Information about these fluids is an invaluable aid in mineral exploration.
Conventional academic methods of analysing fluid inclusions are too slow and tedious to be of practical application in typical mineral exploration activities.
However, the academic data from numerous studies does show that CO2 is an exceptionally important indicator when exploring for most types of gold deposit.
Because the baro-acoustic decrepitation method is a rapid and reliable method to measure CO2 contents in fluids, it can be used to study a spatial array of data and it is an invaluable and practical exploration method.
Measurements of temperatures of fluid inclusions does not usually help in mineral exploration as hydrothermal minerals deposit over a wide temperature range and there is no specific temperature which is indicative of mineralisation. However, if temperatures are available on a large spatial array of samples, then temperature trends may be a useful exploration method to find the hottest part of the system, which is presumably the location of the best economic mineralisation. Baro-acoustic decrepitation is the most practical method to determine temperatures of the large numbers of samples required.
Salinities of fluid inclusions are of limited use in exploration and are difficult to measure. However, they can be used to recognise intrusion related hydrothermal systems.
2. CONTENT
What is Fluid inclusion?
Where and when are they trapped?
Types of fluid inclusions
How it can be studied?
Applications in ore geology
Case studies
Cu
Au
3. FLUID INCLUSION
WHAT? WHERE? WHEN?
Tiny bubbles of fluid trapped in mineral
Composition- host melt
<100 um in size (10 microns generally)
4. HOST MINERALS
• Sphalerite, cassiterite, quartz, calcite,
dolomite, fluorite, halite, Apatite,
Topaz and baryte. (Shepherd 1985)
• Some feldspar in granites also contains
fluid inclusions.
6. Primary fluid inclusion
• Formed during the formation of the enclosing
crystal
• Trapped along growth zones and crystal faces
• Indicates conditions of formation
8. Secondary fluid inclusion
Trapped in the fractures
Caught due to healing of fractures
Occurs as :
• Trails
• Clusters
Cut across grain boundaries
12. Pseudosecondary fluid inclusion
Trapped during formation of host mineral
Occurrence:
• Along trails
• Ends abruptly against grain boundaries or one of
the growth zones
14. COMPOSITION
• The composition of trapped fluid varies
• Vapour or liquid phase- H2O
• Gas phase - CO2, CH2, H2S, Cl, Br, F, I
• Solid phase- S, Na, K, Ca, Mg, Fe and their salts
• E.g. Halite (NaCl)
15. HOW TO STUDY?
Methods used to analyse fluid inclusions
• Manual manipulation under the microscope
• Micro thermometry (-200⁰C to1500⁰C )
• Laser Raman microprobe
• Laser ablation with mass spectrometry
• Baro-acoustic decrepitation
17. • P-T-X of the fluid system.
• Salinity of the aqueous fluid.
• Composition of the fluid.
• Gas composition of the fluid.
• Density of fluid-by Tmf and Th and
isochores
INFORMATION WE CAN GET….
18. PRACTICAL APPLICATIONS
• Mineral Exploration: For hydrothermal
deposits e.g. gold, silver, copper etc.
• CO2
• Salinity
• Temperature
20. FLUID INCLUSIONS IN COPPER
DEPOSITS
• If the hydrothermal fluid is
highly saline, above 23%
NaCl.
• Halite can crystallise upon
cooling with solid crystals
present.
• Such fluids usually occur in
porphyry copper deposits or
the core of igneous intrusion
derived fluid systems.
• Saline inclusions are less
common.
21. FLUID INCLUSIONS IN GOLD
A CASE STUDY
Hollinger-McIntyre mine area, Ontario, Canada
22. EXAMPLES
• The Cowra goldfield in NSW, Australia
• The Woods Point gold deposit, Victoria, Australia
• The Brusson gold mine, northern Italy
• Kalgoorlie area archean deposits, Western
Australia
23. SUMMARY
• Tiny bubbles trapped in crystal of mineral gives information
about its temp., pressure, salinity, density, composition etc.
• If minute crystals are present in the inclusion, such
as halite, sylvite, hematite, or sulphides are present, they
provide direct clues as to the composition of the original fluid.
• Used for petroleum exploration, mineral exploration,
sedimentary facies environment and diagenesis, morphometric
grades, paleohydrogeology, air inclusions in ice cores etc.
• Different methods are used for the analysis and assay of
samples are analysed.
• Used for hydrothermal ore deposits.
24. REFERENCES
• Comparison of decrepitation, microthermometric and compositional
characteristics of fluid inclusions in barren and auriferous mesothermal
quartz veins of the Cowra Creek Gold District, New South Wales, Australia.
J.A. Mavrogenes, R.J. Bodnar, J.R. Graney, K.G. McQueen, Kingsley
BurlinsonJournal of geochemical exploration, 54/3 (1995) pp167-175
• Study of Fluid Inclusions: Methods, Techniques and Applications K. R.
Randive, K. R. Hari ,M. L. Dora , D. B. Malpe and A. A. Bhondwe, Gondwana
Geological Magzine., V. 29(1 and 2), June and December, 2014. pp19-28
• Relation of fluid inclusion geochemistry to wallrock alteration and
lithogeochemical zonation at the Hollinger-McIntyre gold deposit, Timmins,
Ontario, Canada Ted J. Smith, Stephen E. Kesler, CIM Bulletin, April 1985,
pp35-46.
Notes de l'éditeur
Quartz; around 1 billion inclusions in 1 gm of milky qtz
Fluid inclusions are not only studied for mineral exploration but it has many other applications like for petroleum exploration, salinity determination especially for archean sea, sedimentary env and diagenesis, air inclusions in ice cores etc
There are various host minerals which are being observed for mineral exploration such as …………….
Manual manipulation under the microscope- broken open with sharp instrument- gas expansion in oil is observed – tedious and demanding- if halite crystals observed - NaCl > 26%- composition and molar volume
Micro thermometry (-200⁰C to1500⁰C )- temperature of formation- eutectic temp.- first liquid phase- temp of host melt
Laser Raman microprobe- composition of fluid- spectra is obtained and through this we can focus on inclusion leaving the host- composition of non aquous fluids- identification of daughter crystals.
Laser ablation with mass spectrometry- composition including trace elements
Baro-acoustic decrepitation- fingerprint of fluid inclusion content- pressure vs temp graph is obtained