Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
Viewpoints:

How CO2 inclusions form from aqueous fluids

Understanding heterogeneous fluids : why gold is not transported in CO2 fluids

Gold-quartz deposits form from aqueous heterogeneous fluids: NOT from CO2 fluids

Inclusion shapes can prove heterogeneous FI trapping

Disproportional FI trapping from heterogeneous fluids explains gas-dominant systems

A discussion of H2 analysis by mass spectrometry

A mechanism to form H2 in the MS ioniser during analyses


News:

Sangan skarn Fe deposits, Iran

New model 205 decreptiometer

Studies of 6 Pegmatite deposits

A study of the Gejiu tin mine, China


Exploration using palaeo-hydrothermal fluids

Using opaque minerals to understand ore fluids


Understanding baro-acoustic decrepitation.

An introduction to fluid inclusions and mineral exploration applications.



 Interesting Conferences:


AGCC expo, Adelaide, Aust. Oct. 14-18 2018

-----2019-----

ECROFI, June 24-26, Budapest, Hungary

AOGS, Singapore, 28 Jul-2 Aug 2019

SGA, Glasgow Scotland, Aug. 27-30 2019


Comprehensive Geology Conference Calendar


Explaining the formation of CO2-only fluid inclusions

K. Burlinson      April 2018

Introduction

There has been much confusion in the literature over the observation of completely CO2 filled fluid inclusions within quartz, lacking any visible water phase. In many cases the authors have concluded that the inclusions must have formed from  a non-aqueous CO2 fluid. But this is nonsense because  everyone has forgotten that you must first have deposition of silica in order to trap a fluid inclusion and silica is not transported in or deposited from pure CO2 fluids. These CO2 filled inclusions can be formed by heterogeneous trapping from aqueous fluids; there is no need to propose unsubstantiated silica transport and deposition from non-aqueous CO2 fluids!

Because of the extensive immiscibility of CO2 and H2O fluids, low temperature trapping below 250 C and disproportional trapping, the fluid inclusion assemblages (FIAs) may leave no trace of either of the 2 fluid phases, which many authors fail to understand. An aqueous CO2 bearing hydrothermal fluid which is homogeneous at high temperatures will separate into a mixture of 2 immiscible fluids as the temperature reduces. At the trapping temperatures typically observed in hydrothermal gold deposits, even parent fluids with only a small content of CO2 will separate into a heterogeneous mixture of these immiscible fluids, one aqueous and the other carbonic. But depending on the rate of silica deposition and conditions within the fluid only one of the heterogeneous fluids may be trapped, leading to the false assumption of a homogeneous single phase fluid system. It is well known that inclusions trapped from a heterogeneous fluid do not trap the 2 (or more) (possibly immiscible) phases in proportion to their abundance.

By understanding the immiscibility of CO2-H2O fluids it is simple to explain the trapping of CO2-only fluid inclusions from a heterogeneous fluid system in which the aqueous phase transported the essential silica, and probably the elements of economic interest such as gold. The claims that gold and silica are transported in pure CO2 fluids are unsubstantiated and false.

Explanation

The diagram below shows the phase relations of a CO2 - H2O system with Temperature and Pressure. (XCO2 is the mole fraction of CO2) The solvus curves of the miscibility boundary are shown at 4 different pressures. The data is from Tdheide and Franck (1963). (Yes, the isobaric solvus curves do intersect and cross. The CO2-H2O system is complex and non-linear with pressure!) If there is any NaCl present, the immiscibility is greatly increased. Clearly immiscibility is a major issue in such fluids and will affect many gold bearing hydrothermal fluid systems, as they frequently contain CO2.

P T X-CO2 phase diagram solvus
        immiscible fluids

At a pressure of 1 Kbar, the fluid in the region shown in green is a homogeneous single phase. If we consider an initial fluid with just 0.1 mole fraction of CO2 at 350 C, as it cools it will follow the path of the red line down until it intersects the solvus at about 260 C. Here it will separate into two immiscible phases, one dominantly aqueous (liquid, as this is below the critical point temperature of water and at high pressure) and the other  a water saturated CO2 phase (a super-critical fluid because it is above the critical point temperature of CO2 and at high pressure). But inclusion trapping does not occur until silica saturation is reached, assumed in this example to be at 200 C and shown as 2 red crosses in this diagram. From 260 C down to 200 C, both fluid phases evolve and coexist as a mixture in an open fluid system. The inclusions trapped at 200 C will be of either or both of the 2 fluid compositions shown by the 2 red stars on the diagram. Intermediate compositions cannot occur because the 2 fluids are immiscible. (Trapping mixtures of these two immiscible fluids within the same tiny fluid inclusion is highly improbable.) Because the super-critical CO2-rich carbonic phase is often lost and not trapped in inclusions, the original CO2 content of the fluid cannot be determined from the remaining aqueous inclusions trapped at 200 C. In this example, after escape of the gas phase fluid, only the aqueous fluid will be trapped in fluid inclusions and it will seem that the parent fluid was homogeneous and had only a low CO2 content of about 0.05 mole fraction CO2. However this is incorrect because the CO2 rich fluid has been lost before trapping and is not present in any fluid inclusions.

