Fluid Inclusions for exploration - the baro-acoustic decrepitation method

Kingsley Burlinson

P.O. Box 37134, Winnellie, NT, 0821, Australia

An instrument to record acoustic decrepitation of fluid inclusions has been developed, incorporating a standard desktop computer with additional control electronics. The instrument provides about 15 analyses per day on crushed samples so that fluid inclusion data can be obtained for use in exploration programmes at modest cost. The presence of CO₂-rich inclusions can be easily discerned and suites of samples can be compared empirically to discriminate between mineralised and barren samples. Although quartz is the most common mineral used, opaque minerals such as feldspars, iron oxides and sulphides can also be analysed using this technique.

Acoustic decrepitation of quartz samples containing CO₂-rich fluid inclusions gives a distinctive peak at low temperatures from 150^o to 300^oC, whereas samples lacking CO₂-rich inclusions show little or no acoustic decrepitation at these temperatures. This provides an approximate but quick means of determining the CO₂ contents of fluid inclusions, which is particularly relevant in Au exploration. The relationship between CO₂-rich fluids and gold mineralization has been well documented in many deposits including the Abitibi in Canada, the Kalgoorlie region in West Australia and the Victorian goldfields, Australia.

At the Victory mine near Kalgoorlie, Western Australia, several different generations of quartz veins are defined on the basis of orientation and some workers interpret the Au mineralization to be related specifically to the horizontal quartz vein sets. Acoustic decrepitation shows that both horizontal and vertical quartz veins within the ore zones contain CO₂-rich fluids, whereas veins remote from the known ore zones rarely contain CO₂-rich fluids, regardless of their orientations. Determination of CO₂ contents by acoustic decrepitation would be a better guide to mineralization than reliance on the physical orientation of the quartz veins in this deposit.

The acoustic decrepitation method can also be used on opaque minerals, where normal microthermometric methods are inapplicable. Haematite-magnetite systems with and without Au mineralization have been studied at Tennant Creek, NT, Australia; Nevada, USA and the Abitibi province, Canada.

At Tennant Creek, Au occurs in massive haematite-magnetite-chlorite host rocks and acoustic decrepitation shows marked variations at small scales, indicating complex inhomogeneity of fluids within single ironstone bodies which were previously thought to have been of uniform origin. Many of the haematite samples from these deposits show intense decrepitation, indicating abundant fluid inclusions. Had this haematite been derived by supergene oxidation of precursor magnetite, as has been proposed in some studies, the original inclusions in the magnetite would have been eradicated. Thus much of the haematite in these deposits must be of primary origin.

At the Upper Beaver mine in the Abitibi province, Canada, auriferous magnetite displays intense acoustic decrepitation but magnetite from nearby barren ironstones lacks decrepitation. Samples from non-auriferous magnetite and ironstones in Nevada may show decrepitation, but many are inactive. In contrast, skarn magnetite associated with low grade Au-Cu mineralization at Lyon, Nevada shows moderately intense decrepitation with major variations between samples several metres apart, similar to the variability seen in the samples from Tennant Creek.

Although there is little understanding of fluid inclusions in opaque minerals, acoustic decrepitation shows that the iron oxide systems can be quite complex and this technique can aid in discriminating between otherwise indistinguishable ironstones during exploration.

20th IGES, Santiago, 2001

Back to main Contents

The Instrument