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Do "IOCG" deposits form from fluids containing abundant CO₂?

(IOCG deposits = Iron Oxide Copper Gold type deposits)

Kingsley Burlinson,   June 2016, April 2018, April 2020

Introduction

It has been asserted by Professor Murray Hitzman (SEG international exchange lecture, 2016) and others that the fluids from which IOCG deposits form are CO₂ rich. Is this correct? How can we determine the fluid compositions involved in the deposition of these opaque Fe-oxide minerals? The determination of formation fluid composition is done by using fluid inclusions and this is almost always done by micro-thermometry, which requires transparent minerals.  It is not possible to examine fluid inclusions in the opaque iron oxide minerals using microthermometry. (Except for  limited studies of some haematite using infrared light.)

Almost all studies of IOCG deposit fluids are done solely on quartz but in doing so it is assumed that the quartz and Fe-oxides are contemporaneous and formed from a single parent fluid. However many deposit studies show that there are multiple fluid events. Many studies even fail to carry out proper paragenetic studies to validate the assumption of co-genesis of  Fe-oxide and quartz formation.  Some studies even completely fail to mention that they were done entirely on quartz, a serious oversight. The assumption of a single parent fluid forming both the quartz gangue and the Fe-oxide minerals is unsafe.

We should be skeptical of the frequent assertions of CO₂ rich formation fluids as this is almost always based upon observation of fluid inclusions within quartz. To understand Fe-oxide deposits we need to study the fluids in the opaque Fe-oxide minerals. This can be done using baro-acoustic decrepitation, infrared micro-thermometry of some haematite samples (examples below)  or by gas extraction into a mass spectrometer during crushing or thermal decrepitation of Fe-oxide materials.

Baro-acoustic decrepitation of numerous magnetite and haemaite samples from IOCG deposits tentatively suggests that CO₂ is not present in the parent fluids.

Jump to the Conclusions

Discussion

Assertions that IOCG deposits form from CO₂ bearing fluids

• Oreskes & Einaudi, 1992, reported that a few samples within hydrothermal quartz contained some CO₂. They found more CO₂ in a few inclusions within fluorite. Most inclusions observed did not show evidence of CO₂. They calculated that when present, the CO₂ was about 0.6 mole % (X_CO2 0.006).
  

• Bastrakov et al., 2007, studied fluid inclusions in the rare quartz at the Olympic Dam deposit, South Australia. They used laser raman analyses to detect CO₂ in  vapour rich inclusions in either quartz or calcite (host mineral not clearly specified) and reported that they could not detect CO₂ as it was less than the detection limit.
  

• Pollard, 2001, discussed un-mixing of CO₂ bearing fluids as a mechanism to form IOCG deposits in the Cloncurry, Queensland. He refers to a few studies from other papers which observed CO₂ bearing inclusions in iron oxide type deposits. All of these studies refer to fluids within quartz only and no new observations of fluid inclusions were carried out by Pollard.
  

• Morales Ruano et al., 2002, studied inclusions within quartz from the Cu deposits at Moonta, South Australia. They found no significant CO₂ in the fluid inclusions. This deposit is hardly an IOCG type as there is no magnetite or haematite in the late stage Cu-Au mineralized quartz veins.
  

None of these discussions provide significant evidence of the involvement of CO₂ rich fluids in IOCG deposits. Only inclusions within quartz, calcite or fluorite were studies and in some cases the conclusion was that CO₂ was in fact absent. These studies are at best inconclusive in ascertaining the association of CO₂ fluids with the iron-oxide deposition in IOCG deposits.

The above references are listed in References at the end of this page

Fluid inclusion microthermometry in haematite using near infrared illumination

Some haematite is transparent to near infrared light and can be used for microthermometric fluid inclusion studies. But remarkably few studies have been reported in the literature. Luders et. al found that some haematite-quartz veins which carry gold in Brazil do show the presence of CO₂ in inclusions within specular haematite, seen here in sub-images e, g and h.

