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Acoustic decrepitation as a rapid
means
for determining CO2 (and other gas) content of
inclusion
fluids. An explanation of the acoustic decrepitation behaviour of
gas
rich fluid inclusions.
Kingsley Burlinson
Presented at ACROFI I
Nanjing, China, May 2006
The acoustic decrepitation method heats a small monomineralic sample
and counts pressure impulses as the inclusions burst when they
develop
high internal pressures. For aqueous fluids, the decrepitation
temperature is correlated with the homogenisation temperature, but
gas
rich fluids give a distinct and characteristic low temperature
decrepitation peak which can actually be used to identify gas rich
fluid inclusions. This information is useful in exploration for Au
deposits, which are frequently associated with CO2 rich
and
sometimes CH4 rich fluids.
This distinctive decrepitation occurs because the non-aqueous
inclusion
fluids expand according to the gas law and develop internal
pressures
high enough to burst the host mineral grain at temperatures well
below
their homogenisation temperatures. In contrast, aqueous fluids
condense
to a liquid and gas phase during post-entrapment cooling. Upon
subsequent heating their internal pressures do not increase
significantly until after homogenisation to a single phase occurs
and
hence they do not decrepitate "prematurely" as gas rich inclusions
do.
This behaviour is usually regarded as an annoyance in conventional
microthermometric homogenisation studies, but can readily be used as
an
exploration aid to find mineralisation deposited from such gas rich
fluids.
Because all the non-condensing gases follow the gas law with only
minor
differences due to non-ideallity, decrepitation determines the total
gas contents. All of the gases CH4, CO2 and N2
contribute almost equally to this and although the actual
composition
of the gas phase cannot be determined, this is of no detrimental
effect
when used in exploration to search for the gas-rich inclusions often
associated with Au minearalisation
Decrepitation results on samples from Cowra, NSW, Australia, which
have
also been microthermometrically measured for XCO2, show
that
amounts of less than 5% XCO2 are easily distinguished by
decrepitation and amounts as low as 1% may be determinable.
Microthermometric results from one sample at the Southern pit of the
Tanami goldfield, NT, Australia, shows Xgas ranged from 5% to 43% in
18
observations, but decrepitation analsis of an averaged sample shows
only about 1% Xgas. In this case the inclusions containing high Xgas
were restricted to a single late event in the rims of vug fill
quartz
crystals and are numerically insignificant and probably not part of
the
main Au depositional event even though they have received
disproportionate attention in research of this deposit.
Not only does the decrepitation method provide a means of rapidly
and
reliably determining the gas content of inclusion fluids, but it
also
provides an averaged result which can be more useful in exploration
than focusing on an individual quartz formation event, which may
inadvertently not be related to the mineralisation for which one is
exploring.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration.