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Heterogeneous inclusion trapping explains
assemblages of pure CO2 , non-aqueous inclusions.
Spherical fluid inclusion shape can indicate
heterogeneous trapping.
Kingsley Burlinson : June 2014,
September 2015
The Problem:
Many authors who study fluid inclusions and find
samples with only non-aqueous CO2
containing fluid inclusions propose formation models with
deposition from gas phase pure CO2 fluids. But they
forget that the quartz or other silicate, in which the
inclusions are merely a trace occurrence, must have been
transported by this fluid. Because there is no scientific
data to support such silicate solubility in CO2
fluids, unsubstantiated transport theories are often proposed.
These authors invariably ignore or misunderstand the effects of
fluid inclusion trapping from heterogeneous fluids, such as CO2
bubble effervescence in a dominantly aqueous fluid, which can
easily explain these observations. In particular, spherical
non-aqueous CO2 filled inclusions indicate formation
as bubbles within a dominant liquid phase, a clear case of
heterogeneous trapping. In addition, the apparent absence of
aqueous inclusions is not unusual for heterogeneous trapping and
does NOT prove the absence of aqueous fluids during deposition.
There is considerable failure to recognize or understand the
effect of heterogeneous trapping in fluid inclusion studies. There
is some general discussion of this problem here on this site
and also an additional discussion relating
to the misinterpretation of pure CO2 filled
inclusions from the Ashanti gold belt, Ghana. The trapping
of inclusions from heterogeneous fluid systems such as an aqueous
fluid with some bubbles of CO2 gas, does not preserve the original
proportions of the fluid system components. The absence of
aqueous inclusions and overwhelming dominance of CO2
filled inclusions is merely a normal product of heterogeneous
trapping. Many authors propose deposition from a gas phase system
to explain the lack of aqueous inclusions, despite the fact that
no-one can properly explain the critically important transport of
quartz or other silicates in such pure CO2 fluids. An
alternative proposal, that the original water has diffused out of
the inclusions, is also often considered. But the most likely
explanation is heterogeneous trapping.
Identification of heterogeneously trapped inclusion assemblages
The recognition of heterogeneous trapping based solely on
temperature and composition measurements of inclusions is
difficult. But observation of the shape of inclusions (the
inclusion morphology) can provide important evidence of
heterogeneously trapped inclusions. If spherical (or distorted
spherical) gas-filled inclusions are observed, then it is
certain that heterogeneous trapping as a bubble within a
dominant liquid phase has occurred. This is because the only
way to form such a carefully ordered spherical shape is due to the
surface tension of the liquid in which the bubble exists. This
surface tension of the liquid minimizes the surface area of the
gas bubble, and this minimum configuration is a sphere. There is
no mechanism by which spherical gas-filled inclusions can form
during deposition from a homogeneous gas phase. Consequently the
existence of gas filled fluid inclusions of spherical, or
distorted spherical form proves that the system was dominated by a
liquid. This is in complete contradiction of the assumptions seen
in the literature that gas filled inclusions imply an entirely gas
phase deposition. In fact, if such rounded, gas-filled inclusions
occur, it actually disproves that assumption. (See some SEM images of opened fluid inclusions here.
They are quite angular)
This image of the fluid inclusions in the Ashanti gold deposits,
Ghana was published by A. Schmidt-Mumm, Oberthur et. al. in
their paper in Mineralium Deposita.
(From Schmidt-Mumm et. al., Mineralium Deposita, V32, 1997, p110) Original caption: Example from the mineralisation at Ashanti
(Ashanti fissure) showing the commonly high number of exclusively
gaseous fluid inclusions in quartz.
Note the abundant round (spherical) gas-filled inclusions in this
photograph, some of which I have indicated by red arrows. And many
other inclusions show a strongly rounded shape. These
spherical inclusions cannot have formed from a homogeneous gas phase
and were trapped as bubbles within a heterogeneous, liquid dominant
fluid. It is certain that the fluids at Ashanti were in fact liquid
(probably aqueous) dominant and inhomogeneous with an unknown, but
probably only small, amount of CO2 gas bubbles. The
quartz and the gold in this deposit was not transported in pure CO2
as the authors suggest, but almost certainly in the abundant
co-existing water. The original inference of transport and
deposition from pure CO2 fluids made by these authors is
incorrect because they failed to recognize that the fluid system was
in fact heterogeneous. A PTX plot showing the heterogeneous deposition
field is here. The authors report low formation temperatures,
confirming that the fluids were within the heterogeneous field when
deposition occurred. Their concern about the lack of aqueous
inclusions is misguided as this is not unusual for inclusion
assemblages trapped from heterogeneous fluids, as explained by Roedder and elaborated
upon here.
