Colloidal gold in sulphur and citrate-bearing hydrothermal fluids: An experimental study

Abstract

The stability of colloidal gold (gold nanoparticles) in sulphur-bearing and citrate-bearing solutions was investigated at temperatures up to 225 °C using Synchrotron X-ray Near-edge Spectroscopy (XANES), and up to 350 °C with a visual check of colour changes. The citrate-based colloidal gold solutions, with or without colloidal silica in the solution, are only stable up to 225 °C. In contrast, the gold colloids in Na2S solutions are not stable upon heating to 150 °C, but stable up to 300 °C when 0.5–1.5 wt% of colloidal silica is present in the solution. The colloidal gold particles started to aggregate and deposit from the solution with the aggregation and growth of silica particles at 350 °C. The concentrations of gold as colloids in the solutions are up to 0.5 mmol (∼95 ppm), more than three orders of magnitude higher than gold solubility as aqueous complexes under the same condition calculated based on available thermodynamic data. These results provide the first evidence that high concentrations of colloidal gold are stable in sulphur-bearing fluids at elevated temperatures (∼300 °C). This implies that the formation of gold colloids is an effective way to concentrate gold in hydrothermal sulphur-bearing fluids to form high-grade gold ores.