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Abstract

Gold is typically transported as aqueous species, often in CO₂ rich fluids, but some workers have found fluid inclusions in quartz which contain only CO₂ with no visible aqueous phase. This has caused concern in how to explain the trapping of such fluids and the associated deposits. One proposed hypothesis is for gold transport as a CO₂ complex in a super critical CO₂-only fluid, despite the absence of any chemical justification. An even more serious problem is the lack of any explanation for transport and deposition of the silica which actually comprises the majority of the mineral deposit, gold being merely a trace impurity in the quartz veins observed. The actual explanation is that a parent CO₂-rich fluid unmixed into 2 immiscible fluids (2 phases), one dominantly liquid water and the other predominantly gaseous CO₂. Such a heterogeneous fluid mixture can transport and deposit gold and silica from the dominant aqueous phase, while appearing to be only a gaseous CO₂ fluid system. The hypothesis of gold transport in CO₂ fluids is an error based on the  mis-identification of heterogeneous fluid inclusion assemblages and both the gold and silica are in fact transported only in the aqueous phase of the fluid mixture.

The underlying assumption of fluid inclusion studies is that the parent fluid was a homogeneous single phase fluid when trapped. Upon cooling this fluid separates into multiple phases which can be studied to determine the original trapping conditions. But the original fluid composition cannot be determined if the mineralizing fluid underwent phase separation, giving a heterogeneous fluid mixture at the time the fluid inclusions were trapped and it can be very difficult to recognize such heterogeneous fluids during microthermometric studies. In the P-T-X_CO2 phase relationship of CO₂ and water there is a large region of immiscibility and such fluids unmix into binary mixtures. Incorrect conclusions can be made in the study of these CO₂ rich fluids because of failure to fully understand the nature of these common, but difficult to recognize, heterogeneous fluids. Heterogeneous trapping does not necessarily trap both phases in proportion to their abundance and can produce CO₂ only fluid inclusions while simultaneously failing to trap the coexisting aqueous phase. Together with recent thermodynamic studies on the CO₂-water system and an understanding of fluid inclusion formation and morphology it is now clear that the transport and deposition of Au and silica occurs within an aqueous, CO₂ rich, heterogeneous, unmixed fluid and that both gold and silica are transported by the aqueous phase. There is no need to invoke unknown chemistry to transport Au and SiO₂ in pure CO₂ fluids. Nor are convoluted phase separation mechanisms or post entrapment modification explanations required to produce CO₂ only fluid inclusion assemblages.

Introduction

Carbonic only fluid inclusion assemblages (FIAs) have been found at many gold deposits and often there seem to be no closely associated aqueous inclusions. (Schmidt Mumm et. al., 1997; Yao et. al., 2001; Wille and Klemd, 2004; Chi et. al., 2006; Klein and Fuzikawa, 2010). To explain such FIAs some authors propose post entrapment modification with diffusion or leakage of water out of inclusions through the host quartz. Other authors propose un-mixing of a parent aqueous-CO₂ fluid followed by separation of the CO₂ and aqueous phases with trapping of CO₂ fluid inclusions from the CO₂-only fluid. Schmidt Mumm et. al. (1997) proposed that at gold deposits in the Ashanti belt, Ghana, water was never present in the mineralizing fluid and that Au, and of necessity also SiO₂, were transported in a super critical pure CO₂ phase, invoking unexplained chemical species. This has led some subsequent research (Chi et. al. 2006, Klein and Fuzikawa 2010, Goldfarb et. al. 2017) to cite this paper, which appears to give credence to this dubious hypothesis. But none of these proposals are really very satisfactory. The post-entrapment removal of water has been questioned because it does not explain the consistency of inclusions on a deposit scale. The separation of a pure CO₂ fluid prior to trapping introduces an even more serious problem as it fails to explain how the essential host silica is deposited, while the transport of Au in carbonic-only inclusions requires unknown chemistry to transport the Au and also fails to explain the transport and deposition of the essential silica from such a fluid. In all these cases the authors have discounted the idea of heterogeneous trapping from two coexisting phases of an unmixed CO₂-rich fluid because they could not find co-existing fluid inclusions of both phases. They assumed that both phases must be trapped in proportion to their abundance and failed to understand the disproportional trapping which is well known in heterogeneous fluids (Roedder, 1984b; Burlinson, 2013). This disproportionation can be so extreme that only one of the two phases present in the fluid mixture is recognizable in the resulting FIA. The inclusions of one phase may be absent or simply be so small that fluids within them cannot be observed. Such small inclusions are often classified as secondary and are ignored, but they may well be small primary inclusions and should be called indeterminate as they may actually be the second phase of a heterogeneous fluid binary pair. There is no reason why fluid inclusions of both phases must be the same size!