This image of fluid inclusions in the same Ashanti gold deposit,
Ghana was published by Oberthur, Schmidt-Mumm et.al. in Economic
Geology.
(From Oberthur et.al., Economic Geology, V91, 1996, p292) Original caption: Primary gaseous CO2
± N2 ± CH4
inclusions in quartz, Ashanti mine.
These two papers are closely related variants of the same research
with mutual co-authors.
Although far less noticeable in this second image, there seems to be
a bias towards rounded inclusions, albeit misshapen. The inclusion
morphology does not provide conclusive evidence of heterogeneous
trapping in this sample, but does suggest the possibility of
heterogeneous trapping.
Yao et.al also studied the same Ashanti gold deposit and these
images are from their paper in Economic Geology, 2001.
(From Yao et. al., Economic Geology, V96, Nov. 2001, p1620) Original caption: One of the major fluid inclusion types in vein
quartz from the Sansu mine at Ashanti. 2P=primary monophase liquid
CO2 - N2
± CH4 fluid inclusions.
(From Yao et. al., Economic Geology, V96, Nov. 2001, p1617) Original caption: One of the major fluid inclusion types in vein
quartz from basin type granitoid-hosted gold deposits and
prospects at Nhyiaso mine at Obuasi, in the Kumasi basin.2P=primary
monophase liquid CO2 - N2
± CH4 fluid inclusions.
The single phase gas-filled inclusions in these 2 photographs
are quite angular. Only a few photographs are available in this
paper and perhaps the lack of spherical inclusions is due to
observational bias in choosing large, easily studied inclusions. The
morphology of these few inclusions alone cannot resolve the question
of heterogeneous trapping. Yao et. al. also found plenty of aqueous
fluid inclusions and concluded from their work that the CO2
rich inclusions are the result of entrapment from unmixed fluids
resulting from phase separation of an originally homogeneous
aqueous-CO2 fluid. That is to say, trapping from a
heterogeneous system of liquid water containing exsolved CO2
bubbles.
A study of fluids at the Campbell-Red Lake mine, Ontario, Canada
Guoxiang Chi et. al. studied the Campbell-Red lake gold deposit
(Ontario, Canada) and found H2O poor, CO2
dominated fluids.
(From Guoxiang Chi et. al., Mineralium Deposita, 2006, V40 #6-7,
p726) Abbreviated original caption: Various clusters of carbonic fluid
inclusions in quartz with homogenization temperatures to
liquid. From the Red lake mine, Ontario, Canada.
The authors state that aqueous-carbonic inclusions were only rarely
observed and are limited to the low grade gold zones. There is only
the above single suitably scaled photograph of fluid inclusions in
this paper, and all the inclusions shown are wholly CO2.
These inclusions are quite angular and their morphology does not
help to determine if the parent fluid was homogeneous or
heterogeneous. But this single photograph may be a specially chosen
case and may not be representative of the deposit. The angular shape
may be the result of post entrapment re-crystallization caused by an
invisible thin layer of water in the inclusion,
as discussed here.
However the authors did carefully consider 3 possible methods of
origin.
Deposition from a pure CO2 gas phase
Heterogeneous trapping from an aqueous phase, with
inhomogeneous CO2 bubbles.
Trapping of aqueous - CO2 inclusions followed by
preferential leakage of the water to give CO2 only
inclusions.
They discount preferential water leakage but were unable to
distinguish between trapping from a pure carbonic fluid or
heterogeneous trapping from an aqueous dominated CO2
bubble bearing fluid. They express great concern that the
heterogeneous trapping model "does not answer why fluid
inclusion evidence of the aqueous phase is absent or extremely
rare". They subsequently presume that it is necessary that "the
pre-unmixing fluid would be significantly more enriched in
carbonic species .... such that the vapour resulting from phase
separation would be volumetrically predominant and the trapping of
the aqueous phase prevented."
In their discussion of the proposed formation origin by homogeneous
deposition from a pure CO2 fluid, the authors completely
ignore the extremely serious problem of transporting and depositing
large quantities of silica from a pure carbonic fluid.
Diabolically, this silica deposition is a far more serious issue
than the unsurprising lack of aqueous inclusions in heterogeneously
trapped fluid inclusion assemblages, which they (wrongly) think is
the major problem!
