The stability of Au-chloride complexes in water vapor at elevated temperatures and pressures

The solubility of gold in liquid-undersaturated HCl-bearing water vapor was investigated experimentally at temperatures of 300 to 360 degreesC and pressures up to 144 bars. Results of these experiments show that the solubility of gold in the vapor phase is significant and increases with increasing f(HCl), and f(H2O). This behavior of gold is attributed to formation of hydrated gold-chloride gas species, interpreted to have a gold-chlorine ratio of 1: 1 and a hydration number varying from 5 at 300 degreesC to 3 at 360 degreesC. These complexes ld are proposed to have formed through the following reaction: Au-solid + m.HClgas + n.H2Ogas = AuClm.(H2O)(n)(gas) + m/2.H-2(gas), (A1) which was determined to have log K values of -17.28 +/- 0.36 at 300 degreesC, -18.73 +/- 0.66 at 340 degreesC, and -18.74 +/- 0.43 at 360 degreesC. Gold solubility in the vapor was retrograde, i.e., it decreased with increasing temperature, possibly as a result of the inferred decrease in hydration number. Calculations based on our data indicate that at 300 degreesC and f(O2)-pH conditions, encountered in high sulfidation epithermal systems, the vapor phase can transport up to 6.6 ppb gold, which would be sufficient to form an economic deposit (e.g,, Nansatsu, Japan; 36 tonnes) in similar to 30,000 yr. Copyright (C) 2001 Elsevier Science Ltd.