The fluid inclusion observations are better explained as the result of un-mixing of a parent CO₂ rich aqueous fluid into a binary pair of immiscible fluids, an almost pure CO₂ fluid and an aqueous fluid with CO₂, whose compositions can be deduced from the P-T-X_CO2 phase diagram. Inclusions in such a heterogeneous fluid are not trapped in proportion to their abundance in the mixture and there is no need to see inclusions of both fluids. Absence of one of the phases, e.g. the aqueous phase, does not prove the absence of that phase, merely the absence of trapping or the inability to observe it. Variations in the trapping of the two phases can be explained by remembering that this is actually a silica deposit and understanding the mechanisms by which inclusions are initiated, filled with fluid and then sealed into the quartz. The Au and SiO₂ are transported in the aqueous phase of the heterogeneous fluid mixture and there is no need for convoluted explanations of phase separation or post entrapment water leakage and no problem with silica transport, and definitely no need to propose unsubstantiated chemical species to transport either silica or Au as a CO₂ complex. 

CO₂ -water immiscibility

In the phase diagram for CO₂-water (Fig. 1) there is an extensive region of immiscibility within which the fluid will unmix into two distinct and separate phases, one aqueous dominant with minor H₂O and the other a CO₂ dominant fluid with variable H₂O. Depending on the deposit formation pressures, this immiscibilty can occur at up to 300°C (in the absence of NaCl) and occurs in almost all CO₂ containing fluids below 275°C. The presence of NaCl in the fluid dramatically increases the immiscibility, increasing the temperatures at which CO₂ immiscibility can occur.The studies which have found pure CO₂ inclusions all report trapping temperatures mostly within this immiscibility field and it is almost certain that they are observing the CO₂ component of a heterogeneous binary fluid mixture. In particular, Schmidt Mumm et. al. (1997) also found aqueous-CO₂ inclusions upon which to make temperature and pressure measurements for deposit formation. They estimated various pressures from <1.7 kbar to 5.4 kbar, mostly near 2.5 kbar and temperatures from 100°C to 325°C, mostly below 275°C. In the phase diagram for CO₂-water (Fig. 1) it is seen that most of these temperatures and pressures fall within the field of CO₂-water immiscibility and the authors should have been alert to the near certainty that heterogeneous trapping had occurred. However, they stated that selective trapping of gaseous inclusions after phase separation (i.e. a heterogeneous fluid) does not explain the lack of aqueous inclusions and therefore assumed that heterogeneous trapping did not occur. But this assumption that both phases of a heterogeneous system should be trapped is incorrect as disproportionate trapping from such fluids is normal and well known. (Roedder 1984b, Burlinson 2013). Chi et. al. (2006) studied the Campbell-Red Lake deposit in Ontario, Canada, and considered heterogeneous trapping but were so concerned by the absence of aqueous inclusions that they instead proposed fluid separation of a pure carbonic fluid before inclusion trapping, ignoring the serious problem that this fluid could not transport the essential silica. Yao et. al. (2001) noticed this same absence of aqueous inclusions in deposits in the Birimian terrane, Ghana. They identified heterogeneous trapping in various mines which also had aqueous inclusions, but at the Sansu mine they proposed post entrapment modifications to explain the lack of aqueous inclusions.