Rounded gas inclusions in a petroleum reservoir
This image shows both gas filled and aqueous fluid inclusions from
the fracture filling cement in a gas reservoir. Note that many of
the gas filled inclusions, labeled g, are well rounded, near
spherical. Such gas inclusions would certainly have been trapped
from a heterogeneous fluid system which has resulted in near
spherical inclusion morphology.
(From András Fall et. al., AAPG Bull., V96 #12, 2012) Heterogeneously trapped gas inclusions within fracture cement in
sandstone, formed at about 160 C.
Fluid inclusions are usually angular, not spherical in shape. Spherical gas-filled inclusions indicate
heterogeneous trapping.
Many people argue that spherical fluid inclusions are normal, but
this is not so. The numerous photographs of fluid inclusions in
the literature almost invariably show angular irregular or
negative crystal shapes. Authors nearly always comment that the
inclusions they studied were of "negative crystal" shape.
Spherical shaped inclusions are quite uncommon and are the result
of unusual fluid conditions. They are not formed from homogeneous
fluids under normal conditions. The most reasonable explanation
for spherical inclusion shapes of gas-filled inclusions is that
they are the result of trapping of gas bubbles from within a
heterogeneous fluid. Think of the gas bubbles in your champagne -
a heterogeneous system in which liquid dominates. These bubbles
are trapped by interference with the growth of the host mineral, a
completely different mechanism to the formation of inclusions from
homogeneous fluids. (Discussed
here)
These are scanning electron microscope images of opened fluid
inclusions within fluorite and their extremely angular shapes are
quite obvious. Note also the fine striations and steps on the
inclusion walls, resulting from crystal growth irregularities.
Even these minute irregularities are not eliminated by subsequent
re-growth or metamorphism. Inclusions do not become "more
spherical" with aging, but perhaps more angular under the
influence of crystal lattice forces.
Figure 6. Fluid inclusions present in fluorite at the Blockspruit
prospect. SEM image of opened fluid inclusion of type 1 (V+L+S).
FROM: REE Mineralization of the Blockspruit Fluorite
Prospect, Bushveld Granitic Complex, South Africa:
Geochemical, Mineralogical and Fluid Inclusion Studies BY: Mihoko Hoshino*, Robert Moritz, Maria
Ovtcharova, Yasushi Watanabe, Jorge Spangenberg & Benita
Putlitz. (Poster
presentation and abstract at SGA conference, Nancy, France,
2015)
The following 4 SEM images of opened fluid inclusions in quartz
are from Lawrence et al.
Note the preservation of the quartz crystal shape of the
inclusion in C.
These all show good preservation of angular features of the fluid
inclusions, with little or no post-entrapment "rounding".
FIG .6. Secondary electron microprobe (SEM) images of opened,
quartz-hosted, multi-phase H2O-CO2-NaCl-FeCl2
type 4-e inclusions from Gara and Yalea North. A) to D) Images of
the various mineral phases observed in open inclusion cavities.
FROM: A Fluid Inclusion and Stable Isotope
Study at the Loulo Mining District, Mali, West Africa:
Implications for Multifluid Sources in the Generation of
Orogenic Gold Deposits BY: DAVID M. LAWRENCE, PETER J.
TRELOAR, ANDREW H. RANKIN, ADRIAN BOYCE, & PAUL
HARBIDGE IN:Economic
Geology, V108 #2 March 2013, p238
(membership required for full text)
The following SEM image shows opened fluid inclusions as negative
crystal shapes in sphalerite. Note that the inclusions are angular.
FROM:Fluid inclusions in sphalerite as
negative crystals - a case study. BY:Ivan K. BONEV and Kalin KOUZMANOV
IN: European
Journal of Mineralogy(June 2002), 14(3):607-620
(membership required for full text)
*** This is a small scale copy as the journal is "restricted
access" to members only.
Other SEM images of opened fluid inclusions have also been
published and these also show angular, not spherical inclusion
shapes.
Discussion
The recognition of heterogeneous inclusion trapping is an important
issue. Boiling, single component (H2O) fluids are one
case of heterogeneous trapping, which is well studied and can be
recognized from the coexistence of inclusions of very different
liquid-vapour ratios in the same fluid inclusion assemblage. But
immiscible gas effervescence (usually of CO2) from a
multi-component fluid (H2O + CO2) is also a
heterogeneous system and recognition that this is a heterogeneous
fluid is more difficult. However, the gas bubbles in such
heterogeneous systems will be spherical or very rounded because of
the surface tension of the host liquid phase. When these gas bubbles
are trapped, the morphology of the gas-rich fluid inclusions will
reflect their origin as spherical gas bubbles within a liquid host
phase. The resulting fluid inclusions may not be perfectly spherical
because various other processes, including post-entrapment changes,
may affect the shape. Consequently, well rounded and/or spherically
shaped gas-filled inclusions are an indicator that heterogeneous
trapping has occurred.
