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

Thermodynamics shows Au is insoluble in CO2 fluids

Do IOCG deposits form from CO2 rich fluids?

Inclusion shapes can prove heterogeneous 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

Why don't Exploration geologists understand fluid inclusions?

News:

New model 205 decreptiometer

Studies of 6 Pegmatite deposits

A study of the Gejiu tin mine, China

Data on MVT Pb-Zn deposits, Tunisia

Data from Hall and Mt Hope Mo, Nevada

A magnetite study - Bergslagen region, Sweden

Exploration using palaeo-hydrothermal fluids

Using opaque minerals to understand ore fluids

Decrepitation using Fe-oxide opaques

Understanding baro-acoustic decrepitation.

An introduction to fluid inclusions and mineral exploration applications.



 Interesting Conferences:


Futores II, June 4-7, Townsville, Australia

ECROFI 2017, June 23-29, Nancy, France

AOGS 14th, Aug 6-11, Singapore

SGA 2017, Aug. 20-23, Quebec city, Canada

SEG 2017, Sept. 17-20, Beijing, China

Exploration 17, Oct. 21-25, Toronto, Canada

AAG 2017 at RFG2018, June 16-21 2018, Vancouver, Canada


Comprehensive Geology Conference Calendar



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

(IOCG deposits = Iron Oxide Copper Gold type deposits)

Kingsley Burlinson,   June 2016

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.

Jump to the Conclusions


Discussion

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

Transmitted IR light microphotographs of fluid
        inclusions in specular hematite.

Fig.3   c–h: Transmitted IR light microphotographs of fluid inclusions in specular hematite.


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 inclusions in Haematite II-III

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.

large hexagonal fluid inclusions in specularite

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.


carbonic fluid inclusions in quartz associated with the
      above Fe-ox minerals

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

transmitted near IR light microphotographs (b-c) of
      minerals from the Wessels mine, Sth Africa
Near IR microphotographs of haematite from the Wessels mine (Kalahari manganese field, South Africa)

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.

typical Feox decrepitation

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.)

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 different!

***** Work in progress - incomplete *****


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.

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.