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Disproportionate inclusion trapping from
heterogeneous fluids
An explanation of CO2 fluid inclusion
populations lacking aqueous inclusions within a quartz host
mineral.
Kingsley Burlinson, September 2013
The Problem
In 1997 Schmidt-Mumm et. al. published research of quartz
from gold deposits in Ghana in which they found abundant pure CO2
fluid inclusions, but failed to identify any associated aqueous
inclusions. They surmised that the gold must be transported in
pure CO2 fluids by an unknown chemical mechanism.
However, such an assertion completely fails to recognize or
explain the presence of quartz, which comprises some 99.999% of
the mineral material transported in the hydrothermal fluids. (Discussed here)
It is incorrect to assert without any supporting evidence
whatsoever that the gold, and by inference the quartz also, must
have been transported in pure CO2 fluids lacking any
water. This paper is being referenced by other research work in
the literature as proof that gold can be transported in pure CO2
fluids. But this paper alone is completely inadequate to support
the hypothesis of gold transport by pure CO2.
The observations can be explained as merely disproportionate
trapping of CO2 and aqueous phases from
a very normal heterogeneous, aqueous dominated fluid with some
CO2 present as an immiscible gas phase.
Both the quartz and the gold are probably transported in the
dominant aqueous fluids which, due to inclusion trapping
conditions, were only poorly trapped as inclusions, while the
co-existing separate immiscible CO2
fluid phase was efficiently trapped as fluid inclusions.
Heterogeneous trapping is a well known phenomenon and has been
discussed at length by E. Roedder (1984). But it has been
forgotten or ignored in some research papers which can lead to
serious misinterpretation of fluid compositions.
Special note: This discussion page focuses on heterogeneous
fluids which contain an immiscible gas phase, specifically CO2
here. But another common heterogeneous system is of water vapour
within liquid water, found in boiling epithermal deposits and
this very different heterogeneous fluid system is discussed here.
CO2 rich non-boiling heterogeneous
systems
Most fluid inclusions are trapped under conditions where the
parent fluid is a single homogeneous phase due to the high
temperatures and pressures during mineral formation. The study of
fluid inclusions usually relies on this homogeneity to be able to
deduce the original formation conditions from a fluid inclusion
which has subsequently separated into 2 or more phases at room
temperature. Frequently this is a liquid water phase with a low
pressure vapour bubble left behind due to condensation of the
water. There may also be some CO2 in the vapour bubble
or even a separate CO2 liquid phase. This condition is
so common that it is easy to overlook the cases in which the
parent fluid was heterogeneous. In particular, CO2 and
water have a large field of immiscibility and will exist as a
heterogeneous mixture at lower temperatures as shown in this P-T-X
diagram.
Fluid inclusions formed within the 2 phase immiscibility region
will have widely differing compositions depending upon which phase
they have trapped, the gas alone, the liquid alone or a
combination of both phases. Trapping from such heterogeneous
fluids is not uncommon and often occurs in fluid inclusion
populations. But unless recognized, studies of such fluid
inclusion assemblages will give misleading results as the
inclusion assemblages do not preserve the original component
proportions of the fluid system.
E. Roedder (1984, page 29) states:
"The inclusions
that are trapped in crystals growing from a heterogeneous
system of two fluid phases may reflect both nucleation and
wetting phenomena, and very erroneous conclusions can be
drawn if these and other features are not considered. As
an example, consider bubbles of CO2 floating as
the dispersed phase from a crystallizing basaltic melt.
The growing crystals of one (or all) of the phases may
become coated with tiny gas bubbles, each of which shields
that part of the surface with which it is in contact from
further growth, and so it becomes enclosed. Note first
that this process may yield a large number of primary gas
inclusions, without any melt, in a crystal that actually
grew from a melt. This would hardly mislead one into
assuming that the silicate crystals grew from a CO2
fluid. But the identical phenomenon, involving steam
bubbles forming on crystals growing from a hydrothermal
fluid and trapped without any of the fluid, may easily be misinterpreted as
evidence that the crystals grew from a low-density fluid
and hence were one more example of "pneumatolysis." The
distinction can be of more than just academic importance.
