Gold transport in hydrothermal fluids: Competition among the Cl-, Br-, HS- and NH3(aq) ligands

Since the Au(I) and Au(III) ions are not stable in water, gold transport and deposition in hydrothermal ore fluids are dependent on the identity and stability of the predominant aqueous gold complexes. Gold(I) bisulfide (e.g., Au(HS)2−) and in some instances Au(I) chloride complexes are widely acknowledged to account for gold transport in hydrothermal ore fluids. This study investigates the potential of the unconventional ligands Br− and NH3(aq) to increase gold mobility. This was achieved by determining the predominant gold species in hydrothermal fluids with binary mixed ligands (Br−–Cl−, Br−–HS−, HS−–NH3), and measuring their structural properties using in situ synchrotron X-ray absorption spectroscopy (XAS). The capacity of XAS to follow the progress of ligand exchange reactions was demonstrated using Au(III) at room temperature: the Au(III)Br4− complex was found to predominate in mixed Br−–Cl− solutions (Br−/Cl− = 0.2–1), with average ligand numbers derived from XAS data in good agreement with the UV–vis study of Usher et al. (Geochim. Cosmochim. Acta 73, 3359–3380, 2009). At elevated temperatures up to 400 °C at 600 bar, the XAS measurements show that Au(I)–HS− complexes are the only stable gold species in mixed HS−–Br− and HS−–NH3 fluids (HS−/Br− = 0.1; HS−/NH3 = 0.2), suggesting that hydrosulfide is the most important ligand for gold transport in the hydrothermal fluid under our experimental conditions, i.e., hydrosulfide complexes outcompete bromide, chloride and ammine complexes in S-bearing fluids.