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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
fluid inclusions.
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
unsubstantiated.
A discussion of heterogeneous fluid systems and the
often-overlooked additional complexity due to immiscible phases
such as CO2.
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.
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."
Conclusions
The thermodynamic and experimental study of super-critical CO2
fluids indicates that gold is not transported in these fluids.
This confirms the prior rebuttal of such gold transport in the
discussions listed above on this website. The interpretation of
anhydrous CO2 fluid inclusions has been
confused by the serious failure to understand heterogeneous fluid
systems in which there is an immiscible fluid component (CO2).
Such fluid systems behave quite differently to the single
component boiling water heterogeneous fluid systems which are the
basis for almost all of the literature discussions of
heterogeneous fluids. The discussions on this website, listed
above, have long since pointed out this error and are confirmed by
this recent thermodynamic study.