Such a nucleation process should be particularly effective
if sudden small pressure drops cause periodic
effervescence of dissolved gas, or boiling of the fluid,
as has been suggested by Barabanov (1958) for some
so-called "pneumatolytic" tungsten deposits."
Inclusions trapped from multi-phase inhomogeneous fluids do not
preserve the ratio of the original phases. If crystal growth rates
were slow, there may be only very few crystal defects in which to
trap inclusions of the aqueous phase. It is entirely possible that
even the major phase of the parent fluid may not be trapped in
inclusions at all and only a lesser phase, such as CO2
gas bubbles, is trapped from a dominantly aqueous system. Failure
to understand this can lead to incorrect conclusions as stated by
E. Roedder, 1984, page 34:
"Incorrect
inferences are frequently drawn in the inclusion literature
that the amount of vapour phase trapped, or the ratio of the
number of inclusions of vapour to those of liquid phase,
give an indication of the relative amounts of the two phases
present in the original heterogeneous system. A single tiny
inclusion of gas phase indicates gas saturation just as well
as a million such inclusions, but the amount and number of
such inclusions in a sample are merely a result of the
vagaries of the inclusion-trapping processes discussed
above, and do not give any valid indication of the phase
ratios in the original two-fluid system at any given time.
The trapping of gas inclusions may even give misleading data
on the sequence of phase changes with time. Thus if one of a
series of zones of primary inclusions in a given crystal is
made up of gas-rich inclusions, the common interpretation
given in the literature is that gases "played a major role"
in the deposition of that zone. All it may really mean,
however, is that some minor amounts of gas bubbles formed
during the growth of that zone and were preferentially
trapped."
The process of trapping fluid inclusions is not necessarily simple
and is frequently ignored when studying them. However these
processes are particularly important when dealing with trapping from
a heterogeneous fluid.
An explanation of disproportional inclusion trapping from
heterogeneous fluids.
The processes of forming fluid inclusions can be classified into 2
major mechanisms.
Crystal defect type inclusions are growth
irregularities or defects in the crystal, in which some fluid is
accidentally trapped and sealed in by continued crystal growth
around the defect. Such irregularities or defects are
predominantly random and depend on growth rate, saturation
levels, flow rates, impurities and other factors. The fluids
trapped in such inclusions will indicate the composition of the
parent fluid component from which the host crystal grew.
Interference type inclusions are obstructions to the
crystal growth, typically caused by an inhomogeneous phase such
as a bubble of an immiscible gas within a liquid. Such an
obstruction can become attached to the growing crystal surface,
forcing the crystal to grow around it. The obstructing phase
will become trapped as a fluid inclusion, but its composition is
not indicative of the fluid component from which the host
crystal is growing. There may or may not be additional defect
type inclusions which contain the other component(s) of the
heterogeneous fluid system. As stated by Roedder (quoted above),
the bulk fluid composition of the parental heterogeneous fluid
cannot be deduced from such inclusion assemblages because the
components of a multiple-phase system are not trapped in
proportion to their abundances.
Conditions which favour the formation of inclusions by one of these
mechanisms may simultaneously suppress inclusion formation by the
other mechanism, which results in extreme disproportional trapping
of the heterogeneous fluid phases. This can be mistaken for
deposition from a single phase fluid as there may be almost no
evidence of inclusions from the other phase! Quartz veins growing
from slowly moving fluids will facilitate the attachment of bubbles
and at low silica supersaturation concentrations, will also result
in very few crystal defects. The resulting inclusion assemblage will
be dominated by the subordinate inhomogeneous gas phase with very
few or no inclusions of the dominant aqueous phase from which the
quartz is actually depositing. High flow rates and turbulence would
have the opposite effect as bubbles are swept away and crystal
defects become common. The resulting inclusion assemblage will
consist of only the quartz depositing aqueous phase (in crystal
defect type inclusions) with few or no inclusions of the
inhomogeneous gas phase. It is quite possible that inclusion
populations represent only one of the compositional extremities of
the heterogeneous fluid system, rather than representing all
of the heterogeneous fluid phases or approximating the bulk fluid
composition of the multiple-phase fluid system.
