Speciation and solubility of gold in CO2-HCl-H2O fluids: MD simulations and solubility experiments
WEIHUA LIU,1 YUAN MEI,1,2
JOËL BRUGGER,2 ARTASHE MIGDISOV,3
- CSIRO Mineral Resoureces Flagship, Melbourne firstname.lastname@example.org
- Monash University, Melbourne, Australia
- McGill University, Montreal, Canada
Increasing evidence of the abundance of water-poor, CO2-rich fluids in orogenic gold deposits indicates that 'exotic' volatile-rich fluids may have played a role in their formation (e.g., Ashanti Belt, Ghana1; Red Lake, Canada2; Sunrise Dam,Australia3). In water-rich, CO2-bearing fluids, CO2 has been suggested to be a pH buffering agent, optimising gold solubility as gold-hydrosulphide complexes4. However, the role of CO2 for gold mobility in water-poor, CO2-rich supercritical fluids is yet to be explored.
We have conducted preliminary gold solubility experiments and ab initio molecular dynamics simulations to investigate this problem. The solubility experiments have been conducted in a titanium autoclave at 340 ºC with 0.01 m HCl, and the amount of water and CO2 were loaded to the cell to ensure the fluid density is in the vapour phase with CO2 mole fraction (CO2/(H2O+CO2)) ranging from 0.1 to 0.84. The results showed that gold solubility has a negative correlation to the CO2 content in the fluids, i.e., decreasing with decreasing water fugacity.
Ab initio MD simulations of the AuCl0 complex (linear [H2O-Au-Cl]0) in the CO2-H2O system were conducted at 340 ºC with CO2 mole fraction from 0.1-0.98 at densities between 0.78-0.15 g/cm3. The MD simulations indicate that the number of hydration water and H-bonds near the AuCl0 complex decreases systematically with increasing CO2 mole fraction. These results are consistent with the experiments, suggesting that H2O as a polarized molecule plays a more active role than the un-polarized CO2 molecule in the fluids, and hydrated chloride species are the main form for transporting gold in the CO2-H2O-HCl system.
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Goldschmidt 2015 Abstracts