Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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Fluid Inclusion Studies on Opaque minerals

Kingsley Burlinson1, T. Mernagh2, D. Gaboury3 Jiuhua Xu4 and LonghuaLin4

1 Burlinson Geochemical Services, Darwin, NT, Australia,
 2Geoscience Australia, Canberra, ACT, Australia,
 3University of Quebec, Chicoutimi, Quebec, Canada,
 4University of Science and Technology Beijing, Beijing, China

A presentation at the ACROFI-4 conference, Brisbane Qld., August 2012


Fluid inclusion studies are almost always carried out on transparent minerals, usually quartz, despite the fact that the economic minerals of interest are usually opaque. It is then assumed that the opaque minerals formed under the same fluid conditions as the transparent quartz. Although paragenetic studies can sometimes provide justification for this assumption, in many cases it is  either unproven or incorrect to extrapolate the observations on the transparent minerals and infer the same fluid conditions for the opaques minerals. We should try and determine the fluid inclusions which formed opaque minerals from the opaque minerals themselves. This is not easy as there are few methods applicable to opaque minerals. However three methods we can use to study inclusions in opaque minerals are the baro-acoustic decrepitation method, microscope observations using infrared light, or mass spectrometric analyses of gases released during decrepitation of the sample.  These three methods have been applied to the analysis of fluid inclusions in pyrite, magnetite and haematite and are discussed in this presentation.

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The baro-acoustic decrepitation method is described thoroughly at .  It has been used extensively in the former USSR and also used in china and western countries. At present three different instruments are known. The BGS instrument is completely digital and works together with a standard personal computer. The Chinese instrument is a completely analog design, and at least 2 of these are operational in China. At least one Russian instrument of analog design is in use in Lviv, Ukraine.  The analytical instruments in use in Western countries and in China are shown here. The sample is placed in a quartz tube and heated (up to 800 C in the BGS instrument). An analysis takes about 30 minutes. The result is a histogram of decrepitation counts versus temperature in the BGS instrument, or dimensionless "integrated voltage" versus temperature for the Chinese instrument.


Another method is to decrepitate samples either mechanically or thermally in a vacuum and analyse the gases evolved in an associated mass spectrometer. D. Gaboury has constructed  such an instrument which uses thermal decrepitation of the sample. By measuring changes in the vacuum pressure it is possible to quantify the amount of fluids and not merely their chemical composition. (reference: Mass Spectrometric analysis of volatiles in fluid inclusions decrepitated by controlled heating under vacuum. Damien Gaboury, Moussa Keita, Jayanta Guha and Huan-Zhang Lu, Economic Geology v 103, pp 439-443, March-April 2008.)

This is a photograph of the instrument in Chicoutimi, Quebec.


This is a block diagram of the instrument design. The sample is placed in the "Probe" where it is heated to 500 C within a vacuum. An analysis takes 2 to 3 hours.

massspec layout

A typical result from the above mass spectrometric instrument is shown here, for gases evolved from auriferous quartz. Although magnetite has been analysed in this instrument, the results are not available. Note that we are able to quantify the amount of water evolved. Although hydrogen was detected and is shown in this plot, this is an artifact of the analytical method and there is no hydrogen actually in the sample.

gas analyses by ms

This study was carried out on pyrite samples with coexisting quartz on samples from the Mt Charlotte gold mine in Kalgoorlie, West Australia; at the Chessy copper-zinc mine, Lyon, France; and at the Enterprise gold mine, pine Creek, NT, Australia. All the samples were studied using baro-acoustic decrepitation, and several samples from the Mt Charlotte mine were also examined using infrared microscopy on thin sections. These pyrite grains are transparent to infrared light.

Additional studies were carried out on magnetite and haematite samples from the Mengku iron deposits and nearby Cu and Au iron skarns in the Altay region of NW China; at the Tennant Creek Au-Cu Fe deposits, NT, Australia; and on surficial laterite nodule samples from Darwin, Australia. These samples were  analysed by baro-acoustic decrepitation and  several samples from Mengku were also analysed by mass spectrometry of fluids released by thermal decrepitation. 

