Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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Semi quantitative determination of CO2 by decrepitation.


At the Cowra Creek goldfield in NSW, Australia, Mavrogenes et al. carried out a comprehensive study which included an evaluation of decrepitation as an exploration method and compared the acoustic decrepitation data with results from  quadrupole mass spectrometric analyses of the gases emitted from the same samples during heating or crushing. This work confirmed that large quantities of CO2 were released from these samples which also had significant  low temperature peaks on their acoustic decrepitation analyses. An explanation of why these gas-rich fluid inclusions produce low temperature decrepitation peaks is included in this report.


Location of the Cowra Creek study area.
location cowra creek


The five samples which were subjected to both acoustic decrepitation and also Quadrupole Mass Spectrometric (QMS) gas analyses are tabulated below with their measured gas contents (by QMS) and cumulative decrepitation counts up to 300 C.


Sample number
(Mavrogenes)		 Sample number
 (decrepitation)
 CO2 %
 CH4 %
 N2 %
 Total gas %
 Counts to 300 C
 Colour
Q7
 G1430
 3.0
 0.2
 1.4
 4.6
 3913
 red
Q3
 G1414
 2.6
 6.3
 2.0
 10.9
 8173
 green
Q10
 G1433
 5.8
 4.1
 3.4
 13.3
 5742
 blue
Q15
 G1436
 10.2
 4.7
 0.7
 15.6
 16354
 magenta
Q4
 G1415
 8.5
 25.6
 6.3
 40.4
 9678
 cyan


The  analyses of these  samples shows an approximate correlation between total gas content, as measured by QMS and total decrepitation counts up to 300 C. The main exception to this is in sample Q4 (G1415) which had a particularly high CH4 content. The lower than expected decrepitation in this sample might be related to a reaction within the inclusion between CO2 and CH4 to produce C and H2O, which would dramatically reduce the internal pressure of the inclusions and thus  result in less than expected decrepitation at low temperature.


Cowra Ck Au


The decrepitation counts graph has been overlaid on the detailed plot of the gas release as measured by QMS for sample Q15, which has a total gas content of 15.6%. This shows that the low temperature counts are indeed caused by the decrepitation of inclusions in which CO2 is abundant. An additional study carried out by Prof. Damien Gaboury also shows that low temperature decrepitation events release large quantities of CO2 gas.

qms co2 and decrep plot


This data shows that acoustic decrepitation can be used as a rapid and low cost method to determine an approximate gas content measurement of fluid inclusions. This measurement of fluid inclusion gas content can be very useful in exploration for many types of gold deposits and in particular, archaean greenstone belt deposits such as those in Western Australia or the Canadian Abitibi province and also sedimentary slate belt deposits such as in Victoria, Australia or the meguma terrane of Nova Scotia.

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