Rarely, if the carbonic fluid bubbles are small and not flushed out of the system by turbulence in the system, the CO2 rich fluid may be trapped. These conditions may allow the carbonic bubbles to adhere to growing crystal surfaces which interferes with silica deposition and forms carbonic fluid inclusions as silica is forced to deposit around these bubbles. The bubbles would have to remain in place for a long time to be encased in silica. This would give CO2-only inclusions, despite actually forming from a heterogeneous, dominantly aqueous, fluid mixture. Such fluid inclusions would have CO2 contents of 90% and may well appear to be totally carbonic at room temperature because the volume of the condensed liquid water phase would be extremely small and exist merely as an invisible film on the host mineral walls of the inclusion. (A calculation of phase volumes confirming this is here.) These same depositional conditions may give rise to only few and/or only small aqueous fluid inclusions if silica growth is slow and forms well ordered quartz, lacking crystal defects. Such crystal defects are the precursors of the cavities which become fluid inclusions. Small fluid inclusions typically cannot be observed well enough to study them and are overlooked or assumed to be unimportant secondary inclusions. The failure to trap or to recognize these aqueous inclusions leads to the false assumption that only CO2 fluids were present. These competing mechanisms of fluid inclusion trapping act in mutual opposition and it cannot be assumed that both fluid phases should be present in FIAs, and the absence of either type of fluid inclusion in the sample does not prove the absence of that type of parent fluid. The misrepresentation of the parent fluid composition is merely a function of selective trapping of the heterogeneous fluid phases by mutually opposing trapping mechanisms.

The carbonic fluid inclusions may also suffer post-entrapment water removal. The carbonic inclusions typically have a very high internal pressure. The 0.9 X-CO2  inclusions trapped at 1 Kbar and 200 C  in the above example have a molar volume of about 50 cc/Mole (density  0.83 g/cc) and still have an internal pressure of more than 300 bar at 100 C and 1 bar room conditions. This pressure differential could facilitate preferential diffusion or migration of  H2O  out of the inclusion because the size of H2O molecules is much less than of CO2 molecules. This would result in more CO2  enriched, "water free" inclusions.

The apparent absence of aqueous inclusions in the sample and the absence of any visible water within carbonic fluid inclusions could be misinterpreted as deposition from a non-aqueous CO2 fluid. However as seen in this example, the parent fluid was in fact dominantly aqueous with a composition of 0.95 mole fraction H2O and 0.05 mole fraction CO2 at the trapping temperature. It would be quite wrong to infer formation from CO2-only fluids, despite the fact that only carbonic inclusions were observed in the sample.


If NaCl (or other salts) are present in the fluid, the CO2  immiscibility is greatly increased and this plot shows the significant effect of salt on the solvus, greatly increasing the immiscibility region.
P-T-X Co2 phase solvus diagram at
          2 Kbar for saline solutions

Conclusions

The extensive immiscibility between water and CO2 is often ignored during fluid inclusion studies merely because it is difficult to identify the presence of these heterogeneous systems based solely on observations of fluid inclusion assemblages in the samples. It is also wrong to assume that the CO2-H2O heterogeneous system is similar to the boiling water heterogeneous system. (The immiscible H2O and CO2 phases behave very differently to the miscible heterogeneous liquid-water / water-vapour system documented in the literature.) It is often assumed that inclusions trapped from heterogeneous fluid systems must trap both component fluids giving bimodal FIAs, but this is incorrect (long since explained by E. Roedder, 1984) because the carbonic fluid (a gas) is usually lost and not trapped in inclusions. It is also often assumed there should be a continuum of compositions between the 2 end member aqueous and carbonic fluids. However that would require trapping of mixtures of the immiscible fluids within individual fluid inclusions, which is improbable. The oft assumed criteria for identification of heterogeneous trapping do not apply to a heterogeneous immiscible fluid system. It is also essential to consider the transport and deposition of silica in order to seal up fluid inclusions, something which is ignored by almost all authors. Silica is not transported in non-polar CO2 fluid, not even if it is super-critical. And few authors consider the multiple mechanisms of fluid inclusion trapping which may act in mutual opposition to give extreme disproportional trapping from heterogeneous fluids. 

With careful understanding of the immiscibility phase diagram for CO2 - H2O with temperature and pressure, it is clear that CO2 only inclusions are easily explained as trapping from heterogeneous fluids in which water is the dominant phase. It is counter-intuitive  to propose the formation of CO2-only inclusions from CO2-only fluid as such hypotheses fail to explain the essential deposition of the host mineral phase, silica.

CO2-only fluid inclusions are trapped from heterogeneous aqueous, CO2-rich fluids which have undergone phase separation due to immiscibility. These fluids may be incorrectly recognized because of disproportional trapping of the separate phases. Such heterogeneous fluids are very common (although difficult to recognize in samples) because there is an extensive Pressure - Temperature - Composition (P-T-X) region of immiscibility in the H2O-CO2 system which includes most of the P-T region in which hydrothermal gold deposits form.

A proposed method to help identify heterogeneous trapping of immiscible fluids is to observe the shape of the carbonic inclusions. Rounded inclusions are indicative of trapping as an immiscible gas bubble within a liquid second fluid, proving heterogeneous trapping, as discussed here.


Index to discussions of heterogeneous H2O-CO2 systems.