1979; Mann, 1984). Other studies have shown that the crystal structure of secondary Au is a function of the environment, e.g., the salinity and content of organic matter in ground- and soil waters, in which they are forming (Gray et al., 2008). Rainfall and temperature patterns closely relate to the species distribution of resident plants and microbes as well as their activities, e.g., the excretion of organic components such as cyanide and amino acids, which may contribute to the mobilization and precipitation of Au, and affect its dispersion and re-concentration (Reith et al., 2007; Viles et al., 2008; Fairbrother et al., 2009; Drenovsky et al., 2010). Understanding the processes affecting the transformation of Au and the conditions under which they occur will improve our understanding of Au mobility in temperate environments. This will improve exploration techniques for economically significant Au deposits. Hence, the aim of this study is to relate the morphological features of secondary Au, in particular of nano-particulate and μ-crystalline Au, to supergene transformation processes and environmental conditions. To achieve this, Au grains were sampled from a variety of secondary deposits from different environmental settings on New Zealand's South Island.

33

2. Materials and methods 2.1. Description of field sites Gold grains for this study were collected from six climatically and geologically different sites across the South Island of New Zealand (Fig. 1). The Shantytown site is a commercial Au operation on a placer deposit (Fig. 1). The Shantytown placer is located west of the Alpine Fault, 10 km south of Greymouth alongside Infants Creek. Primary Au is derived from hydrothermal quartz-vein systems containing Au mineralization, which have formed as a result of the uplift of the Southern Alps (Koons and Craw, 1990). Subsequently, fluvial and glacial erosion on both sides of the Alps has led to the formation of Au placers in current streams and rivers (Koons and Craw, 1990). The climate at Shantytown is temperate with mean annual temperature of 12.1 °C and a mean annual rainfall of 2700 mm (Tait et al., 2006). The rainfall pattern is different to Central Otago and Southland sites, and sustains dense temperate rainforest vegetation, which may provide different biogeochemical conditions for the mobilisation and precipitation of Au. For comparison, eluvial Au grains from a highly weathered quartz-vein deposit near Reefton were obtained from a local prospector (Fig. 1). The temperature and rainfall at Reefton are comparable to those at Shantytown (Tait et al., 2006). The Parker Road site is located in pastoral lands of the Waimumu district of Southland (Fig. 1). A detailed description of the geology, mineralogy and physiogeography of the site is given in Falconer et al. (2006). Climate at the site is cool temperate, with a mean annual temperature of 10.0 °C and a mean annual rainfall of 930 mm (Tait et al., 2006). Basement rock is comprised of the Murihiki terrane greywacke, overlain by Oligocene marine sedimentary rocks, and Tertiary non-marine strata that are dominated by the Gore Lignite Measures (Isaac and Lindqvist, 1990). Samples from the Parker Road site were collected from two adjacent stratigraphic horizons that contain Au grains; these are the Waimunu Quartz Gravels and the underlying Gore Lignite Measures. Gold grains were collected from unconsolidated conglomerates at the Kawarau Gorge and from stream sediments close to Arrowtown in Central Otago, New Zealand (Fig. 1). The climate at these neighbouring sites is a little drier than at the Parker Road site with a mean annual rainfall of 735 mm, and a mean annual temperature of 9.1 °C (Tait et al., 2006). The source of primary Au in both placer systems is the Otago Schist belt, which has numerous mesothermal vein systems shedding Au into placers throughout Central Otago (Craw and Norris, 1991; Falconer et al., 2006).

Fig. 1. Location of sampling sites on the South Island of New Zealand.

34

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

Gold grains were also collected from the Orepuki Beach (OB) placer located on the southern tip of the South Island, New Zealand (Fig. 1). This site rests on a fragment of Palaeozoic Gondwana; consisting of granitic rocks intruding meta-sedimentary rocks of lower Palaeozoic age. Erosion of these eastern Fjordland granites supplied the majority of heavy mineral sands and gravels, which are deposited along the beaches of the south coast (McCann and Craw, 2009). The average annual rainfall (1300 mm) and annual temperature (9.9 °C) are similar to some of the other sites (Tait et al., 2006); however, the exposure to high winds and storm surges as well as the chloride-containing sea water may affect surface morphologies. 2.2. Sample collection, preparation and analyses Gold grains from Shantytown, Arrowtown and Orepuki Beach were collected in November 2009. Parker Road and Kawarau Gorge samples were collected in March 2010. Between 20 and 30 grains were collected from each site according to a field sterile procedure aimed at preserving delicate secondary structures (Reith et al., 2010). Gold grains were washed and stored in sterile physiological solution (0.9 wt.% NaCl), and transported over ice to the laboratory. Reefton samples were collected by a local prospector, who had washed them in tap water. Samples were screened using optical microscopy (Nikon DXM1200 stereomicroscope), and six grains representing typical morphologies for each site were chosen for further analyses. Four Au grains from each site were washed in deionized water to remove salt, mounted on C-tape and air dried in a dustfree environment. Samples were analyzed using a field emission scanning electron microscope (FEG-SEM, Philips XL-30), images were collected in secondary electron and backscatter imaging mode at 10 and 20 kV, respectively. Selected samples were further analyzed using a focused ion beam secondary electron microscope (FIB-SEM; Helios NanoLab DualBeam, FEI, Netherlands). Images were collected at 3 to 15 kV with sectioning and cleaning carried out at 30 kV and 21 nA and 20 kV and 2.8 to 0.34 nA, respectively. The instrument was equipped with an energy dispersive spectrometer (EDS) operating at 15 to 20 kV using a 10 mm 2 Sapphire Si(Li) EDAX detector; this was used to collect element maps across milled sections. The remaining grains were set in epoxy resin, polished with 1 μm diamond paste, coated with a 15 nm thick C film and subjected to electron microprobe analysis (EPMA) using a Cameca SX51 Microprobe (Cameca, France) equipped with five wavelength dispersive spectrometers (Adelaide Microscopy, University of Adelaide). Data collection and reduction were performed with the SAMx package (Note: because Au grain whole mounts edge were used and effects can appear). Analyses were conducted at 20 kV and 19.89 nA with a 1 μM diameter beam. The grains were analyzed for (detection limits in parenthesis in wt.%): Al (0.06), Si (0.05), S (0.05), Fe (0.08), Ni (0.09), Cu (0.11), As (0.25), Pd (0.25), Ag (0.25), Au (0.28), Hg (0.6) and Bi (0.5). All the elements were calibrated on a mixture of minerals and pure metal standards from Astimex. Aluminum, Si, and Fe were calibrated on natural garnet, S and Cu on chalcopyrite, As on arsenopyrite, Bi on a Bi-telluride, Ni, Pd, Ag, Au on pure metal standards, and for Hg calibration HgS was used. 3. Results 3.1. Gold grains from the West Coast (Shantytown and Reefton) Gold grains from Shantytown are between 0.2 and 0.8 mm in diameter. They are rounded, flattened, platy grains displaying folded re-flattened edges that are between 10 and 50 μm thick (Fig. 2). Grain surfaces are highly textured with profuse cavities (Fig. 2B). Electron microprobe maps of polished grains showed wide rims composed of high-purity Au (100 wt.%), with an average of 99.4 wt.% Au across the grain (Fig. 2H, I). Towards the interior, increasing concentrations of Ag were detected, with maximum concentrations of Ag of 2.3 wt.%

