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The Crystallography, Metallography and Composition of GoldRobert M. Hough1, Charles R. M. Butt1 and Jrg Fischer-Bhner21811-5209/09/0005-0297$2.50 DOI: 10.2113/gselements.5.5.297

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old is an element, a metal and a mineral. In nature, gold most commonly occurs as an alloy with silver and, more rarely, with palladium, mercury and copper, and ranges in size from nanoparticles to nuggets weighing 70 kg. Crystallography, metallography and composition control the colour of the alloy, how it will deform, how it will behave at high temperature and how it reacts. These properties offer insights into how gold deposits have formed and been altered, whether under hydrothermal or Earth-surface conditions.

Native gold nearly always occurs as an alloy with Ag; natural alloys with Cu, platinoids, Hg, Pb, Sn, Sb and Bi are much rarer. Gold and Ag have the same atomic radius and so can form a continuous alloy series; when the Ag content exceeds 20 wt%, the alloy is termed electrum. The variation in the Au/Ag ratio of native gold has led to the composition being described in terms of fineness, Keywords : gold grains, electrum, alloy, annealing, gold nuggets which is represented by the formula 1000 wt% Au / (wt% Au + wt% Ag). INTRODUCTION In effect, this describes the gold content of the alloy in parts Elemental gold is relatively rare (~1 ppb) in the Earths per thousand and is a key parameter used to characterize mantle and continental crust and is a trace constituent gold in hypogene, alluvial and supergene environments. (~1 ppm) in some meteorites (Palme and ONeill 2003). As From a few weight percent up to 5060 wt% Ag, the alloy a metal, gold has a high melting point, is soft, dense, highly retains its malleability and ductility, but properties such malleable and ductile, and is an excellent conductor. As a as colour, microstructure, hardness, work-hardening during mineral, gold is an isometric (face-centred cubic) phase deformation and response to heating are altered. with a close-packed atomic structure. It takes many crystal habits, including cubes, octahedra, dodecahedra, triangles Much of the literature on gold deposits ignores the gold itself, other than giving a chemical analysis. Gold composition and hexagons. Native gold is found dominantly as small is discussed in some contexts, but golds crystallography (micrometre- to millimetre-sized), finely disseminated, and metallography are less commonly considered. This paper mostly polycrystalline particles and grains and, more rarely, as single crystals. It also occurs as polycrystalline highlights the significance of these properties to studies of dendrites, wires, plates, sheets and nuggets (Fig. 1). Many gold ore systems. of these are exquisite forms and are a result of twinning controlled by the {111} 60 twin law; for example, wires (Fig. 1d) are formed by repeated twins. Gold also occurs as a component in other minerals, including tellurides (calaverite, petzite), selenides and sulfides, and as a trace component in, for example, pyrite, arsenian pyrite and arsenopyrite. The extensive studies of arsenian pyrite from Carlin-type deposits revealed that incorporation of Au into the pyrite structure is facilitated by As. In this case, Au can be invisible and in solid solution, with a maximum Au/ As molar ratio of 0.02 (Reich et al. 2005); above this ratio, though, gold can be imaged directly by high-resolution transmission electron microscopy as nanoparticles of metallic Au (Palenik et al. 2004). Such nanoparticles in arsenian pyrite have been shown (experimentally) to become unstable above 370C; however, if they coarsen during heating, the particles are stable up to 650C (Reich et al. 2006). These observations set an upper limit on the temperature at which the nanoparticles (1. The gold typically contains 520% Ag, but in some deposits, it is essentially pure whereas, in others, the Ag content may exceed 50%. In general, the Au/Ag ratio of primary gold has been little studied. Most reported Au/ Ag ratios are 4 mm, nuggets were commonly discovered during the early gold rushes in North America and Australia. By far the majority came from alluvial deposits and regions with a deeply weathered, residual regolith. The Victorian goldfield of southeastern Australia was the most prolific source, producing both the greatest numbers and the largest. By 1910, at least 1200 nuggets weighing, each, more than 20 troy ounces (622 g) had been documented; many exceeded 1000 oz (31.1 kg), and the two largest, the Welcome and the Welcome Stranger, were 2218 oz (~69 kg) and 2315 oz (72 kg), respectively. Large nuggets are still being found today (Fig. 5).

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Figure 5

This 8 kg gold nugget (Kingower, Victoria, Australia) was found in the near surface with a metal detector in 1995.