It is quite possible to grow crystals with almost zero growth
defects. An example is the semiconductor industry which routinely
grows very large silicon crystals which are free of defects, a
necessity for electronic chips. Although this is done from a melt,
it would be possible to grow quartz with very few defects (and thus
very few fluid inclusions) from a hydrothermal fluid if conditions
were right. Such crystals with very low inclusion abundances do
occur in nature (eg in Brazil). But it would be wrong to infer that
these crystals grew without fluid involvement merely because of the
lack of inclusions.
During quartz vein formation, pressure reductions (for example,
during earthquakes) could cause the effervescence of bubbles of CO2
from a homogeneous aqueous-CO2 fluid whose pressure and
temperature was close to the immiscibility field. If the fluid flow
was gentle and silica deposition slow, these bubbles would be
efficiently trapped as interference type inclusions, but at the same
time crystal defects would be rare, resulting in very few inclusions
of the aqueous phase as they only occur in the rarely formed crystal
defect type inclusions. This disproportionate trapping of the 2
fluid phases present could lead to misinterpretation of the bulk
fluid composition.
Experimental limitations
The failure to observe additional fluid phases in a heterogeneous
fluid system is exacerbated by the standard protocol for the study
of fluid inclusions. It is normal to completely ignore very small
inclusions (less than about 5 microns across) as it is very
difficult or impossible to make precise observations of the fluid
phases within such small inclusions. And the many inclusions whose
primary / secondary paragenetic origin cannot be ascertained are by
default assumed to be secondary and are ignored. Only large,
demonstrably primary inclusions are used in most fluid inclusion
studies. But this results in the vast majority of inclusions in a
sample being ignored for experimental practicality reasons. These
ignored inclusions are not necessarily irrelevant and may actually
prove that the fluid was heterogeneous.
Sealing up the inclusion
There is so much focus on the fluids within the inclusion that the
mechanism of sealing the inclusion to actually trap the fluids being
observed is usually overlooked. But this is a critical process
without which the fluid inclusion would not even exist. This
requires simultaneous deposition of the host mineral, usually
quartz, either from the fluid being observed, or from one which
co-existed with that fluid and which may not be trapped in
inclusions or recognized. This fluid must have been transporting and
depositing the host mineral phase at the time the inclusion was
trapped. It is logically incorrect to propose a fluid in which the
host mineral is insoluble and the observation of only such fluids
implies the existence of additional fluids which have been
overlooked and ignored. (Secondary or pseudo-secondary inclusions
might form by solid state re-crystallization of a pre-existing host
mineral with little or no fluid phase transport.)
E. Roedder (1984, page 35) briefly mentions the problem of inclusion
fluids in which the host mineral is insoluble:
"The enigma of inclusions of fluid having zero
solubility for the host. One fascinating but seldom
considered paradox in the formation of inclusions from
immiscible fluid pairs concerns the exact mechanism whereby
fluid inclusions can become sealed even though they apparently
contain only a fluid in which the host crystal is
essentially insoluble. The usual explanation is that the fluid
that is eventually trapped was present as the dispersed phase
(i.e., globules) in a continuous phase, and that
crystallization of the host mineral took place only from
the continuous phase."
