Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 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.
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