The thermodynamics of the CO₂-water system is quite complex but has been thoroughly described by Diamond (2001). Pichavant et. al. (1982) state that because of the extremely low mutual solubility of CO₂ and NaCl, it can be expected that the miscibility gap(s) in the ternary system H₂O-CO₂-NaCl will be drastically enlarged with regard to those in the systems H₂O-NaCl and H₂O-CO₂ . The addition of small amounts of NaCl to the H₂O-CO₂ system greatly increases the region of fluid immiscibility (Bodnar et. al. 1985). The extensive P-T-X_CO2 immiscibility is a major challenge in fluid inclusion studies but is well known and all such CO₂ rich fluids, particularly near pure CO₂ fluids, should be considered as quite probably unmixed phases of a heterogeneous fluid system.

This diagram (Fig. 2) from Larryn Diamond (2001), shows the complexity of the CO₂-H₂O system and the extensive immiscibility field (shaded). This diagram does not show the effect of NaCl, which dramatically increases the CO₂ immiscibility. This figure is merely diagrammatic to emphasize important features near 0° C and to avoid the further complexity of showing actual values.

Fig. 2  Schematic P–T–X model of equilibrium phase relations in the system CO₂ –H₂O (not to scale). The topology is true except for
the critical point of pure H₂O , which is drawn for clarity at a lower pressure than the critical point of pure CO₂ . The region
of liquid–liquid immiscibility, the clathrate dissociation surface, and the compositional planes of solid CO₂ and clathrate are shaded. (original author's caption)

This actual plot (Fig. 3) of the P-T-X_CO2 system in the immiscibility region of interest shows compositions without any NaCl. The addition of even small amounts of NaCl dramatically increases the immiscibility region but actual thermodynamic data to show this effect is very limited and not readily available in the literature. The data for this plot has been reprocessed from experimental data by Tödheide and Franck (1963).  Note that the extent of immiscibility of CO₂ initially decreases as the pressure increases, but then reverses and increases with further pressure increases. (Refer to Fig. 1 also.)

Transport of the host mineral phase (quartz)

 The fluid must be capable of transporting the host mineral phase, silica in this case, as these are in fact silica deposits. The gold is merely an interesting impurity. Fluid inclusion studies are actually observing the host mineral, in most cases quartz and the inclusions are minute crystal imperfections in the host quartz, from which we try to infer the fluid conditions which transported and deposited the quartz. It is then inferred, based on paragenetic observations, that these fluid conditions also relate to the gold impurities which are of interest. Roedder (1984a) discussed the enigma of inclusions of fluid having zero solubility for the host and points out that the trapped fluid must have been a dispersed phase within a second continuous phase, that is, heterogeneous trapping. The host quartz phase exerts primary control over the trapping of fluid inclusions. Depending on the hydrothermal fluid conditions, degree of silica supersaturation and many other factors the quartz may or may not trap inclusions and also controls the size of inclusions. It is logically incorrect to ignore the host phase mineral simply because it is not of economic importance. There is no attempted explanation of, or known mechanism to transport large quantities of silica in pure CO₂ fluid and the hypothesis that pure CO₂ fluids can transport gold and silica is untenable.

Fluid inclusion morphology

The fluid inclusion morphology shown in the published photo by Schmidt Mumm et. al. (1997) shows an abundance of round (spherical) fluid inclusions (Fig 4). Such inclusions are formed by the surface tension of a liquid phase enclosing a gas bubble. This shape is not normal for fluid inclusions in quartz, where the inclusion shape is controlled by the crystal structure of quartz.