Clearly, there must be a fluid present which is capable of
transporting and depositing the host mineral phase in order for a
fluid inclusion to be enclosed and trapped. Proposing a pure CO2
fluid as the only fluid present in a quartz host mineral, as
Schmidt-Mumm et. al. have done, requires that a mechanism for the
transport and deposition of silica from this same fluid must be
explained. As silica is not known to be transported in pure CO2
fluids, then there must be another (probably aqueous) phase present
and the fluid system must be heterogeneous.
The failure to identify or consider this additional aqueous phase is
a serious experimental oversight.
Summary
Quartz containing essentially pure CO2
inclusions and lacking aqueous inclusions can be explained as the
product of deposition from a heterogeneous fluid comprised of
mostly water with a few CO2 bubbles. Slow
growth from a heterogeneous, aqueous, silica-bearing fluid which
also carried CO2 bubbles would facilitate
attachment of the small CO2 bubbles to
the quartz surfaces while simultaneously suppressing the formation
of crystal growth defects. This would produce quartz with many CO2
inclusions of the subordinate gas phase which are formed by
obstruction of the crystal growth, but very few aqueous
phase inclusions of the dominant silica
bearing fluid because of the lack of crystal defects necessary
to trap them. The gas inclusions are likely to be
spherical in shape as their shape is controlled by the surface
tension of the liquid host fluid. The quartz observed in Ghana by Schmidt-Mumm et. al. (1997)
is best explained as the product of disproportionate trapping from
a heterogeneous fluid. There is no need to suggest the transport
of gold and silica by pure CO2 fluids as
the system was actually dominated by normal aqueous fluids, which
simply happen to be poorly trapped in inclusions. This has mislead
the authors into thinking that no aqueous fluids were
present. Although Schmidt-Mumm et. al. did consider the
possibility of heterogeneous trapping, they wrongly presumed that
this could not explain the lack of aqueous inclusions. But as Roedder (quoted above) has stated, fluids
trapped from heterogeneous fluids are not trapped in proportion to
their phase abundances. It is entirely possible that
aqueous fluid inclusions are absent simply because of gentle
growth conditions and lack of crystal defect type inclusions,
while interference type inclusions (the CO2 gas phase
in this case) will simultaneously be efficiently trapped.
In their paper, Schmidt-Mumm et. al. published a photograph of the
fluid inclusion assemblage they studied. This shows abundant
spherical or near spherical inclusions which is in fact clear
evidence that this assemblage of inclusions was trapped from a
liquid dominant host phase as heterogeneous bubbles, as discussed in detail here. (Spherical
inclusions are formed due to the surface tension of a host liquid
phase and cannot be formed from a purely vapour phase or pure CO2
fluid.)
In addition, the failure to explain the transport and deposition
of the host mineral (quartz) in pure CO2 fluids
confirms that a concurrent silica-bearing aqueous phase must have
been present. This aqueous phase quite probably also transported
the gold and there is no need whatsoever to propose the transport
of gold in pure CO2 fluids.
In the absence of proper consideration of the effects of
disproportionate trapping from a heterogeneous fluid and failure
to explain the critically important transport of the host
mineral quartz, this paper fails to provide any reasonable
evidence for the "new category of ore forming fluids" that it
postulates.
Considerable care needs to be taken when interpreting such
heterogeneous fluid inclusion assemblages to avoid serious
misinterpretation of the composition of the inferred parent fluid
systems.
Edwin Roedder (1984) Fluid Incusions Reviews in
Mineralogy, Volume 12, Mineralogical Society of America.
A. Schmidt-Mumm et al. (1997) High CO2
content of fluid inclusions in gold mineralisations in the
Ashanti Belt, Ghana: a new category of ore forming fluids?
Mineralium Deposita, 1997, V32, p107-118
R. J. Bodnar, T.J. Reynolds and C.A. Kuehn, 1985. Fluid
inclusion systematics in epithermal systems, In:
Geology and geochemistry of epithermal systems, Reviews in
Economic Geology, Volume 2, Society of Economic Geologists.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 