occurring in hotspots, otherwise Ag concentrations lay between 0.5 and 1 wt.%. Field emission scanning electron microscopy revealed highly complex surface structures with many scratches, flattened, abraded surfaces and carved striated channels, which are indicative of physical damage following transport in the environment (Fig. 2A). Areas that were not physically damaged are covered by extensive branched networks of Au and flat step-like Au sheets (Fig. 2B, arrows). Grains are covered by polymorphic layers, consisting of branching and budded Au conglomerates, organics, silica, aluminosilicates and Fe-oxides (Figs. 2B, 3A–C). Associated with the surface hyphae-like structures were observed that were most likely derived from fungi (Fig. 2C, arrows). Other parts of the grains were coated with a mixture of minerals and organics derived from the surrounding soils (Fig. 2C. Within these coatings Au nano-particles are highly abundant (Fig. 2D). Nano-particles are predominantly spheroidal and between 20 and 100 nm in diameter (Fig. 2E, F), accumulations of nano-particles forming μ-crystals also occur (Fig. 2G, arrows). To obtain a detailed insight into the structure of the polymorphic layer and the underlying secondary Au sheets, FIB-SEM-(EDXA) analysis was used in an area undamaged by physical transport (Fig. 3). This technique allows for the precise milling of sections across fragile layers without disturbing internal structures; the section can be imaged and chemically characterized (Wirth, 2004; Fig. 3B, C). A polymorphic layer consisting of secondary Au intermixed with carbonaceous and siliceous materials of approximately 10 μm depth overlies a layer of coarsely crystalline Au (Fig. 3B–D), which is followed at a depth of 5 μm by thin bands (some b0.5 μm deep) of μ-crystalline Au (Fig. 3A, E, F). Coarse Au displays crystal orientations with sharp distinct boundaries (Fig. 3D), whereas the underlying μ-crystalline Au is made up of crystals with indistinct crystal boundaries (Fig. 3E, arrows), displaying non-linear, crystalline orientations in elongated, thin bands (Fig. 3F, arrows). The long, elongated bands are orientated horizontally with respect to the sample surface, dipping to the right of the section into depth (Fig. 3F). They display a mylonitic style texture, in which bands are comprised of parallel horizontally aligned μ-crystals (Fig. 3F). The bands do not affect the crystal orientations on either side of the mylonitic zone, with μ-crystalline orientations remaining continuous on either side. Gold in the polymorphic layer is nano-particulate or μcrystalline (Fig. 3D). While some of the μ-crystalline Au has preserved crystal boundaries of the underlying coarser Au, crystal orientations from other particles do not follow underlying orientations. Beneath the polymorphic layer numerous etched pits were observed on the surface of the grain (Fig. 3A, arrows). The Reefton grains are up to 10 mm in diameter (Fig. 4A) and had a highly texture surface consisting of two distinct morphologies (Fig. 4). The first is sponge-like and porous, and consists of Au-rich, highly textured surfaces with many holes, pits and deep rifts and silica inclusions (Fig. 4B, C, D, arrows); exposed surfaces have been smoothed and rounded by physical weathering. The second morphology is silica-rich matrix containing abundant nano-particulate and μ-crystalline Au that forms large conglomerates resembling ‘bacterioform’ Au (Fig. 4D, E, F). Nano-particles range from 10 to 500 nm in size and are rodshaped, cuboctahedral and spheroidal (Fig. 4F). Abundant triangular and hexagonal platy μ-crystals, which are between 1 and 2 μm in diameter and conglomerates of μcrystals are present (Fig. 4E, F, G); EDXA showed that surface consisted of 99.9 wt.% Au, other metals were not detected. 3.2. Gold grains from Southland (quartz pebble conglomerates and Gore Lignite Measures, Parker Road) Gold grains from the Parker Road quartz pebble conglomerate (QPC) are between 0.1 and 1 mm in diameter (Fig. 5A). Optical microscopy revealed that most grains had flat and smooth surfaces with numerous cavities. The edges of the grains are flattened, between 10 and 20 μm thick and sub-rounded to rounded; in some

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

35

Fig. 2. Electron micrographs and quantitative EPMA maps of Au grains from the Shantytown, showing (A) size and typical morphologies; (B) the highly textured grain surfaces; (C,D) backscatter electron (BSE) micrographs showing soil and fungal-hyphae with abundant nano-particulate covering Au grain surfaces; (E,F,G) individual and aggregated nano-particulate Au in the polymorphic layer; (I,J) EPMA map of Au and Ag concentration in polished section of a Au grain (I; maximum conc. of Au 100 wt.%; J; maximum conc. of Ag 2.3 wt.%).

