Thermodynamic studies show that Au is NOT transported in pure CO2 fluids
Kingsley Burlinson: November 2016
Since 1997 assertions have been made that gold might be
transported in and deposited from pure CO2
(non-aqueous) fluids. These assertions have been based on
incorrect interpretation of CO2 only fluid inclusions
within quartz, supposedly lacking contemporaneous H2O
Recently (2015) Liu et. al. have undertaken thermodynamic
studies, including gold solubility experiments, to measure the
solubility of gold in pure CO2 fluids. They presented a paper at the
Goldschmidt conference in 2015 and their data indicates that
gold does NOT dissolve in pure CO2 fluids and
that hydrated chloride species transport the gold.
In 1997 Schmidt-Mumm et. al. published a paper on a fluid
inclusion study at the Ashanti gold mine, Ghana. They recognised
pure CO2 fluid inclusions and did not identify any
associated aqueous inclusions. They cursorily dismissed the
possibility of trapping from a heterogeneous fluid (a
physical mixture of liquid water and CO2 gas bubbles)
and asserted that gold could perhaps be transported in
super-critical anhydrous CO2 fluids. I have discussed
this erroneous research in 4 other sections on this website and
shown conclusively that their own photographs of their fluid
inclusion samples prove that heterogeneous fluids containing water
and CO2 gas bubbles were present and that the assertion
of gold transport in anhydrous CO2 fluids was
Gas dominated inclusion assemblages may be trapped from aqueous dominant heterogeneous fluids by disproportional trappingA explanation why heterogeneous fluids do not trap inclusions from all phases or in proportion to their phase abundance.
Spherical or well rounded gas-filled fluid inclusions prove that heterogeneous trapping occurred from a dominantly liquid phase fluid.A proof that pure gas fluid inclusions were actually trapped from heterogeneous water-CO2 fluids and NOT from homogeneous anhydrous CO2 fluids.
Following after that 1997 publication, pure CO2 fluid inclusions were found in other gold deposits, most notably Campbell-Red Lake in Canada, and these were also mis-interpreted to indicate gold transport in anhydrous CO2 fluids. The lack of critical analysis of the conclusions in these studies led to the funding and conduct of research into the thermodynamic behaviour of gold in super-critical CO2 fluids at CSIRO in Canberra, Australia.
This research work by Liu et.al. is summarised in the abstract of the paper presented by them at the Goldschmidt conference in 2015.
The authors conducted autoclave experiments and determined that gold solubility decreased as the CO2 mole-fraction of the fluids increased. They also carried out Molecular Dynamics simulations and concluded 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."