Fig. 4 Original photograph of fluid inclusions from Schmidt Mumm et. al. 1997, with added arrows pointing to a few of the many spherical inclusions. Such spherically shaped inclusions are abnormal in quartz and are formed by trapping a gas bubble within a host liquid phase, proving the presence of a dominant liquid phase and heterogeneous trapping. (Modified after Schmidt Mumm et. al., 1997)

Fluid inclusion studies usually report that inclusions are of negative quartz crystal shape, or they can be of irregular shape if silica deposition was fast. Any subsequent recrystallization actually enhances the crystalline shape. A scanning electron microscope (SEM) image of an opened fluid inclusion is shown in Fig. 5 and numerous additional SEM photographs of opened fluid inclusions (Burlinson 2014b) show the strong negative crystalline shape control of inclusions.

Fig. 5 SEM image of an opened fluid inclusion in quartz. Note the angular shape of the inclusion, controlled by the crystal structure of the host quartz. (From Lawrence et. al. 2013)

The numerous spherical, CO₂-only inclusions seen in the photo in the original published paper are clear proof that these inclusions formed as gas bubbles within a dominant liquid (aqueous) phase from which the silica deposited around the gas bubble. The importance of inclusion shape was completely unrecognized in the original published research. But not all gas filled inclusions are spherical and some studies (Chi et. al. 2006) show sub-rounded or somewhat angular gas filled inclusions. The process of trapping an inclusion is not instantaneous and it takes a long time for the enclosing silica host mineral to deposit around the gas bubble until it is completely sealed. The silica will preferentially deposit in locations controlled by the crystal structure, but is blocked by the presence of the gas bubble. Depending of the silica 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. But the end result is still a pure CO₂ fluid inclusion which was trapped from a dominantly aqueous fluid.

Competing mechanisms to trap fluid inclusions

Inclusions are formed by two entirely different processes which results in disproportionate trapping of heterogeneous fluids. For homogeneous fluid trapping, fluid inclusions are the result of random crystal defects during the deposition of quartz. These defects grow into crystal cavities which seal over into fluid inclusions containing some of the homogeneous fluid. But fluid inclusions also form when foreign items, such as a heterogeneous gas bubble, force the quartz to deposit around them by interference with normal crystal growth. Conditions which favor 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 deposition rates, will also result in very few crystal defects. The resulting FIA 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 aqueous phase (in crystal defect type inclusions) with few or no inclusions of the inhomogeneous gas phase. To trap a gas bubble within a liquid the bubble must remain in place for some time. Small, fluid inclusion sized bubbles have very low buoyancy and can remain in place long enough to become fluid inclusions. The presence of natural surfactants may also facilitate bubble stabilization on the growing quartz surfaces, enhancing the trapping of gas bubbles. The resulting disproportionate trapping can easily be so extreme that only one of the phases of a heterogeneous system may be observed in the resulting FIA (Burlinson, 2013). Schmidt Mumm et. al. (1997) failed to understand this disproportionate trapping and actually assumed that failure to trap both phases proved that the fluid was homogeneous and therefore water was absent. This is the critical error in their work as absence of one of the fluid phases in a FIA (in this case, the aqueous inclusions) cannot be used to prove the absence of a second phase in the parent fluid. This lack of aqueous fluid trapping was the only evidence that Schmidt Mumm et. al. (1997) offered against heterogeneous trapping and is inconclusive. Other authors have also wrongly interpreted the absence of one of the unmixed fluid phases in FIAs as evidence against heterogeneous trapping.

Many workers become entangled trying to explain the formation of CO₂ only inclusions and argue in favor of physical phase separation of CO₂ from the H₂O (Wille and Klemd 2004, Chi et. al. 2006, Klein and Fuzikawa 2010). But this is self-defeating as it is not possible to deposit silica from the resulting pure CO₂ fluid and hence no fluid inclusions can be formed at all, much less pure CO₂ inclusions. Deposition of the mineral host phase (silica) is the primary requirement of fluid inclusion formation, only then can concerns about the inclusion contents be addressed. The pure CO₂ inclusions must be formed within a silica transporting phase which is most probably liquid water. This is easily accomplished by the trapping of heterogeneous CO₂ bubbles. The real issue is to understand why aqueous inclusions seem to be absent, which is merely disproportional trapping and is a function of numerous variables in the hydrothermal fluid system. And aqueous inclusions may not actually be absent, simply overlooked because they are too small to be properly examined or assumed to be secondary inclusions.