cases folding of edges led to a pronounced thickening (Fig. 5A). From the edges angular tears of up to 100 μm that intersect into the grain (Fig. 5A, arrow). Electron microprobe maps show high‐purity Au rims (up to 100 wt.%) with increasing concentrations of Ag (up to 11.2 wt.%) towards the core of the grains. Gold and Ag concentrations are inversely proportional (Fig. 5F, G). Concentrations of other metals, including Hg, were below detection. Field emission scanning electron microscopy revealed carved channels containing prominent striations representing slicken slides (Fig. 5B, arrows). In addition, numerous cavities that were up to 20 μm in diameter were observed on the surface, these are filled with secondary Au, as well as small numbers of individual Au nanoparticles, that are often associated with aluminosilicates (Figs. 5B, 6A). Individual Au nano-particles are between 10 and 200 nm in size with a spheroidal morphology (Fig. 5C). Abundant sheet-like Au composed of conglomerates of nano-particulate Au covered grain surfaces (Fig. 5C, D). These conglomerates form branched networks of pure, porous Au, which coat mineral inclusions, such as silica and

aluminosilicate with layers of lacelike branched networks of secondary Au (Fig. 5D, arrows). Other commonly encountered features are dissolution textures, which expose crystal boundaries of individual Au crystals (Fig. 5E). To obtain a more detailed insight into the internal structure and composition of these grains, FIB-SEM-(EDXA) was used in topographically low area of the grains. A polymorphic layer of up to 10 μm width, consisting of randomly oriented Au nano- and μ-crystals and Au sheets, organics, aluminosilicate and quartz, was observed (Fig. 6A arrows). In another FIB-milled region a thin layer (1–2 μm) of porous nano-crystalline Au is followed by a band of high‐purity μ-crystalline Au that is approximately 5 μm wide (Fig. 6A, B). This is followed by a band of μ-crystalline Ag-rich Au, which itself is underlain by a region of large Au crystals overlying an area of porous nano-crystalline Au and surrounding a large inclusion of siliceous material (Fig. 6B, C). Large Au crystals are successively replaced by μcrystalline Au as well as porous nano-crystalline Au, as shown using ‘slice and view’ milling, in which a thin (100 nm) slice of material

36

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

Fig. 3. Electron micrographs (A,D,E,F) and element maps (EDXA, B,C) of a FIB-milled section on the Shantytown Au grain from Fig. 2A, showing: (A) polymorphic layer and crystal orientations through the section; (B) RGB map of Au, Ag, and Si (red, green, blue, respectively) distribution in the section; (C) RGB map of Au, Ag, and C (red, green and blue, respectively) of the polymorphic and directly underlying layers; (D) crystal orientation of Au-particles in the polymorphic layer; (E) underlying coarse crystalline Au; (F) fine vertical banding of μ-crystalline Au with mylonitic style texture.

was cut away every time, before a new image is collected, producing a 3-D representation of these features (Supplementary material 1). Gold grains from the underlying Gore Lignite Measures display similar morphologies to grains from the Quartz Pebble Conglomerates (Fig. 7). Grains are between 0.3 and 1.0 mm in diameter, sub-rounded to rounded with prominent folded edges, flat surfaces and numerous cavities (Fig. 7A, B). Grain edges are between 10 and 20 μm thick, rounded and flat (Fig. 7A). Electron microprobe maps of polished whole mounts show high‐purity Au rims (up to 100.0 wt.%), with Ag increasing towards the interior of the grains up to concentrations 8.6 wt.%; Au and Ag concentrations were inversely proportional (Fig. 7F–I). While the gradient of Ag concentration across some of the grains was sharp and high Ag concentrations were only detected in two distinct regions, Ag in other grains was more evenly distributed (Fig. 7F–I); concentrations of other elements, except Si, were below detection. Four grains were imaged using FEG-SEM and displayed morphologies analogous to grains from the QPC, i.e., moderately flat, smooth surfaces containing numerous carved striated channels (Fig. 7A, B, arrows), with some secondary Au in topographically low lying regions (Fig. 7C). While smooth flat grain surface areas are due to stepped

flat sheets of Au, the predominant morphologies on the highly structured grains are due to extensive networks of branched and budding secondary Au (Fig. 7C). These grains contain many cavities, which hold abundant nano-particles, finely branched Au filaments and aluminosilicates (Fig. 7D, arrows). Nano-particles are between 10 and 500 nm is size and spherical to cuboctahedral in shape (Fig. 7D). Mineral inclusions are coated by layers of thin branched networks of secondary Au derived from apparent nano-particle conglomeration (Fig. 7E). Both the flat stepped Au sheets and porous branched networks of Au in topographic lows display abundant dissolution textures that expose the crystal boundaries of individual sub-grains (Fig. 7E, arrows). 3.3. Gold grains from Central Otago (Arrowtown and Kawarau Gorge) Gold grains from Arrowtown are between 0.2 and 1.2 mm in diameter (Fig. 8). Most grains (70%) are flat and rounded to elongated (type 1; Fig. 8A). Approximately 30% of grains are angular with sharply defined edges (type 2; Fig. 8B). Type 1 grains display smooth flat surfaces with numerous cavities containing mineral inclusions, e.g., aluminosilicates and secondary Au (Fig. 8A). Surfaces consist mostly

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

37

Fig. 4. Optical (A) and electron (B–G) micrographs of size and morphology of the Reefton Au grains. (A) Size and overall morphology of a Reefton Au grain; (B,C,D) SE and BSE images of the highly textured surfaces with many holes, dissolution pits and deep rifts and inclusions of silica of the grain; (E,F) morphology of Au nano-particles in a silica-rich matrix; (G) conglomerate of μ-crystals in a silica-rich matrix.

of flat step-like Au sheets (Fig. 8C, arrows). Individual nanoparticulates and μ-crystals have been observed in the cavities but are rare compared to abundances at the other sampling sites (Fig. 8D arrows). On the surface of one of the grains an extended network of hyphae-like microbial growth covering 200 μm 2 was observed (Fig. 8E). Electron microprobe element maps of polished whole-grain mounts show that the concentrations of Au and Ag are homogenous throughout the grains with maximum concentrations of Ag of 5.8 wt.% (Fig. 8G, H). Gold and Ag concentrations are inversely proportional, other metals were not detected. Rims of highly pure Au (100 wt.%) were narrow. Type 2 grains are blocky and display sharp angular edges to slightly rounded edges. Surfaces are smooth with few cavities (Fig. 8B). Nano-particulate and μ-crystalline Au was not observed. One of the grains contained a large negative crystal, which has an exceptionally smooth surface (Fig. 8B, arrow). Rare cavities contain branched network styles of Au which appear to be the result of dissolution rather than aggregation, similar to Erlebacher (2001; Fig. 8F, arrow)

