Abstract The solubility of gold has been calculated in the high-temperature (290°C) hydrothermal fluids of the Ohaaki-Broadlands geothermal system. If the dihydrosulfidogold(I) complex (Au(HS) − 1 ) is assumed to account for the gold in solution, the calculated solubility is 11.1 μg kg −1 which is in reasonable agreement with the measured value of 1.5 μg kg −1 . The concentration of gold in solution as AuCl 2 −1 is very small (1.2 x 10 −7 μ g kg −1 ) and this species is unimportant in the transport of gold in these ore-forming fluids. If gold is present in solution as Au(HS) 2 - , single-step adiabatic flashing of the Ohaaki-Broadlands deep fluids (t = 290°C) leads initially to an increase in gold solubility, thus preventing gold precipitation until a temperature of 277°C is reached. If, however, the gas phase is removed (open system) at any intermediate temperature between 290° and 277°C, the gold solubility drops rapidly with further boiling and phase separation with gold deposition occurring within 5° to 10°C of the fractionation temperature. By contrast, simple conductive cooling causes immediate minor gold precipitation but does not lead to the dumping of essentially all the gold in solution over a small temperature interval. It has also been shown that with fluid fluxes characteristic of active hydrothermal systems, 10 6 oz of gold may be transported into the boiling zone of an ore-depositing system over short periods of less than 1,000 years.