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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
Overview
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.
The baro-acoustic decrepitation method is described thoroughly at
http://appliedminex.com . 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.
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.
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.
Core samples from drillholes in the Mt Charlotte deposit are
strongly
silicified and contain abundant coarse pyrite of about 3mm
grainsize.
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.
Other pyrite samples from this deposit vary in opacity and can be
difficult to work with.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
Various magnetite and haematite hosted deposits show the
decrepitation
response for iron skarns and hydrothermal deposits.
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.
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.
Conclusions
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.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