Grains from Kawarau Gorge are between 0.5 and 1.0 μm in diameter and sub-rounded to rounded (Fig. 9A). Edges show no obvious folding and surfaces display no or carved striated channels. The grains show both flat, step-like Au sheets (Fig. 9B, arrows) and abundant nano-particulate Au within protected cavities (Fig. 9C); no recognizable dissolution features, such as grain boundary dissolution, were observed. The numerous cavities contain aluminosilicates and abundant associated nano-particulate Au, but no branched networks of fragile, secondary Au (Fig. 9D). Gold nano-particles are 10 to 200 nm in diameter and spheroidal or cuboctahedral (Fig. 9E). 3.4. Gold grains from Orepuki Beach (Southland) The grains are between 0.4 and 0.8 mm in diameter, and included flat, rounded platy grains (Fig. 10A) and elongated, thicker grains (Fig. 10B). The flat platy grains have very thin grain edges, that are smooth and rounded, while the elongated grains have thick folded edges, which are also smoothed and rounded but with rougher

38

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

Fig. 5. Secondary electron micrographs (A-E) and quantitative element maps (EPMA, F,G) of Au grains from the Parker Road Quartz Pebble Conglomerates showing (A) typical size and overall morphology; (B) surfaces damaged by pitting and scratching; (C) secondary flat, step‐like, Au sheets covering silica inclusions; (D) sheet-like secondary Au composed of conglomerates of nano-particulate Au; (E) dissolution grain boundary dissolutions; EPMA maps of Au (F; max. conc. 100 wt.%) and Ag (G; max. conc. 11.1 wt.%).

surface textures (Fig. 10A, B). Electron microprobe element maps of polished grains showed that one of the studied grains contained an Ag-rich core, with maximum with Ag concentration of 28.6 wt.%, underneath a wide rim of pure Au (up to 100 wt.%; Fig. 10G, H). In another grain Ag hotspots of 2.0 wt.% only occurred in an area close to the rim (Fig. 10I, J). In both grains Au and Ag concentrations are inversely proportional; other metals were not detected. The surface texture of all grains contains numerous cavities of deeply etched pits, cracks and valleys displaying well developed grain boundary dissolution features; valleys are occur preferentially along grain boundaries (Fig. 10C, D). Scratches and striations were not observed. High resolution FEG-SEM revealed that numerous (>10 μm - 2), small (b100 nm) etching pits occur across the surfaces of some grains (Fig. 10B, C). Abundant nano-particulate and nanocrystalline Au that is associated with organic films was observed in protected cavities (Fig. 10D, E). Nano-particles are 5 to 100 nm in size and predominantly spheroidal; conglomerates of nano-particles were also observed (Fig. 10F). 4. Discussion All grains display dissolution, re-precipitation and coupled dissolution/re-precipitation textures, similar to those described by Falconer and Craw (2009). A key difference between dissolution and aggregation features is that dissolution acts preferentially at surface defects and sub-grain boundaries. During dissolution sharply defined, sub-grain boundaries develop mottled textures, and sub-grain cores become isolated (Falconer and Craw, 2009). Aggregation textures develop through repeated nucleation of Au nano-particles to

form spheroidal particles, with further aggregation leading to polyspheroidal aggregates and chain structures. Where Au precipitates via aggregation, sheets of secondary Au can form, but the individual buds often remain clearly distinguishable (Falconer and Craw, 2009). A characteristic feature of all studied grains (except for the very recently been liberated type 2 grains from Arrowtown), independent of the predominance of dissolution or re-precipitation features, is the presence of nano-particulate Au. Manufactured Au nano-particles have received significant attention in recent years due to their unique electronic, photonic and catalytic properties that have led to novel technical and biomedical applications (Hough et al., 2011). Similarly, the nano-particulate fraction of Au in ore systems is increasingly recognized as an important component of economic deposits, leading to the formation of geochemical anomalies and high-grade supergene accumulations (Williams-Jones et al., 2009; Hough et al., 2011). Hough et al. (2008) have observed that pure Au nano-particles and b200 nm sized, nano-particulate Au plates were formed during the weathering of an Au mineralization in arid Western Australia. They concluded that Au nano-particle formation was driven by the inorganic reduction of mobile Au-complexes linked to evaporation, and the formation of evaporate minerals, e.g., barite and halloysite (Hough et al., 2011). In the wet-temperate climates of New Zealand evaporation is an unlikely driver of Au nano-particle formation. Hence, other formation mechanisms need to be explored. A number of early studies have shown that the reduction of Au(III)-complexes and the formation of colloidal Au occurs through the interaction with organic matter, e.g., low molecular weight organic acids (LMWOAs), along with fulvic and humic acids (Fetzer, 1934, 1946; Ong and Swanson, 1969). The

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

Fig. 6. Secondary electron micrographs (A,B) and element maps (EDXA, C) of two FIBmilled section of a Au grain from the Parker Road Quartz Pebble Conglomerates, showing: (A,B) the polymorphic layer and crystal orientations through the section; (C) RGB map of Au, Ag, and Si (red, green, blue, respectively) distribution in the section.

authors of these studies hypothesized that Au colloids are important contributors for the mobility of Au in supergene environments; however, due to analytical limitations at the time this could not be verified. Using advanced μ-analytical techniques, we can now confirm

