inclusion morphology as an indicator of hetrogeneous trapping, pure co2 fluids and gold transport deposition

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)

Jump to the conclusions

Studies of fluids at the Ashanti gold mine, Ghana

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.

fluid inclusions at Ashanti gold, Ghana

(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.

frouded fi ashanti gold by Yao

(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.

angular fi ashanti gold by Yao

(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.

campbell-red lake canada co2 inclusions

(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."

However, both of these concerns are misguided as the authors have not understood that heterogeneous trapping does NOT trap the fluid components in proportion to their abundance, as discussed here. It is not only possible but quite likely that the aqueous phase may not be trapped at all. The trapping of an inhomogeneous gas bubble and the parent aqueous fluid happen by completely different, independent mechanisms which do not act in unison and may in fact act in mutual opposition, excluding the aqueous inclusions while simultaneously efficiently trapping the CO₂ gas inclusions.

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.

heterogeneous inclusions in diagenetic oilfield cement

heterogeneous inclusions in diagenetic oilfield cement

(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.

sem image of opened inclusion in fluorite
sem image of 4 fluid inclusions in fluorite

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.

inclusion in quartz

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.

Disproportionate inclusion trapping from heterogeneous fluids

Discussion of the differences between boiling and immiscible gas heterogeneous systems.

Thermodynamic experiments show that gold is NOT transported in anhydrous CO₂ fluids

Heterogeneous inclusion trapping explains assemblages of pure CO2 , non-aqueous inclusions.