Identification of fluid immiscibility

Ramboz et. al. (1982) outlined three general criteria for the identification of fluid immiscibility. These are: (1) contemporaneous trapping of two different fluid types reflected in fluid inclusion characteristics, (2) contrasting total homogenization into both the liquid and vapor phases over the same range of temperature and (3) the same entrapment pressure at the homogenization temperature of both fluid inclusion types (i.e. if one inclusion type decrepitates before homogenizing, the other type must behave similarly). These criteria may apply to the boiling water heterogeneous system, but they ignore the problem of disproportional trapping which is common in the CO₂-water system and which may trap only one of the fluid phases. Using these criteria would result in highly disproportional heterogeneous systems being incorrectly classified as homogeneous. In addition, the decrepitation criteria of item 3 does not apply to CO₂ rich fluids, which are known to have anomalously low decrepitation temperatures caused by the CO₂ and explained by Burlinson (2005). The Ramboz criteria can prove the occurrence of heterogeneous fluids, but absence of these criteria does not prove the converse, ie, homogenous fluids, as this can simply be disproportional trapping of the heterogeneous phases. (Additional discussion of the Ramboz criteria problem is here) Microscope observation procedures used to identify boiling water immiscible systems are not directly applicable to water-CO₂ immiscible systems because of the common behaviour of disproportional trapping. (Additional discussion here.)

Boiling and immiscibility heterogeneous systems are different

The expected inclusion assemblages of heterogeneous fluid systems are frequently based upon the observations of heterogeneous boiling water systems. But Burlinson (2014a) has explained that heterogeneous trapping of water and steam in a boiling hydrothermal system behaves very differently to heterogeneous trapping of the immiscible phases in the CO₂-water system. Bodnar et. al. (1985) provide a comprehensive review of fluid inclusion techniques for studying epithermal boiling fluid systems. They explain that boiling systems should trap inclusions with variable liquid/vapor ratios, representing the end-member liquid and vapor phases as well as inclusions with random mixtures of these phases. Plots of the liquid-vapor phase ratios, or the observed homogenization temperatures should show a bimodal distribution. But that discussion applies specifically to single-component heterogeneous systems (boiling aqueous systems). It is also difficult to apply as often the vapor phase inclusions are absent or their homogenization temperatures cannot be measured. In boiling aqueous fluid systems, silica deposition and fluid inclusion trapping and sealing are very rapid, so inclusions represent the complete range of phase mixtures as they are sealed before phases can migrate and agglomerate. And both interference type-inclusions and crystal defect-type inclusions are abundant in this rapidly deposited quartz. As an aqueous hydrothermal fluid boils, changing much of the liquid to vapor, the volume of the liquid is drastically reduced and all the solutes in the parent fluid, including silica, are concentrated into this reduced liquid volume. The silica concentration exceeds saturation, but cannot become supersaturated because of the great turbulence in the boiling fluid and so silica is deposited rapidly at a rate to match the rate of boiling. Under these conditions of rapid deposition, the quartz is full of crystal defects, which become fluid inclusions when they are sealed over by continuing quartz growth and interference-type inclusions are also trapped. The defect-type inclusions trap the parent fluid liquid phase, as well as mixtures of the liquid and gas phases. Gas filled inclusions are also trapped by interference with quartz deposition. Overall these boiling systems are dominated by defect-type inclusions which contain both phases from the parent fluid. Consequently the boiling fluid inclusion assemblage data could give bimodal data distributions. But there is no certainty that all boiling systems will show bimodal data distribution plots and the lack of such bimodal plots cannot prove the absence of boiling.