39

that Au nano-particles are a main factor of contributing to the mobility of Au in temperate environments. Polymorphic layers observed on New Zealand grains show similarities to those observed on Au grains from Australia (Reith et al., 2006, 2010; Fairbrother et al., 2012-this issue). Similar to the Australian grains, polymorphic layers in this study contained secondary Au, suggesting that active biogenic processes might drive the formation of nano-particulate and other secondary morphotypes (e.g., Figs. 3A, 6A, 9C). In particular, as biomineralization of Au has been shown to be kinetically favoured over abiogenic adsorption and reduction on (in)organic phases, e.g., Fe or Mn-oxides or soil organics (Gray, 1998; Gray et al., 1998; Reith et al., 2009; Kenney et al., 2012). Cyano-, sulfide-oxidizing-, sulfate-reducing, and metallophilic bacteria have been shown to rapidly precipitate nano-particulate Au from aqueous Au(I/III)-complexes (Lengke and Southam, 2005; Lengke et al., 2006a, 2006b; Lengke and Southam, 2007; Reith et al., 2009). Reductive precipitation of these complexes appears to improve survival rates of those bacteria that are capable of detoxifying their immediate cell environment by detecting, excreting and reducing Aucomplexes; these include Salmonella enterica, Cupriavidus metallidurans, Plectonema boryanum (Reith et al., 2007). Other bacteria have been shown to gain metabolic energy by utilizing Au-complexing ligands, such as thiosulfate, which is utilized by Acidothiobacillus ferrooxidans (Lengke and Southam, 2005). Over time, nano-particles of Au are released from the cells and deposited on cell surfaces and bulk solutions (Lengke and Southam, 2006, 2007). Ultimately, these nano-particles contribute to the formation of μm-size octahedral Au crystals, framboid-like structures (~1.5-μm diameter) and mm-sized Au foils (Lengke and Southam, 2006, 2007), similar to those observed in this study (Figs. 2B, 4G, 5C, 7E). Fungi have also been shown to reductively precipitate Au-complexes under in vitro conditions (Nakajima, 2003). However, only few a studies have provided field evidence for a link of Au and fungi (e.g., Borovicka et al., 2010). In our study hyphae-like structures were observed on grains from two sites, Shantytown and Arrowtown, supporting a possible link between fungi and Au biomineralization in temperate environments. Generally, Au grains from all sites display strong links to environmental and climatic conditions they are currently experiencing, as well as to the time and distance travelled from their respective sources. While purely secondary Au grains have been observed, most placer grains originate as primary grains, which are transformed under supergene conditions (Reith et al., 2010). In particular, the formation of a layer of high-purity secondary Au is indicative of supergene transformation processes (Hough et al., 2007; Reith et al., 2010). A number of processes are likely to contribute to the formation of these layers of secondary Au. These include (bio)precipitation (as discussed above) and self-electro-refining of the Au (Larizzatti et al., 2008). During self-electro‐refining, which is a process occurring due to fluctuating redox-conditions, Au and Ag undergo oxidative dissolution and Au, as the more inert metal, re-precipitates (Larizzatti et al., 2008). As the aggregation of Au forms into sheets, over time continual dissolution and precipitation yield a crystalline mylonitic structure parallel to the surface structure with indistinct crystalline boundaries composed of μ-crystalline material. Hence, the width of the highpurity Au rim of transformed primary grains, can be sued as a yardstick for the extent of supergene transformations the grain has experienced. Grains from the Parker Road site have been transported in alluvial channels, and evidence of physical damage was observed. Optical microscopy showed features including the flat platy habit, folded and flattened edges, damage (i.e., carved striated channels and edge tears) and rounded thickened edges. The inclusions of silica and aluminosilicates that have been observed may have been worked into the Au surface by physical reworking. However, high-resolution FEG-SEM imaging revealed a different story, with fine branched networks of Au in protected areas and flat, step‐like, Au sheets covering

40

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

Fig. 7. Electron micrographs (A–E) and quantitative element maps (EPMA, F–I,) of Au grains from the Gore Lignite Measures, showing their typical size and morphology (A,B); (C) secondary flat, step-like, Au sheets made up of budded conglomerates covering a polymorphic layer; (D) nano-particulate Au in the polymorphic layer: (E) overgrowths of secondary Au on a silica inclusion; quantitative EPMA maps of Au (F,H; max. conc. max. conc. 100 wt.%) and Ag (G,I; max. conc. 8.6 wt.%).

silica inclusions (Fig. 5C). In cavities; fine branched networks and overgrowths and display a low level of grain boundary dissolutions (Fig. 5E). These would have been destroyed if the inclusion was pressed into the surface by post-depositional physical processes; hence secondary Au can be interpreted as recent precipitation and aggregation features. The features described are consistent with precipitation of Au on to the surface, covering and removing damage, creating an Au-rich rims. Gold grains from the underlying (older) Gore Lignite Measures, displayed a lesser degree of physical damage caused by physical reworking, and a larger proportion of nanoparticle and secondary Au formation. A strong dependence between the morphological evolution of Au grains and their position in lateritic weathering profiles has also been observed in the Brazilian study (Larizzatti et al., 2008). Here progressive (bio)geochemical weathering has led to a decrease in grain size, rounding and increasing relative Au content upwards within the lateritic profile. These factors suggest an environment in which Au mobilization exceeds re-precipitation (Larizzatti et al., 2008). These results suggests that Parker Road grains occur in an ‘active’ environment, in which, other

than in the Brazilian example, post‐depositional Au precipitation rates exceed those of Au dissolution. Shantytown Au grains are different to Parker Road grains. While samples show the similar effects caused by physical movement, Au concentrations throughout the grains are far higher with an average concentration of 99.4 wt.%, suggesting that these grains have undergone a higher degree of supergene transformation (Fig. 2H, I). In addition higher concentrations of Au nano-particles were observed in the surrounding soil materials (Fig. 2C, D). This suggests that the strong orogenic precipitation at the site might lead to rapid changes in redox-potentials, which would support rapid electro-refining. In addition, very active plant and microbial communities will be present under these conditions, which may also increase the dissolution and re-precipitation rates. This demonstrates that grains have been extensively transformed in supergene environments, and suggests that a larger proportion of the grains may have aggregated from nanoparticulate Au. To observe the intricate interplay of physical and biogeochemical processes leading to the formation of polymorphic layers as well as

