Do "IOCG" deposits form from fluids containing abundant CO2?
(IOCG deposits = Iron Oxide Copper Gold type deposits)
Kingsley Burlinson, June 2016
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.
Jump to the Conclusions
Fluid inclusion microthermometry in haematite using near
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
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
Fig.3 c–h: Transmitted IR light microphotographs of ﬂuid inclusions in specular hematite.
- 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
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
Large fluid inclusions with hexagonal shape in specular haematite. The fluid inclusions in the left-hand side contain a small solid saturation phase. Insets g1 and g2 are enlargements showing solid inclusions that formed after heating. In g1 two solid phases formed after heating to 400°C and subsequent cooling. In g2 a single solid phase formed after heating and cooling.
Primary aqueous carbonic fluid inclusions in quartz at 25°C. Tm(ice) = 16.6°C and Th(total) = 149° C.
The authors state that: "The quartz veins from the analyzed samples cut across the metamorphic schistosity (S1) or interfinger with the banded microstructure of the hematite ores. They envelop all the early minerals, including specularite plates and are the product of late, aqueous carbonic hydrothermal fluids of low salinity (less than 8 wt % NaCl equiv), with total homogenization temperatures of the fluid inclusions of approximately 330°C. These fluids are of uncertain age and origin and did not participate in oxidation of magnetite or Fe mineralization processes." The CO2 rich fluids seen in the quartz are apparently a late stage post Fe-oxide event.
This pair of images are From:
Fluid inclusion studies in cogenetic hematite, hausmannite, and gangue minerals from high-grade manganese ores in the Kalahari manganese field, South Africa.
BY: Volker Luders, Jens Gutzmer & Nicolas J. Beukes.
Economic Geology Vol.94, 1999, pp.589-596, Fig. 3
Near IR microphotographs of haematite from the Wessels mine (Kalahari manganese field, South Africa)
- 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
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
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.)
Mass spectrometric analysis of the gas released during sample
crushing would resolve this ambiguity but no such studies have
been reported in the literature.
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. (Plans have been made to perform such a study.)
Refer to Mineralium deposita 51/1 Saunders et al. - isotopic data that ore and gangue are different!
ALSO Mindep 50:7 p847 - FI images in apatite, siderite,
qtz and carbonates of feox-apatite, fluids for Fe and late Au are
***** Work in progress - incomplete *****
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
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. But the young's modulus of magnetite and haematite
are about double that of quartz, so it is not clear that CO2
fluids within Fe-oxides would cause the same characteristic 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.
Recent studies using stable isotopes of Cu (Saunders et al, Mineralium Deposita 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.