decrepitation, gold deposits

European Current Research on Fluid Inclusions (ECROFI-XIX)

An updated understanding of Acoustic emission decrepitation

Burlinson, Kingsley

Burlinson Geochemical Services Pty. Ltd., Darwin, NT, Australia.

(Poster presentation)

The premature demise of acoustic emission decrepitation as a fluid inclusion study technique was mainly because of an inability to relate the temperature of onset of massive decrepitation with the formation temperature. But much of this problem was in fact due to a lack of understanding of the importance and behaviour of CO₂ in fluid inclusions. From the gas law, it is clear that CO₂ rich fluid inclusions will develop high internal pressures at low temperatures, resulting in decrepitation well below their formation temperatures. Although this behaviour is a hindrance in determining formation temperatures it means decrepitation data can easily be used to detect CO₂ rich inclusion populations, which is very useful in mineral exploration for Au deposits because of the commonly documented association between Au and CO₂rich fluids. (Fig 1)

Other gases such as CH₄ behave just like CO₂ and so they contribute to this low temperature decrepitation effect. Plots of the equation of state for various gases show that they all result in high inclusion pressures and low temperature decrepitation. Statements by some authors that CH₄ does not cause fluid inclusion decrepitation are incorrect and contradict the gas law.

Schmidt-Mumm (1991) asserted that sounds measured in decrepitation experiments were dominated by cyrstallographic and grain boundary effects. This is incorrect as the instruments used measure a pressure pulse in the air column between the sample and sensor. Changes in crystal structure or grain boundary movements simply cannot generate large enough pressure pulses to be detected. Only the rupture of fluid inclusions with subsequent release of high pressure gases or a steam explosion from superheated water can generate the pressures necessary for detection. And because secondary inclusions leak gradually or open at low temperatures, they fail to generate sufficient pressure to be detected. Consequently acoustic decrepitation side-steps the entire problem of secondary inclusions and their potential mis-identification in microthermometric studies.

A comparison of fluid inclusion abundance counts in thin section with decrepitation of the same samples shows that only some 0.5% of inclusions larger than 8 microns across decrepitate and are detected during analysis. Despite this, replicate analyses of aliquots of the same sample give consistent and reliably reproducible results.

Acoustic decrepitation has been incorrectly maligned and although it is not a high precision method, it gives consistently reproducible fluid inclusion population temperatures and an indication of CO₂ + CH₄ gas contents. As it is fast and cheap it is ideal for use in exploration or for preliminary scanning in conjunction with conventional microthermometric studies.

co2

Fig. 1. Quartz from Ballarat decrepitates at low temperatures between 180 - 300 C as it contains abundant CO₂ rich inclusions, while quartz from Dongping lacks CO₂ rich inclusions and does not decrepitate until 370 C, its approximate Tf.

REFERENCES

Schmidt-Mumm A. (1991) Phys Chem Minerals. 17:545-553.

Talk presentation of this poster

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