But not all gas filled inclusions are spherical, as seen in some of
the images above. Are such angular gas filled inclusions
evidence of formation from a homogeneous gas phase fluid, or could
they also form from heterogeneous trapping of a liquid - gas
mixture? I suggest that it is also possible for heterogeneous
trapping to form sub-rounded or somewhat angular gas filled
inclusions and that even these inclusions may be formed in a liquid
rather than a pure gas phase. The process of trapping an inclusion
is not instantaneous and it takes a long time for the enclosing
silicate mineral to deposit around the gas bubble until it is
completely sealed. The silicate molecules will preferentially
deposit in locations controlled by the crystal structure, but are
blocked by the presence of the gas bubble. Depending of the silicate
saturation level of the liquid, diffusion rates, fluid flow rates
and numerous other variables, both the crystal structure and bubble
interference will control deposition and affect the overall shape of
the inclusion. In addition, the gas bubble may not remain in place
depending on fluid turbulence and bubble coalescence. Bubbles may
move about or leave completely to be later replaced by another
bubble. This would allow some crystal faces to partially develop
before the final sealing of the inclusion occurs, leading to a
mixture of angular and rounded shapes to the inclusion cavity.
Inclusions which appear to be filled with a single gas phase most
likely contain an invisible layer of liquid water adhering to
the inclusion walls. The film of water in these inclusions
will allow recrystallization of the silica over geological time
which will lead to increasing angularity of the inclusions. Instead
of expecting inclusions to be rounded, the real issue is to
understand why the inclusions are not angular, because angular
cavities have lower crystal surface energy than spherical cavities.
The lowest surface energy of crystals (or cavities within crystals
for fluid inclusion shapes) occurs with surfaces conforming to the
crystal shape. Spherical fluid inclusions have higher surface
energies and will re-crystallize to give crystal structure
controlled surfaces, becoming more angular, if there is enough water
in the inclusion to allow silica dissolution, transport and
re-precipitation. This mechanism cannot happen in non-aqueous
inclusions, which will therefore remain spherical.
In contrast, homogeneous deposition would normally give rise to
negative crystal shaped or irregular inclusions, but not spherical
or strongly rounded inclusions. The above SEM images of opened fluid
inclusions indicate that any post-entrapment changes are dominated
by the crystal structure, with the inclusions retaining angular and
euhedral (negative crystal) shapes and they do not become rounded.
Conclusions
The shape of fluid inclusions can indicate the occurrence of
heterogeneous trapping. Spherical or well rounded gas-filled
fluid inclusions indicate that the inclusions were
trapped as a gas bubble within a dominantly liquid host phase.
Gas filled inclusions do not necessarily imply that they formed
from a solely gas phase fluid. The most common source for these
inclusions is in fact from heterogeneous trapping of subordinate
gas bubbles within a dominant liquid phase. Such heterogeneous
trapping can be difficult to recognize during fluid inclusion
studies and is frequently not recognized and misunderstood. The
shape of the inclusions may assist in recognition of heterogeneous
trapping because spherical or well rounded inclusions must have
formed by the trapping of a gas bubble within a liquid host phase.
The spherical shape of inclusions is caused by the surface tension
of a liquid which hosts a heterogeneous gas bubble phase, and
which constrains the gas bubble to have the minimum surface area
for its volume, i.e. a sphere. This bubble is then trapped by
interference with silicate deposition of the host mineral,
typically the quartz being studied. Other silicate deposition
mechanisms do not produce spherically shaped inclusions and
instead give inclusion shapes controlled by the crystal structure
of the host mineral, giving rise to negative crystal shaped
inclusions, or else completely random shapes if deposition is
fast, or some combination of these shapes.
A common misconception is that heterogeneous trapping should trap
fluids from both of the phases present. But the trapping mechanism
of the separate phases are different and completely independent as explained here, and there is
absolutely no need for both phases to be represented in the
resulting fluid inclusion assemblage. In addition,
experimental restrictions such as the need to work with large
inclusions and to only study inclusions whose paragenesis can be
determined also lead to the failure to recognize a coexisting
aqueous phase even when those inclusions do exist in the
inclusion assemblage.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