Transmitted IR light microphotographs of fluid inclusions in specular hematite.
FROM: Genesis of itabirite-hosted Au–Pd–Pt-bearing hematite-(quartz) veins, ́Quadrilatero Ferrıfero, Minas Gerais, Brazil: constraints from fluid inclusion infrared microthermometry, bulk crush-leach analysis and U–Pb systematics. BY: Volker Luders, Rolf L. Romer, Alexandre R. Cabral, Christian Schmidt, David A. Banks & Jens Schneider
Mineralium Deposita (2005) 40:289 Fig. 3

• c      Aqueous fluid inclusion with solids of unknown composition in specular hematite from a barren vein in the low-strain domain of the QF, Fabrica.
• d      Aqueous fluid inclusion with a solid of unknown composition in specular hematite from a jacutinga-style vein, Gongo Soco.
• e,f)   Negative crystal-shaped aqueous fluid inclusion with a solid (blue circle in e and f) and aqueous-carbonic inclusion (red circle in e and f) in a specular hematite grain, Itabira.
• g      Aqueous fluid inclusion with solids and aqueous-carbonic inclusion in specular hematite, Itabira.
• h      Aqueous-carbonic inclusion with a small solid from a cluster of multi phase aqueous and aqueous-carbonic inclusions.
  

NOTE the small size of most inclusions, usually only 5 microns
Other studies of inclusions within haematite do not show the presence of CO₂.

The next 3 images are From:
The origin of hematite in high-grade iron ores based on infrared microscopy and
 fluid inclusion studies: the example of the Conceição mine, Quadrilátero Ferrífero, Brazil
BY: Carlos Alberto Rosière & Francisco Javier Rios
Economic Geology, (2004) Vol. 99, pp. 611–624. Fig 4

Primary two-phase fluid inclusions typical of Hm II crystals, enclosed in an Hm II-III grain. Some of the inclusions are elongated parallel to the basal plane and decrepitated at 345°to 350°C.

• b)     Primary fluid inclusions decorating growth zones in hematite.
• c)     Necking-down of fluid inclusions in hematite.

Again, the inclusions lack evidence of CO₂ in the fluids.

The few studies of inclusions within haematite using infra-red microscopy do confirm that some fluids are CO₂ rich, but in other cases the fluids lack CO₂ and there are too few studies to draw an overall conclusion about the typical compositions of IOCG forming fluids.

Opaque mineral analysis by baro-acoustic decrepitation

Numerous decrepitation analyses of Fe-oxide minerals have been carried out from many deposits and much of that data is presented on this website.
An overview of decrepitation of opaque minerals is here  and another summary is here  and an overall comparison of many deposits is here.
Results from various deposits are listed here   and data from the Bergslagen area in Sweden is here.

Examples of decrepitation from various FeOx deposits are shown here. Decrepitation can be intense and occurs in both haematite and magnetite minerals.

This data shows that Fe-oxides do retain fluid inclusions and decrepitation can provide information about formation temperatures.

Fe-oxides generally lack the low temperature decrepitation peak near 300 C seen in quartz containing CO₂ rich fluid inclusions. This may be interpreted as evidence that Fe-oxides do not usually contain CO₂ rich fluids. However, the Young's modulus of magnetite (and also haematite) is much higher than that of quartz. The increased strength of the Fe-oxide minerals could withstand higher internal inclusion pressures before decrepitation occurs, leading to typically higher decrepitation temperatures than in quartz. The low-temperature decrepitation peak caused by CO₂ fluids in quartz could be shifted to higher temperature or even be absent in Fe-oxide minerals due to their higher Young's modulus. (A discussion of the dependence of decrepitation upon the young's modulus of host minerals is here.)

Note that the magnetite sample from the Agrium carbonatite (blue) does show decrepitation below 300 C. This may be caused by CO₂ rich fluid inclusions which would not be unexpected in a carbonatite deposit. This suggests that decrepitation may be valid for detecting CO₂ rich fluids in magnetite and consequently that most IOCG deposits did not form from  CO₂ rich fluids as they do not decrepitate below 350-400C.

Mass spectrometric analysis of gases released during sample crushing.