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

41

Fig. 8. Electron micrographs (A–F) and quantitative element maps (EPMA, G,H,) of Au grains from Arrowtown showing: (A,B) their typical sizes and morphologies; (C) typical surface morphologies; (D) fungal‐hyphae coating parts of the grain; (E) a polymorphic layer containing few Au nano-particles; (F) showing dissolution features within a surface pit; EPMA maps of Au (G; max. conc. max. conc. 100 wt.%) and Ag (H; max. conc. 5.6 wt.%).

dissolution, re-precipitation and re-crystallization of Au, FIB-SEMEDXA was used on key-samples from Shantytown and Parker Road. The sequence of compositions and crystalline forms in the subsurface of Au grains from Parker Road, suggest dissolution and reprecipitation under supergene conditions. FIB-milling has shown that the grains have previously been folded, as indicated by the large inclusion of porous Au and silica that are the likely remnants of grain surface processes, which had led to the formation of

secondary sheet-like Au. On the newly formed outside surface of the grains active processes have, since the folding, led to the purification of the Au via loss of Ag, and dissolving and re-precipitating Au forming a new layer of secondary nano-crystalline Au. On Shantytown grains a polymorphic layer similar to the layer observed at the Kilkivan site was observed. Sectioning through the polymorphic layer revealed that aggregates of nano-particulate Au occur (Fig. 3B). These are likely the result of nano-particle aggregation,

42

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

Fig. 9. Electron micrographs (A–E) and element maps (EPMA, F–I,) of Au grains from Kawarau Gorge showing (A) their typical size and morphology; (B) secondary flat, step-like, Au sheets covering extensive areas on the grains; the distribution of Au nano-particles (C) and conglomerates of Au nano-particles in the polymorphic layer (D); (E) morphologies of Au nano-particles.

with further aggregation leading to the formation of Au films and small grains (Lengke et al., 2006a, 2006b), similar to those observed on Au grains in this study (Fig. 3B). Based on our analyses the crystal features underlying the polymorphic layer of Shantytown grains reflect secondary Au formation and re-crystallization rather than features of primary Au. Arrowtown grains display fluvial transport features, e.g., carved channels with striations. Generally they are blocky and have seen very little Ag dissolution, but do show narrow rims of pure Au (Fig. 9F). The high Ag concentration in the interior and narrow Aurich rims, suggest that these grains have only recently been liberated from the primary source, but since then have experienced active Ag/ Au dissolution and possibility Au re-precipitation in surface environments (Fig. 8F). The differences in sample morphologies suggests that different primary Au sources provided materials to the placer, leading to different travel distances and times, and hence difference in physical alteration and rounding. They also suggest that samples have experienced different post deposition weathering conditions; one

leading to active reductive precipitation and the other a very mild oxidative dissolution condition. The Reefton samples show highly developed surface dissolution morphologies. Surfaces contain numerous dissolution pits, is Aurich, likely due to Ag dissolution, and contains carved striated channels indicative of physical damage. These grains are partly comprised of quartz-rich matrices which contain Au nano-particles, platelets and μ-crystals of various shapes. These have likely been formed in a quartz‐vein hosted system, in which Au platelets reflect either rapid precipitation of Au from a supersaturated solution, or have aggregated from nano-particulates during the formation of the vein hosted system (Hough et al., 2011) The Orepuki beach placer grains display remnants of alluvial features and current aeolian deformation, in combination with biogeochemical transformations mediated by seawater and microbial communities. Flat, rounded, platy grains had preserved alluvial features, whereas aeolian deformation led to the formation of toroidal morphologies and cylinder-shaped toroids. These results confirm

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

43

Fig. 10. Electron micrographs (A–F) and quantitative element maps (EPMA, G–J,) of Au grains from the Orepuki, Beach placer, showing their typical size and morphologies (A,B); (C) extensive dissolution features on the grain surfaces; (D,E) distribution and morphology of Au nano-particles in carbonaceous exopolymeric layer covering cracks and pits; (F) Au nano-particles in polymorphic layer; EPMA maps of Au (G,I; max. conc. max. conc. 100 wt.%) and Ag (H,J, max. conc. 2.0 wt.% and 28.6 wt.%, respectively).

the results of earlier work by McCann and Craw (2009), who studied a sequence of beach placer deposits along the south coast of New Zealand's South Island. They found a strong correlation of Au morphologies to environmental parameters, in particular the strong southerly winds, size and exposition of the beaches and sand dunes (McCann and Craw, 2009). Biogeochemical effects are observed on the surfaces, which are pock-marked with etched dissolution pits (Fig. 10C), and highly developed grain boundary dissolution features, (Fig. 10D). The extensive dissolution and etch features appear to be an effect of the influence of sea water on the grain surfaces. Due to the high chloride content of seawater, Au-chloride complexes may form (Usher et al., 2009). These appear to have reductively precipitated by extracellular polymeric substance (EPS) and bacteria therein, which may explain the abundance of nano-particles in extracellular polymeric substances. (EPS; Fig. 10D–F). Extracellular polymeric substance are mostly composed of polysaccharides secreted by bacteria, and support the attachment of cells to surfaces and the formation of biofilms (Harrison et al., 2007). The presence of extensive extracellular polymeric layers can lead to the temporary or permanent immobilisation of metal ions and complexes, decreasing the amount of toxic Aucomplexes reaching the cells (Harrison et al., 2007; Reith et al., 2007).

