October 2009

The properties of gold change dramatically at the nanoscale. Gold nanoparticles and nanoporous gold have remarkable optical and catalytic properties, which are being investigated for applications in biomedicine and manufacturing. Using a variety of synthetic methods, the shape, size and properties of these materials can be optimized, making them a powerful platform for the development of new contrast agents for optical imaging of biological tissues, photothermal destruction of tumours and catalysis of various reactions.

The biosphere catalyzes a variety of biogeochemical reactions that can transform gold. Microbial weathering contributes to the mobilization of gold by releasing elemental gold trapped within minerals and by solubilizing gold via oxidation-promoting complexation. Subsequent microbial destabilization of gold complexes coupled with bioprecipitation and biomineralization can immobilize gold, completing the cycle. Secondary gold can occur as colloidal particles, crystalline gold and bacteriomorphic structures, the latter being a controversial form of ‘biogenic’ gold.

Gold 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.

Auriferous porphyry Cu deposits are restricted to convergent plate settings, whereas epithermal precious metal deposits form at extending convergent plate settings and in rifts. Both deposit types are linked to magma carrying metals and ligands that rises to form an upper-crustal magma chamber. Magma convection and fractionation lead to volatile exsolution and collection in the apical parts of the chamber, from which exsolved hydrothermal fluid ascends to form either a porphyry Cu–Au deposit associated with stocks and dykes generally at 2–5 km depth, or an epithermal deposit associated with coeval volcanic rocks at depths of

Gold concentrations in mineable deposits range from ~1 to 100 parts per million (ppm), or higher in bonanza deposits, in comparison to an average crustal abundance of ~1.3 ppb. A diverse range of elements may be associated with Au (La, Ce, U, V, Cr, Mo, W, Fe, Co, Ni, Pd, Pt, Cu, Ag, Zn, Hg, B, Tl, C, Si, Pb, As, Sb, Bi, S, Se, Te). A common mineralogical association is gold and quartz, but gold accumulations are also found with carbonates, carbon, feldspars, Fe sulfides and oxides, base metal sulfides, Fe ± Co ± Ni arsenides, and Fe ± Mg ± Ca ± V ± Cr silicates.

Although gold is a noble metal and is effectively insoluble even in strong acids, we have known for nearly 500 years that it can be concentrated to mineable levels by being transported as dissolved species in crustal fluids (indeed, most economic gold deposits owe their origin to this mode of transport). From alchemy and later experimental chemistry and geochemistry, we have developed an understanding of the solubility and speciation of gold in aqueous liquids and other crustal fluids. This knowledge informs us about the processes that promote the transport of gold in the Earth’s crust, result in exploitable gold deposits and lead to the remobilization of gold in the surficial environment.

Gold has fascinated people of most cultures since earliest times because of its colour, seeming immutability and ease of fabrication into exquisite objects. Due to gold’s rarity, its principal functions have been as currency and a store of wealth. Most of the gold ever mined has been hoarded, and rich discoveries over the past 160 years have increased the notional global amount per capita two to five fold. This rate of inflation is minor in comparison to that of currency, so gold still performs its historical financial role. Research is directed at discovering new deposits to support this role, and at finding practical applications commensurate with its monetary cost.