Analysis of Gold(I/III)-Complexes by HPLC-ICP-MS Demonstrates Gold(III) Stability in Surface Waters

Abstract

Understanding the form in which gold is transported in surface- and groundwaters underpins our understanding of gold dispersion and (bio)geochemical cycling. Yet, to date, there are no direct techniques capable of identifying the oxidation state and complexation of gold in natural waters. We present a reversed phase ion-pairing HPLC-ICP-MS method for the separation and determination of aqueous gold(III)-chloro-hydroxyl, gold(III)-bromo-hydroxyl, gold(I)-thiosulfate, and gold(I)-cyanide complexes. Detection limits for the gold species range from 0.05 to 0.30 μg L(-1). The [Au(CN)2](-) gold cyanide complex was detected in five of six waters from tailings and adjacent monitoring bores of working gold mines. Contrary to thermodynamic predictions, evidence was obtained for the existence of Au(III)-complexes in circumneutral, hypersaline waters of a natural lake overlying a gold deposit in Western Australia. This first direct evidence for the existence and stability of Au(III)-complexes in natural surface waters suggests that Au(III)-complexes may be important for the transport and biogeochemical cycling of gold in surface environments. Overall, these results show that near-μg L(-1) enrichments of Au in environmental waters result from metastable ligands (e.g., CN(-)) as well as kinetically controlled redox processes leading to the stability of highly soluble Au(III)-complexes.

Biological and Geochemical Development of Placer Gold Deposits at Rich Hill, Arizona, USA

“…Indeed, some are reactive Mn-oxide biominerals (e.g., birnessite), capable of mobilization of gold by oxidizing Au(0) and Au(I) to Au(III) complexes, thereby driving the transformation of gold nuggets [47,48]. Both manganese and iron occur at elevated levels in the coating of the highly transformed black placer gold (Figure 6b), while iron is much lower and manganese very low for the red placer gold.…”

Gold Behavior in Supergene Profiles Under Changing Redox Conditions: The Example of the Las Cruces Deposit, Iberian Pyrite Belt

“…These include (AuCl/Br4) -at <4 pH, (AuOH4) -at <10 pH, and hydroxidehalide-mixed complexes at intermediate pH conditions. Ta et al (2014) reported such Au 3+ -halidehydroxide complexes in natural hypersaline waters.…”

“…As a general rule, Au 3+ halide complexes can only be transported short distances downward through the weathering profile due to low stability in less acidic environments (Mann, 1984;Saunders, 1993); hence, these complexes are not stable in the new system conditions. At neutral pH, however, Au 3+ -halide ligands can be hydrolyzed to Au 3+ -hydroxide-halide complexes, increasing their solubility (Krupp and Weiser, 1992;Usher et al, 2009;Ta et al, 2014). Alternatively, they could be reduced to Au 1+ -halide complexes that are also more stable at neutral pH (Krupp and Weiser, 1992;Usher et al, 2009).…”

“…Alternatively, they could be reduced to Au 1+ -halide complexes that are also more stable at neutral pH (Krupp and Weiser, 1992;Usher et al, 2009). In addition, in areas within the supergene profile under alkaline conditions, Au could be mobilized as Au 3+ -hydroxide complexes, which are the most soluble ligands under these conditions (Krupp and Weiser, 1992;Usher et al, 2009;Ta et al, 2014). These mechanisms are sufficient for the transport and precipitation of Au throughout the weathering profile.…”

“…Secondary Au ores occur mainly as a result of dissolution and redistribution of Au during the weathering of primary sulfides. Supergene Au mobility occurs via complexation with organic-, halide-, hydroxide-, halide-hydroxide and sulfur ligands, which are dependent on weathering conditions (Boyle et al, 1975;Mann, 1984;Webster, 1986, Groen et al, 1990Benedetti and Bouleguè, 1991;Freyssinet et al, 2005;Tagirov et al, 2006;Usher et al, 2009;Ta et al, 2014). In addition, biological activity can contribute to dissolution-precipitation processes of Au during weathering in the near-surface environment (Reith et al, , 2007Southam et al, 2005Southam et al, , 2009Hough et al, 2008;Fairbrother et al, 2012).…”