However, immiscible gas fluid systems (multi-component), which did not boil, do not deposit silica so quickly and this can give a very different appearance to the FIAs. This difference in the rate of silica deposition can lead to preferential trapping of one of the two phases present because the liquid rich inclusions occur as defect-type inclusions and the gas rich inclusions occur as interference-type inclusions which are trapped by different and independent mechanisms. Slow silica deposition will give very few crystal defects and therefore no way to trap the dominant liquid phase as inclusions. But gas bubbles will be efficiently trapped by interference with the deposition of quartz, which is forced to deposit around the bubbles, which then become pure gas filled inclusions. This fluid system is prone to extreme disproportional trapping. Consequently it is unlikely that plots of such immiscible gas FIA data would be bimodal, despite this being a true heterogeneous fluid system. It is quite possible that only fluid inclusions of the minor CO₂ gas phase are trapped with no inclusions of the dominant aqueous phase, or that only aqueous inclusions are trapped as the gas phase was flushed through before it could be trapped in inclusions.

In the boiling system, the two phases are metastable, with large thermal transfers of latent heat taking place, causing turbulence and rapid silica deposition as the liquid volume is reduced. In the immiscibility system the phases exist together in complete stability with no thermal transfers. Conclusions or expectations drawn from the boiling heterogeneous system cannot be directly applied to the immiscibility heterogeneous system.

Thermodynamic experiments on super critical CO₂

Weihua Liu et. al. (2015) conducted experiments to examine the solubility of gold in super critical CO₂. They state that the results showed that gold solubility has a negative correlation to the CO₂ content in the fluids� and 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. The gold would have been transported in water rather than CO₂. That work has been summarized by Burlinson (2016) and Mei et. al. (2017) have published a detailed paper of this work.

Summary & Conclusions

Quartz vein hosted gold deposits, including those in West Africa, are formed from aqueous hydrothermal fluids which had relatively high CO₂ contents, but no more than mesothermal gold deposits in other gold provinces. The suggestion of Au transport in pure CO₂ hydrothermal fluids made by Schmidt Mumm et. al. (1997) was based on a failure to understand heterogeneous trapping at temperatures and pressures within the immiscibility field of water-CO₂ mixtures. None of their observations prove the absence of water and the spherical morphology of the inclusions in the photograph in their published paper actually proves that a dominant liquid phase, almost certainly water, was present. Hence the entire concept of transport of gold and silica in pure CO₂ fluids has no factual basis. In addition, the many convoluted explanations in the literature to explain pure CO₂ inclusions are unnecessary and actually compound the problem by failing to understand the requirement to deposit silica (or another host mineral phase). Such pure CO₂ fluid inclusions are merely heterogeneous gas bubbles trapped within a dominant aqueous phase which deposited the host mineral. The absence of trapping of the aqueous phase is a result of disproportional trapping due to the complexities of initiation and growth of fluid inclusions in the host mineral phase. Identification of FIAs trapped from immiscible heterogeneous fluids is frequently erroneous because of the failure to understand inclusion deposition mechanisms, the necessity of transporting the host mineral, disproportional trapping or that heterogeneous fluids due to boiling are very different to unmixed immiscible fluids. Almost all CO₂ containing fluids will be affected to some degree by the extensive immiscibility of CO₂ and water, particularly if even small amounts of NaCl are present, which must be carefully considered when interpreting these fluid systems.

References

Bodnar, R.J., Reynolds, T.J. and Kuehn, C.A., 1985. Fluid inclusions systematics in epithermal systems. In Geology and geochemistry of epithermal systems, Berger, B.R. and Bethke, P.M. eds. Reviews in Economic Geology, v 2. The Economic Geology publishing company.