The best way to be certain of the CO₂ contents of Fe-oxide minerals is by mass spectrometric analysis of the gas released during either crushing or thermal decrepitation of mono-mineralic haematite or magnetite.

But no such analyses have been found in the literature to date. 

Two attempts to carry out mass spectrometric analyses of inclusion fluids have been made.

Analysis attempt 1

Five samples of magnetite were analysed by mass spectrometry with this equipment by D. Gaboury to try and determine the fluid composition. Three samples were from Mengku, Altay area, China, one from the Lyon deposit, Nevada, USA and one from The Upper Beaver deposit, Ontario, Canada. The samples were chosen to be monomineralic magnetite with significant baro-acoustic decrepitation. The decrepitation results of the samples submitted for mass spectrometer analysis are shown in the 2 graphs below.

No actual data files from the analyses of these samples are available. D. Gaboury reported merely that "In short, in all 5 samples, there is no gas release related to fluid inclusion decrepitation."  This comment seems strange as it implies there was not even water present in the gases released from these samples. The equipment used by D. Gaboury does detect water release, as seen here in quartz samples.  This instrument uses thermal decrepitation of the sample to open the fluid inclusions.

There should have been some gas detected as these samples did show significant decrepitation. The decrepitation instrument does not detect counts caused by mineralogical effects and it is certain that fluid inclusions are present.

Analysis attempt 2

Another attempt to analyse the gases released from magnetite samples by mass spectrometry was carried out by N. Blamey.  The four samples analysed were from Afton, British Columbia, Canada, Upper Beaver, Ontario, Canada, Tennant Creek, Northern Territory, Australia and the Mt. Wright iron mine, Labrador, Canada. The samples were again selected to be monomineralic magnetite with substantial decrepitation.

Decrepitation results of the 4 samples submitted for the second attempt of  mass spectroscopic examination of fluid inclusion contents.

No actual data files of the analyses of these samples are available. N. Blamey reported merely that "Only one of the samples gave me gas; the material is so fine that it would appear that the inclusions have been compromised". Neither the gas found nor the sample it came from were identified. The samples provided were part of the same material that was decrepitated and was sieved to <420 and >200 microns. It was not so fine grained that the fluid inclusions were compromised as can be seen from the above decrepitation graphs of these samples.

Why did the mass spectrometric analyses fail to detect any inclusion fluids?

It is clear from the decrepitation results that there are inclusions in the magnetite samples, although perhaps less numerous than in typical quartz samples. And in all cases, the sample material decrepitated and analysed by mass spectrometry was identical. Decrepitation counts are not due to crystallographic or mechanical effects and repeat analyses of already analysed samples give no counts. Only irreversible events are counted, so it is certain that decrepitation counts are caused by fluid inclusions.

Perhaps the inclusions in magnetite are so small that they do not release enough gas to be measured in the mass spectrometer. In the photographs of FIs in haematite (above) most inclusions are very small, often just 5 microns across. Decrepitation of such small inclusions would give very little gas to analyze. In the mass spectrometer, the flux of analyte into the ioniser must exceed the rate of evacuation by the vacuum system. A small gas quantity released from small inclusions may well be evacuated so fast that it is not detected in the mass spectrometer. Typically, spectrometers require more than a milligram of sample.

It seems that mass spectrometer analyses of fluids released by thermal decrepitation on magnetite must be done on much larger samples than normal to compensate for the apparently small size and low abundance of inclusions in magnetite and haematite. In both these studies the analyst did not provide details on the sample size they used.

Conclusions

There have been very few FI studies of haematite by infrared microthermometry.  CO₂ rich fluids have been seen in one study, but in others the haematite lacks CO₂ while adjacent quartz is CO₂ rich, indicating different fluid events.

There is only minor evidence for the presence of CO₂ bearing fluids in these deposits and all of that is based on fluid inclusion studies in quartz or other gangue minerals and is at best very questionable evidence for the presence of CO₂ fluids in the formation of the Fe-oxide and economically interesting minerals of concern.

Most fluid information on IOCG deposits is actually derived from FIs within quartz. Often there is no paragentic study and it is uncertain that the quartz and Fe-oxides are actually deposited from the same fluid.

