The observation of non-aqueous carbonic fluid
inclusions within auriferous
quartz does not prove that gold
must have been transported by such fluids. BY: Kingsley Burlinson,
September 2011
Recently there have been a number of papers inferring the
transport and deposition of gold by pure CO2 fluids
in mineralizing hydrothermal systems. These conclusions have
been based on the observation of fluid inclusions which lack a
visible aqueous phase and the failure to observe (or document)
aqueous fluid inclusions in the same sample of quartz. These
authors usually refer back to several previous publications to
justify their own inference of such fluids as being dominant in
the system under study. However all these papers fail to deal
with the issue of the origin of the quartz in which all the
studies were carried out.
Many authors refer back to the paper "High CO2
content of fluid inclusions in gold mineralisations in the
Ashanti Belt, Ghana: a new category of ore forming fluids?" by
Schmidt-Mumm et al in mineralium Deposita, 1997, V32, p107-118.
But there are serious issues that need to be considered and
which are not dealt with in that work.
The real problem with this and with all subsequent work
purporting to prove that Au is transported in (essentially)
non-aqueous fluids is the complete failure to observe that the
inclusions being studied are within quartz. Everyone is so
intent on seeing the microscopic fluid inclusions and discussing
the ppm levels of gold in the system that they have failed to
see that the system is in fact about 99.999% quartz (with 10 ppm
gold). It must be remembered that the fluid inclusions being
observed are trapped entirely within quartz and that
silica deposition must have been essentially contemporaneous
with the presence of the non-aqueous fluid under study in order
for the inclusion to seal and trap that fluid. However,
silica is not known to be transported by or deposited from
non-aqueous CO2 fluids. Based on the
"like dissolves like" guidelines for solubility, it
is unlikely that silica could be transported by super-critical
CO2, because CO2 is a non-polar molecule
and would not be expected to dissolve silica which is soluble in
polar water. Silica dissolves in water primarily because of
hydrogen bonding with the protonic solvent, water, but
non-protonic CO2 lacks the capability to form such
hydrogen bonds and it is quite unlikely that super-critical CO2
could dissolve silica. It is disingenuous and probably
misleading for authors to infer the transport of silica in
super-critical CO2 as a mechanism to try and justify
their conclusions.
Although most authors make a comment that quartz is not known to
be transported in non-aqueous CO2 fluids, the all too
obvious conclusion is that there really was a dominant aqueous
fluid present in all these systems and that the aqueous fluid
transported and deposited the overwhelmingly dominant quartz
mineralization and quite possibly also the gold. And for some
reason the aqueous fluids failed to give rise to identifiable
fluid inclusions, or that the aqueous phase inclusions present
have been ignored.
The real problem here is to understand why the dominant aqueous
fluid is not being recognized and reported in the research
results.
Microscope observation of fluid inclusions is a tedious and
often boring task and there is a strong tendency to focus ones
attention on the unusual observations. So large inclusions, or
ones with unusual compositions invariably attract attention. The
myriad of small inclusions are often too small to allow the
contents to be seen clearly and are ignored. Or they are
considered to be secondary inclusions and unimportant. In many
cases the paragenesis of the small inclusions is unclear and
established protocol is to ignore inclusions which do not meet
the very strict observational criteria to confirm their origin.
So these inclusions are routinely ignored simply because they
cannot be classified. But that does not mean the fluids they
represent are inconsequential! It means the research is
incomplete!
If a fluid inclusion study were done on our favourite sugary,
fizzy soft-drink, we would see that the "inclusions" (bubbles)
present are almost entirely CO2 with a very small
water content. So should we conclude that the "mineralization"
(sugar) present must have been transported by pure CO2
fluids? Of course not, the host medium (water) is not irrelevant
in understanding this system, even though the water itself is
rather common and boring. Similarly, the host quartz medium for
the fluid inclusions and gold in a mineralized quartz vein is
not irrelevant. To understand the system one must explain the
origin of all the species present and silica (as quartz) is the
dominant component which must not be ignored merely because it
is common or boring. It is a key part of the mineralizing system
and it was not transported by pure CO2 fluids. The
ubiquitous existence of the quartz suffices to assert the
dominant involvement of aqueous fluids in the system.
Observations of CO2 rich fluids are not wrong. They
probably demonstrate clearly that at some stage during the
millions of years of activity of the hydrothermal system there
was at least one CO2 rich fluid phase event. But for
how long did this last, how important was it in the overall
formation and was it actually the phase that carried the
economically interesting mineralisation? Or was it just a
sideshow to the main aqueous fluids which actually transported
the silica and other minerals of interest?
Hydrothermal systems are not simple stable fluid events. Most
studies describe multiple different fluid inclusion assemblages
and there are clearly different fluids present at different
times. But there is still a reluctance to understand just how
variable such systems are as would be witnessed by visits to the
geyser fields at Yellowstone park or other surface outcrops of
present day hydrothermal systems. The fluids can change in mere
minutes. And typical mineralizing quartz systems were active for
millions of years, so there was ample opportunity for numerous
fluid compositional changes. Our observations are hardly
representative of the immense amount of variability that really
does occur in hydrothermal systems. It is naive to therefore
think that observation of some interesting CO2 rich,
non-aqueous inclusions in a quartz vein is adequate to prove
that an aqueous phase was never present at all and that the
economic mineralization (and the quartz itself) must therefore
have been transported in this probably fleetingly transient
fluid event.
Observations of non-aqueous CO2 fluids in quartz
veins do not prove that aqueous fluids were absent. Until there
is evidence (currently lacking and unlikely) that silica can be
transported in and deposited from such non-aqueous fluids, then
the mere fact that the observations are being made on fluid
inclusions hosted within quartz is enough to be quite certain
that aqueous fluids were not only present but were actually
dominant as they transported and deposited the quartz which
comprises some 99.999% of the mineral matter present.
In my own baro-acoustic decrepitation analyses of samples the
results reflect the entire assemblage of inclusions present in 1
gram of sample. Using this method at the Brusson gold mine in Italy
I identified 3 distinctly different inclusion assemblages and
their spatial distribution at differing depths within the vein
system. (Burlinson, 2010) Because one
of these assemblages terminated at the same mine level as the
gold mineralisation terminated it was possible to develop a
depositional model to explain the high level cutoff of both this
quartz type and the gold mineralization. Understanding the
change in quartz deposition at this level was crucial in
realising why the gold mineralisation terminated at the same
level. Previous studies of this mine had been unable to provide
anything more than a vague explanation of this mineralisation
cutoff despite several PhD doctorates, numerous fluid inclusion
measurements and extensive stable isotope work. It is always
appropriate to understand the quartz vein itself when studying
the genesis of a minor mineral phase within a quartz vein.
It is unacceptable to ignore the overwhelming presence of
quartz, merely because its market value is less than that of
some minor metallic constituent with which it is associated. And
given the presence of quartz we can be pretty certain that
aqueous fluids were not only present but also important, or even
essential!