Burlinson, K. 2005. Why CO₂-rich inclusions decrepitate at low temperatures http://appliedminex.com/decrep/general/pt.htm

Burlinson, K., 2013. Disproportionate inclusion trapping from heterogeneous fluids. http://www.appliedminex.com/viewpoint/disprop.htm

Burlinson, K., 2014a. Contrasting types of heterogeneous hydrothermal fluids. http://www.appliedminex.com/viewpoint/hetfitype.htm

Burlinson, K., 2014b. Spherical fluid inclusion shape can indicate heterogeneous trapping. http://www.appliedminex.com/viewpoint/fincmorph.htm

Burlinson, K., 2016. Thermodynamic studies show that Au is NOT transported in pure CO₂ fluids. http://www.appliedminex.com/viewpoint/au-notin-CO2.htm

Chi, G., Dubé, B. and Williamson, K., 2006. Formation of the Campbell-Red Lake gold deposit by H₂O poor, CO₂-dominated fluids. Mineralium Deposita v. 40 p. 726-741.

Diamond, L.W., 2001. Review of the systematics of CO₂-H₂O fluid inclusions. Lithos v. 55, p. 69-99.

Goldfarb, Richard J., Anne-Sylvie André-Mayer, Simon M. Jowitt and Gavin M. Mudd, 2017. West Africa: The World's premier paleoproterozoic Gold province. Economic Geology, v. 112, p.123-143.

Klein, E.L. and Fuzikawa, K. 2010. Origin of the CO₂-only fluid inclusions in the palaeoproterozoic Carará vein-quartz gold deposit, Ipitinga auriferous district, SE-Guiana shield, Brazil: Implications for orogenic gold mineralization. Ore geology reviews v. 37 p. 31-40.

Lawrence, D.M., Treloar, P.J., Rankin, A.H., Boyce, A. and Harbridge, P. 2013. 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. Economic Geology v. 108 p. 238.

Liu, W., Mei, Y., Brugger, J., Migdisov, A., and Williams-Jones, A 2015. Speciation and solubility of gold in CO₂-HCl-H₂O fluids: MD simulations and solubility experiments. Goldschmidt 2015 abstracts. Also at http://www.appliedminex.com/decrep/general/gold-CO₂-liu.htm

Mei, M., Liu, W., Brugger, J., Migdisov, A.A. and Williams-Jones, A.E. 2017. Hydration is the key for gold transport in CO₂-HCl-H₂O vapor. ACS Earth and Space chemistry, v. 1, p. 368-375.

Pichavant, M., Ramboz, C., and Weisbrod, A. 1982. Fluid immiscibility in natural processes: use and misuse of fluid inclusion data (I). Chemical Geology, v. 37 p. 1-27.

Ramboz, C., Pichavant, M. and Weisbrod, A. 1982. Fluid immiscibility in natural processes: use and misuse of fluid inclusion data (II). Chemical Geology, v. 37 p. 29-58.

Roedder, E., 1984a. Fluid Inclusions. page 35. Reviews in Mineralogy v 12, Mineralogical Society of America.

Roedder, E., 1984b. Fluid Inclusions. pages 29 & 34. Reviews in Mineralogy v 12, Mineralogical Society of America.

Schmidt Mumm, A., Oberthür, T., Vetter, U. 1997. High CO₂ content of fluid inclusions in gold mineralizations in the Ashanti Belt, Ghana: a new category of ore forming fluids? Mineralium Deposita, v. 32, p. 107-118.

Tödheide K. and Franck E.U.  1963. Das Zweiphasengebiet und die kritische Kurve im Kohlendioxid-Wasser bis zu Drucken von 3500 bar. Z. Phys. Chem. N. F. 37, 387-401.

Wille, S.E. and Klemd, R. 2004. Fluid inclusion studies of the Abawso gold prospect, near the Ashanti Belt, Ghana. Mineralium Deposita v. 39 p. 31-45.

Yao, Y., Murphy, P.J., and Robb, L.J. 2001. Fluid characteristics of granitoid-hosted gold deposits in the Birimian Terrane of Ghana: Fluid inclusion microthermometric and raman spectroscopic study. Economic Geology v. 96 p. 1611-1643.

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