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 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 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 and low temperature trapping below 200 °C, the fluid inclusion assemblages (FIAs) may leave no trace of the aqueous phase, 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) 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 an aqueous fluid system which 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.  (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 almost all gold bearing hydrothermal fluid systems, as they frequently contain CO2.

PTX co2-h20 phase diagram solvus, with coolingpaths

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 coexist as a mixture in an open fluid system. The super-critical CO2-rich carbonic phase is often lost or not trapped in inclusions and the original CO2 content of the fluid cannot be determined from only the aqueous inclusions trapped at 200 °C. It will seem that the parent fluid had only a low CO2 content of about 0.05 mole fraction CO2 but this is incorrect because an indeterminate amount of CO2 may have been lost in an immiscible phase prior to inclusion trapping.

It seems to be uncommon, but it is possible that the carbonic phase could be trapped. This could happen in fluids with little turbulence and in which the immiscible carbonic bubbles are small. Such 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. This would give CO2-only inclusions, despite actually forming from a heterogeneous, dominantly aqueous, fluid mixture. 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 we call fluid inclusions. Such small inclusions typically cannot be observed well enough to study them and are overlooked or assumed to be unimportant secondary inclusions. The failure to recognize these aqueous inclusions leads to the false assumption that only CO2 fluids were present. Because of these competing mechanisms of fluid inclusion trapping 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 fluid phases by mutually opposing trapping mechanisms.

If a carbonic inclusion is trapped at 200 °C (the right hand red cross on the diagram), it will contain less than 0.1 mole fraction of H2O (XCO2 >0.9). As it cools to room temperature, it will separate into 2 immiscible phases within the inclusion, as shown by the blue path lines. The aqueous phase would be quite small and contain only 0.04 mole fraction of H2O. At room temperature this liquid volume would be so small that it is unlikely to be recognized in the CO2 dominated inclusion. It would occur as a invisible thin aqueous film, weakly bonded to the silica walls of the inclusion, giving the appearance that water was absent from these fluid inclusions and leading to the false assumption of formation from a non-aqueous fluid.

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.

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, but this is incorrect (long since explained by E. Roedder, 1984). 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. 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 and also 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.


Index to discussions of heterogeneous H2O-CO2 systems.