Mt Charlotte gold mine, Kalgoorlie, WA.

charlotte loc map

Core samples from drillholes in the Mt Charlotte deposit are strongly silicified and contain abundant coarse pyrite of about 3mm grainsize.

charlotte core

Fluid inclusions can be seen in the Mt Charlotte pyrite using Infrared light microscopy. In the pyrite, some of these seem  to contain 3 phases with liquid, gas and a daughter crystal present. But only 2 phase inclusions of liquid with a large gas bubble occur in the inclusions within the coexisting quartz. This suggests that the fluids which formed the pyrite are not the same as those that formed the immediately adjacent quartz.

charlotte 130

Other pyrite samples from this deposit vary in opacity and can be difficult to work with.

charlotte 125

charlotte 301

The core samples from Mt Charlotte were crushed to <420 microns grainsize and separated into heavy (SG >3) and light fractions (SG <3) using TBE. Traces of carbonate were removed by reaction with HCl. The heavy fractions were comprised entirely of pyrite, and the light fractions were all the silicates, dominantly quartz. Baro-acoustic decrepitation of the quartz and pyrite fractions of each sample give very different results with strong fluid inclusion decrepitation in the pyrite, but weak or no decrepitation of the quartz fractions. This again indicates that the fluid conditions for pyrite deposition were not the same as for the coexisting quartz.

charlotte 1095

charlotte 1096

An additional sample from the Enterprise gold mine at Pine Creek, NT, Australia was also studied and separated into pyrite and quartz fractions.  The decrepitation of the 2  mineral fractions are similar at low temperature up to about 450 C, but differ at higher temperatures. Both fractions contain CO2 as indicated by the low temperature decrepitation from 150 C to 350 C. The 2 coexisting minerals seem to have formed from similar but not identical parent fluids.

pine creek py-qtz

Pyrite samples from various deposits worldwide show considerable differences, reflecting the different depositional conditions of the pyrite. Samples from the gold mine at Kori Kollo, Bolivia, the copper-zinc mine at Chessy, France as well as from the lead-zinc mine at Woodcutters, NT show intense decrepitation indicating their origin in hydrothermal systems. Samples from sedimentary deposits at Nairne, South Australia and Rio Tinto, Spain have little decrepitation. The sample from Broken Hill, NSW is from an amphibolite facies metamorphic environment and this metamorphic event may have altered the fluid inclusions.

various pyrite

Chessy copper-zinc mine, France

This description of the Chessy mine is from T. McCann (ed), The geology of central Europe, V2.
At Chessy, chalcopyrite and sphalerite are associated with baryte averaging 2.5 wt% Cu, 10 wt% Zn and 15 wt% Ba (Bril et al 1994). The mineralisation occurs in two effusive acid volcanic units (mainly submarine lava flows) characterized by their dacitic to rhyolitic composition (Lacomme et al 1987: Milesi & Lescuyer 1993). The ore bodies comprise a central zone with alternating beds of pyrite, sphalerite and thin volcanic flows surrounded by a baryte rim. The main ore body is rooted in quartz-pyrite-white mica stockworks. Similar stockworks occur at several volcanic levels, demonstrating the longevity of hydrothermal activity in the Chessy area (Lacomme et al 1987).

chessy locations

Baro-acoustic decrepitation analyses of the coexisting quartz and pyrite fractions show only limited  similarity, with significant differences, particularly the presence of low temperature CO2 decrepitation on the pyrite fractions.

chessy py-qtz pair

District wide analyses of pyrite show it has potential for use as a mineral exploration method. Although the pyritic host St Antoinne formation extends over at least 200 Km, baro-acoustic decrepitation of the pyrite shows the best response in the Chessy area and only shows the presence of CO2 rich inclusions within the actual open pit mine area. Analyses of the coexisting quartz do not provide useful exploration targets as do the pyrite analyses.

chessy region

Mengku Iron and skarn deposits, Altay, NW China.