The origin of nuggets has been debated for over 160 years, but very little scientific study has been carried out. Many prospectors and geologists consider most nuggets to be secondary, formed in alluvial or weathering environments. Their relative abundance in these environments compared to their rarity in lode deposits, the rounded or mamillary form of some nuggets and the overgrowths on others, as well as observations of delicate structures and small pristine crystals in recent sediments support a supergene origin. The early debate was summarized by Liversidge (1893) who, although demonstrating that gold could be dissolved in groundwater and reprecipitate under surface conditions, nevertheless concluded on geological grounds that nuggets in alluvium and weathered rock are of primary origin. He also showed that nuggets are internally polycrystalline (Liversidge 1897). Unfortunately, Liversidges conclusions and, in particular, his petrographic observations and techniques were forgotten. Wilson (1984) incorrectly concluded that nuggets from Western Australia had accreted around secondary Fe oxides and clays, but Wilson did not etch his specimens and saw only featureless, shiny surfaces, not the crystals, the twins and the relationship of secondary minerals to crystal boundaries and the exterior (Fig. 6). E lEmEnts

A section of a nugget from Bayleys Reward, Coolgardie, Western Australia, imaged by optical microscopy: (A) polished, (B) polished and etched. Etching reveals a polycrystalline structure, with twinning diagnostic of annealing. The ferruginous material is not enclosed but entered through numerous interconnecting voids that are open to the exterior. Specimen courteSy of QueenSland muSeum, BriSBane

Figure 6

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The inclusions actually occupy voids dissolved by weathering solutions that had permeated along these boundaries, over a distance of 30 mm in some large nuggets. The dissolved gold was leached, some precipitating elsewhere as secondary crystals. Recently, Hough et al. (2007) studied nuggets from a wide range of geological and secondary environments, mainly in Australia, and subsequently re-examined those studied by Liversidge and Wilson. Nearly all are AuAg alloys similar in compositions to primary gold, with Ag depletion on crystal boundaries open to the exterior. The nuggets have eventextured, polycrystalline fabrics, with many crystals exhibiting coherent and incoherent twinning (Fig. 6). Petrovskaya (1973) provides numerous examples of hypogene and alluvial gold from Russia with similar crystallographic features. These textures and fabrics are the same as those observed in metallographic studies, where they are considered characteristic of thermal annealing of AuAg alloys at >250C. These similarities conclusively demonstrate a hypogene origin for the nuggets. The relative abundance of nuggets in surficial environments is due to physical concentration by chemical wasting and fluvial transport, not by chemical growth. Nuggets are weathering, not forming, in the supergene environment.

CONClUsIONsAs gold is both a mineral and a metal, any study needs to take into account both its crystallography and its composition because these play a key role in understanding its physical properties and behaviour (i.e. its metallography). Metallography provides insights into the features displayed by natural gold, such as grain size, annealing fabrics and colours of alloys. These, in turn, may assist in exploration, by identifying the provenance of detrital particles, and in paragenesis, by providing constraints relating to the thermal and tectonic history of deposits. We are only now beginning to realize the extent of the distribution of nanoparticulate gold in ore systems. Potentially, nanoparticulate gold may play an important role in the transport and deposition of gold in both hydrothermal and supergene environments and lead to improved understanding of the genesis of deposits in these environments.

ACKNOWlEDGMENTsWe are indebted to our late colleague Ernie Nickel for many insightful discussions over the last few years. Ernie passed away whilst this paper was being completed. Financial support was provided by the CSIRO Minerals Down Under National Research Flagship.

REFERENCEsBowell RJ (1992) Supergene gold mineralogy at Ashanti, Ghana: Implications for gold dissolution and re-precipitation. Mineralogical Magazine 56: 545-560 Boyle RW (1979) The Geochemistry of Gold and Its Deposits. Geological Survey of Canada, Bulletin 280, 584 pp Chapman RJ, Leake RC, Moles NR (2000) The use of microchemical analysis of alluvial gold grains in mineral exploration: experiences in Britain and Ireland. Journal of Geochemical Exploration 71: 241-268 Clough DM, Craw D (1989) Authigenic gold-marcasite association: evidence for nugget growth by chemical accretion in fluvial gravels, Southland, New Zealand. Economic Geology 84: 953-958 Cobley CM, Xia Y (2009) Gold and nanotechnology. Elements 5: 309-313 Corti CW (1999) Metallurgy of microalloyed 24 carat golds. Gold Bulletin 32(2): 39-47 Degussa AG,