In organic matter-rich beach sediments, such as those at Orepuki Beach, sulfate reduction is the main form anaerobic metabolism (Winfrey and Ward, 1983). During sulfate reduction, sulfate is reduced to hydrogen sulfide, which can lead to mobilization of Au via the formation of Au(I)-thiosulfate complexes(Fitz and Cypionka, 1990). Sulfate-reducing and sulfide-oxidizing bacteria have been shown to reductively precipitate Au(I)-thiosulfate complexes and deposit native Au inside the cells as spherical nano-particles, as also discussed above (Lengke and Southam, 2005, 2006, 2007). A link to the sulfur cycling has also been observed in a study by Falconer et al. (2006), who assessed the isotope composition of the sulfide minerals, marcarsite and pyrite occurring at the Belle Brook and Parker Road sites (Falconer et al., 2006). The study has shown that sulfides at both sites are predominantly diagenetic. The authors concluded that they were formed from hydrogen sulphide resulting from microbial sulfate reduction. They also concluded that changes in redox‐conditions driven by active sulfur cycling are likely to drive Au cycling at these sites. Transported Au grains are commonly used in mineral exploration for Au to pinpoint a primary source (Hough et al., 2007). Hough et al. (2007) noted that the morphology of grains, and in particular those found in stream sediments, is not diagnostic for origin or distance,

44

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45

but rather provides a record of the supergene transformations the grains have undergone. This study confirms these findings and provides a biogeochemical process model, for the initial and subsequent stages of Au grain transformation and nano-particle formation in temperate environments (Fairbrother et al., 2012-this issue). In combination with current geological and physicochemical conditions, this model provides a baseline for the interpretation of macro-, micro-, and nano-morphologies of Au grains from New Zealand. 5. Conclusions Gold nano-particle formation as well as surface morphologies of Au grains are the result of current supergene transformations. They determine the surface morphologies and compositions of grains in placer settings, rather than primary processes. Gold nano-particles are drivers of Au dispersion in temperate environments. Subsurface analyses using FIB-SEM-EDXA is an important tool for the study of Au mineralization processes, as it can provide a detailed insight into the complex history of physical deformation, mobilisation, precipitation and re-crystallisation of secondary Au. These techniques showed that Au grains used in this study displayed dissolution, re-precipitation and coupled dissolution/re-precipitation as well as re-crystallisation textures. This shows that the combined model of supergene Au grain transformation and Au nano-particle formation introduced by Reith et al. (2010) is also valid in temperate environments. Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.chemgeo.2012.05.021. Acknowledgments The authors acknowledge the following individuals and institutions for their contributions: the Australian Research Council (LP 100200102), The University of Adelaide, The Institute for Mineral and Energy Resources and the Centre for Tectonics, Resources and Exploration, CSIRO Land and Water (CSIRO Julius fellowship to S.A. Wakelin) (TRaX Record 220), The South Australian Museum, Adelaide Microscopy; A. Gregg for assistance with the fieldwork, D.M. Falconer, J. Smith, J. Eden and Shantytown Heritage Park for providing access to sites and samples as well as infrastructure support; L. Green and A. Netting for help with the micro-analyses. The editor J. Fein and the two anonymous reviewers provided helpful suggestions on this manuscript. References Aspandiar, M.F., 2004. Mechanisms of metal transfer through sedimentary overburden. In: Gee, D. (Ed.), 2004 Minerals exploration seminar; abstracts. Cooperative Research Centre for Landscape Environments and Mineral Exploration, Bentley, West. Aust., Australia, pp. 22–24. Borovicka, J., Dunn, C.E., Gryndler, M., Mihaljevic, M., Jelinek, E., Rohovec, J., Rohoskova, M., Randa, Z., 2010. Bioaccumulation of gold in macrofungi and ectomycorrhizae from the vicinity of the Mokrsko gold deposit, Czech Republic. Soil Biology and Biochemistry 42, 83–91. Boyle, R.W., 1979. The geochemistry of gold and its deposits. Geological Survey of Canada Bulletin 280. Craw, D., Norris, R.J., 1991. Metamorphogenic Au-W veins and regional tectonics: mineralisation throughout the uplift history of the Haast Schist, New Zealand. New Zealand Journal of Geology and Geophysics 34, 373–383. Drenovsky, R.E., Steenwerth, K.L., Jackson, L.E., Scow, K.M., 2010. Land use and climatic factors structure regional patterns in soil microbial communities. Global Ecology and Biogeography 19, 27–39. Erlebacher, J., Aziz, M.J., Karma, A., Dimitrov, N., Sieradzki, K., 2001. Evolution of nanoporosity in dealloying. Nature 410, 450–460. Fairbrother, L., Brugger, J., Shapter, J., Laird, J., Southam, G., Reith, F., 2012. Supergene gold transformation: biogenic secondary and nano-particulate gold from arid Australia. Chemical Geology 320-321, 17–31 (this issue). Fairbrother, L., Shapter, J., Pring, A., Southam, G., Brugger, J., Reith, F., 2009. Effect of the cyanide-producing bacterium Chromobacterium violaceum on ultraflat gold surfaces. Chemical Geology 265, 313–320. Falconer, D.M., Craw, D., Youngson, J.H., Faure, K., 2006. Gold and sulphide minerals in tertiary quartz pebble conglomerate gold placers, Southland, New Zealand. Ore Geology Reviews 28, 525–545.