Iron is mined from an apparently stratigraphic horizon within high grade metamorphic rocks. There is also Pb-Zn and Au-Cu mineralisation nearby which seem to be skarns. There is continuing debate about the genesis of these deposits which could be of sedimentary stratigraphic or hydrothermal replacement origin. Samples were obtained from the locations shown on this map and analysed by baro-acoustic decrepitation to check for the presence of hydrothermal fluid inclusions in the magnetite.

mengku location

At the Menkgu iron mine, an extensive pattern of samples was analysed. All the samples were separated into magnetic and non-magnetic components before analysis. The magnetic samples were comprised almost entirely of magnetite and the non magnetic samples were mostly of pyroxenes and garnet. There was a small amount of quartz, and up to 25 % non-magnetic sulphides in some samples.

mengku pit sample
There are major differences between the baro-acoustic decrepitation results on different magnetic separates of the same sample. However it is unclear how much of this is because sulphides decrepitate differently due to mineral strength and oxidation issues during the analysis.

mengku mags-nonmags

Samples from the east end of the mine have a low temperature decrepitation peak cause by CO2 rich fluid inclusions, clearly indicating the presence of hydrothermal fluids.

mengku with co2

Most of the Mengku samples have only low intensity of decrepitation in the magnetic sample fractions. But there are considerable differences in decrepitation along a strike length of 2 Km, further suggesting that the deposits are not purely stratigraphic but have undergone substantial modification and perhaps upgrading by hydrothermal fluids.

mengku west-east

Three of the Mengku samples were selected for analysis using mass spectrometry on the gases evolved during decrepitation. The results of these analyses are not available and it is claimed that only SO2 and H2S were observed, but no water or CO2.  Those results seem to be erroneous as it is very unlikely that there was not even water present.

mengku mass spec

The Qiaoxiahala skarn deposit is about 100 Km SE of the Mengku deposit. Three analyses of the same sample show intense decrepitation of the magnetite which is consistent with the deposit being a skarn.

qiao... skarn mags

Various magnetite and haematite hosted deposits show the decrepitation response for iron skarns and hydrothermal deposits.

various feox skarns

We should not assume that all the mineral components of a sample form at identical fluid conditions. At the Great Bear Magmatic zone in Canada, magnetic and non-magnetic fractions of a surface ironstone show very different decrepitation patterns indicating different fluid conditions of formation within this single sample.


For comparison with hydrothermal magnetite, samples of surficial laterite nodules from Darwin were also analysed. the nodules were hand selected highly ferruginous, rounded pisolites about 5-10 mm across.

darwin laterite

The crushed samples were separated into a magnetic and non-magnetic fractions comprised of iron oxides, with rare quartz grains in the non-magnetic fraction. The magnetic fraction gave no decrepitation at all and the non-magnetic fraction has very minor decrepitation caused by traces of quartz and other silicates. This indicates that surficial magnetites and iron oxides do not give a baro-acoustic decrepitation response, in contrast to hydrothermal and skarn derived magnetites.

lat nodule


By using baro-acoustic decrepitation, infrared light microscopy or mass spectroscopy on the gases evolved during sample decrepitation, we can study the fluid inclusions and formation conditions of opaque minerals, specifically pyrite, magnetite and haematite in this study.  The use of opaque minerals is essential  in many mineral systems which lack transparent minerals such as quartz.  But even when transparent minerals are present, this study has shown that there can be significant differences in the fluids which formed the different minerals. We therefore need to exercise caution in assuming we can  extrapolate the results from studies of quartz to infer the formation conditions of coexisting opaque minerals.

Most minerals of economic interest to the mining industry are opaque and it is preferable to try and directly analyse these opaque minerals, complemented by information from coexisting transparent minerals. 

Baro-acoustic decrepitation of pyrite can be used to locate potentially mineralized zones in regionally extensive rock units, given a sufficient number of samples and an extensive spatial array of sample points.

Baro-acoustic decrepitation of magnetite can be used in the same way, and is also useful to distinguish between hydrothermal and sedimentary magnetite. This distinction is important because hydrothermal magnetite is an important exploration target for associated gold, copper zinc and other metals.

Opaque minerals pose significant problems for the study of their contained fluid inclusions. But they are of great economic importance in most mineral deposits and we should try to use them directly where possible rather than studying only the transparent quartz, which does not always form under the same fluid conditions as the associated opaque minerals of interest.