Baro acoustic decrepitation of haematite and magnetite almost always lacks the low temperature decrepitation peak caused by CO₂ rich fluid inclusions hosted in quartz suggesting that CO₂ rich fluids are not involved. But the young's modulus of both magnetite and haematite is about double that of quartz, so it is not conclusive that CO₂ fluids within Fe-oxides would cause the same characteristic low-temperature decrepitation peak as seen in quartz.

No mass spectroscopic analyses of gases extracted during crushing or thermal decrepitation of Fe-oxides have been found in the literature. Attempts to analyze gas extracted from 9 magnetite samples by thermal decrepitation have failed to find any gas in 8 of the samples, not even water! One sample contained unidentified gas.

The inability to measure gas extracted form these samples which have substantial decrepitation responses is probably because insufficient fluid was released from the fluid inclusions. It seems that inclusions in haematite and possibly magnetite may typically be smaller than inclusions in quartz, hence containing less fluid. Baro-acoustic decrepitation of magnetite samples also is usually less intense than quartz samples, suggesting fewer inclusions. These effects could reduce the fluid amount to less than is required to perform the mass spectrometric analysis. Much larger samples may be needed to analyse magnetite fluids. As the analysts involved have not provided the actual data files it is not possible to be certain of this explanation and further more careful mass spectroscopic work with larger samples is required.

Recent studies using stable isotopes of Cu (Saunders et al, Mineralium Deposita 2016, V51 #1) have confirmed different fluid sources for ore and gangue minerals in epithermal Au-Ag deposits. The authors state: "This conclusion has implications for fluid inclusion and isotope studies that have focused on using the gangue minerals for analysis, if those minerals do indeed have principally different sources." This is a serious concern for Fe-oxide deposits as FI studies are almost always done only on the quartz gangue minerals.

There is no clear evidence that CO₂ bearing fluids are coeval with or involved in the deposition of Fe-oxide minerals and the associated Cu and Au minerals.

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References

Bastrakov EN, Skirrow RG, Davidson GJ (2007)  Fluid evolution and origins of iron oxide Cu-Au prospects in the Olympic Dam District, Gawler Craton, South Australia. Econ. Geol. 102:1415-1440.

Morales Ruano S, Both RA, Golding SD (2002) A fluid inclusion and stable isotope study of the Moonta copper-gold deposits, South Australia: evidence for fluid immiscibility in a magmatic hydrothermal system. Chem. Geol. 192:211-226

Oreskes N, Einaudi MT (1992) Origin of hydrothermal fluids at Olympic Dam:preliminary results from fluid inclusions and stable isotopes. Econ. Geol. 87:64-90

Pollard PJ (2001) Sodic (-calcic) alteration in Fe-oxide-Cu-Au districts: an origin via un-mixing of magmatic H₂O-CO₂-NaCl ± CaCl₂-KCl fluids. Mineralium Deposita 36:93-100

Saunders, J.A., Mathur, R., Kamenov, G.D., Shimizu, T. and Brueseke, M.E.  (2016), New isotopic evidence bearing on bonanza (Au-Ag) epithermal ore forming processes.  Mineralium deposita 51:1 p1

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Appendix - additional FI photographs in iron oxide associated minerals.

FROM:  Mineralium deposita 50:7 p847 2015  Yu et. al.
d,e,f & j are within quartz.    a,b & c are within apatite.    j,h &i are within siderite    k is in calcite     l is in dolomite
NOTE the small size of most inclusions, usually only 5 microns

Author's figure caption:
a,b two-phase inclusion in stage 2 apatite,
c three-phase inclusion (L+V+S) in stage 2 apatite,
d two-phase inclusion in quartz as cavity fillings,
e three-phase inclusion in quartz as cavity fillings,
f vapor-phase inclusion in quartz as cavity fillings,
g vapor-phase inclusion in siderite,
h three-phase inclusion in siderite,
i two-phase inclusion in siderite,
j two-phase inclusion in quartz as vein,
k two-phase inclusion in calcite as vein,
l two-phase inclusion in dolomite as vein
   S=solid phase