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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 AuCl2−1 is very small (1.2 x 10−7 μg 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, 106 oz of gold may be transported into the boiling zone of an ore-depositing system over short periods of less than 1,000 years.

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