Falconer, D.M., Craw, D., 2009. Supergene gold mobility; a textural and geochemical study from gold placers in southern New Zealand. Society of Economic Geologists 14, 77–93. Fetzer, W.G., 1934. Transportation of gold by organic solutions. Economic Geology 29, 599–604. Fetzer, W.G., 1946. Humic acids and true organic acids as solvents of minerals. Economic Geology 41, 47–56. Fitz, R.M., Cypionka, H., 1990. Formation of thiosulfate and trithionate during sulfite reduction by washed cells of Desulfovibrio desulfuricans. Archives of Microbiology 154, 400–406. Gray, D.J., 1998. The aqueous chemistry of gold in the weathering environment. Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Open File Report, 38. Wembley West, Australia. Gray, D.J., Lintern, M.J., Longman, G.D., 1998. Chemistry of gold-humic interactions. Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Open File Report, 43. Wembley West, Australia. Gray, D.J., Sergeev, N.B., Britt, A.F., Porto, C.G., 2008. Supergene Mobilization of Gold and Other Elements in the Yilgarn Cratron — Final Report. Cooperative Research Centre for Landscape Evolution and Mineral Exploration, Open File Report 228. Wembley West, Australia. Haferburg, G., Kothe, E., 2007. Microbes and metals: interactions in the environment. Journal of Basic Microbiology 47, 453–467. Harrison, J.J., Ceri, H., Turner, R.J., 2007. Multimetal resistance and tolerance in microbial biofilms. Nature Reviews Microbiology 5, 928–938. Hough, R.M., Butt, A.R.M., Reddy, S.M., Verrall, M., 2007. Gold nuggets: supergene or hypogene. Australian Journal of Earth Sciences 54, 959–964. Hough, R.M., Noble, R.R.F., Hitchen, G.J., Hart, R., Reddy, S.M., Saunders, M., Clode, P., Vaughan, D., Lowe, J., Gray, D.J., Anand, R.R., Butt, C.R.M., Verrall, M., 2008. Naturally occurring gold nanoparticles and nanoplates. Geology 36, 571–574. Hough, R.M., Noble, R.R.P., Reich, M., 2011. Natural gold nanoparticles. Ore Geology Reviews 42, 55–61. Isaac, M.J., Lindqvist, J.K., 1990. Geology and lignite resources the East Southland Group, New Zealand. New Zealand Geological Survey Bulletin 101. Kenney, J.P.L., Song, Z., Bunker, B.A., Fein, J.F., 2012. An experimental study of Au removal from solution bu non-metabolizing bacterial cells: Part 1 aquesous chemistry results. Geochimica et Cosmochimica Acta 87, 51–60. Koons, P.O., Craw, D., 1990. Gold mineralisation as a consequence of continental collision: an example from the Southern Alps, New Zealand. Earth and Planetary Science Letters 103, 1–9. Larizzatti, J.H., Oliveira, S.M.B., Butt, C.R.M., 2008. Morphology and composition of gold in a lateritic profile, Fezenda Pison “Garimpo”, Amazon, Brazil. Journal of South American Earth Sciences 25, 259–376. Lengke, M.F., Fleet, M.E., Southam, G., 2006a. Morphology of gold nanoparticles synthesized by filamentous cyanobacteria from gold(I)-thiosulfate and gold(III)-chloride complexes. Langmuir 22, 2780–2787. Lengke, M.F., Ravel, B., Fleet, M.E., Wanger, G., Southam, G., 2006b. Mechanisms of gold bioaccumulation by filamentous cyanobacteria from gold(III)-chloride complex. Environmental Science and Technology 40, 6304–6309. Lengke, M.F., Southam, G., 2005. The effect of thiosulfate-oxidizing bacteria on the stability of the gold-thiosulfate complex. Geochimica et Cosmochimica Acta 69, 3759–3772. Lengke, M.F., Southam, G., 2006. Bioaccumulation of gold by sulfate-reducing bacteria cultured in the presence of gold(I)-thiosulfate complex. Geochimica et Cosmochimica Acta 70, 3646–3661. Lengke, M.F., Southam, G., 2007. The deposition of elemental gold from gold(I)-thiosulfate complex mediated by sulfate-reducing bacterial conditions. Economic Geology 102, 109–126. McCann, R., Craw, D., 2009. Gold and Platinum Deformation, Aeolian Deformation and Teroid Development in Beach Placer Deposits, Southland. AusIMM New Zealand Branch Conference, pp. 1–13. Mann, A.W., 1984. Mobility of gold and silver in lateritic weathering profiles: some observations from Western Australia. Economic Geology 79, 38–50. Mossman, D.J., Reimer, T., Durstling, H., 1999. Microbial processes in gold migration and deposition: modern analogues to ancient deposits. Geoscience Canada 26, 131–140. Nakajima, A., 2003. Accumulation of gold by microorganisms. World Journal of Microbiology and Biotechnology 19, 369–374. Ong, H.L., Swanson, V.E., 1969. Natural organic acids in the transportation, deposition and concentration of gold. Colorado School of Mines Quarterly 64, 395–425. Reith, F., Etschmann, B., Grosse, C., Moors, H., Benotmane, M.A., Monsieurs, P., Grass, G., Doonan, C., Vogt, S., Lai, B., Martinez-Criado, G., George, G.N., Nies, D., Mergeay, M., Pring, A., Southam, G., Brugger, J., 2009. Mechanisms of gold biomineralization in the bacterium Cupriavidus metallidurans. Proceedings of the National Academy of Sciences 106, 17757–17762. Reith, F., Fairbrother, L., Nolze, G., Wilhelm, O., Clode, P.L., Gregg, A., et al., 2010. Nanoparticle factories: biofilms hold the key to gold dispersion and nugget formation. Geology 38, 843–846. Reith, F., Lengke, M.F., Falconer, D., Craw, D., Southam, G., 2007. Winogradsky Review: the geomicrobiology of gold. ISME Journal 1, 567–584. Reith, F., McPhail, D.C., 2006. Effect of resident microbiota on the solubilization of gold in soil from the Tomakin Park Gold Mine, New South Wales, Australia. Geochimica et Cosmochimica Acta 70, 1421–1438. Reith, F., Rogers, S.L., McPhail, D.C., Webb, D., 2006. Biomineralization of gold: biofilms on bacterioform gold. Science 313, 233–236. Southam, G., Lengke, M.F., Fairbrother, L., Reith, F., 2009. The biogeochemistry of gold. Elements 5, 303–307.

F. Reith et al. / Chemical Geology 320-321 (2012) 32–45 Tait, A., Henderson, R., Turner, R., Zheng, Z., 2006. Thin plate smoothing interpolation of daily rainfall for New Zealand using a climatological rainfall surface. International Journal of Climatology 26, 2097–2115. Usher, A., McPhail, D.C., Brugger, J., 2009. A spectrophotometric study of aqueous Au(III) halide-hydroxide complexes at 25–80 °C. Geochimica et Cosmochimica Acta 73, 3359–3380. Viles, H.A., Naylor, L.A., Carter, N.E.A., Chaput, D., 2008. Biogeomorphological disturbance regimes: progress in linking ecological and geomorphological systems. Earth Surface Processes and Landforms 33, 1419–1435.

45

Williams-Jones, A.E., Bowell, R.J., Migdisov, A.A., 2009. Gold in solution. Elements 5, 281–287. Winfrey, M.R., Ward, D.M., 1983. Substrates for sulfate reduction and methane production in intertidal sediments. Applied and Environmental Microbiology 45, 193–199. Wirth, R., 2004. Focused Ion Beam (FIB): a novel technology for advanced application of micro- and nanoanalysis in geosciences and applied mineralogy. European Journal of Mineralogy 16, 863.876.