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Rapid fluid inclusion data for exploration (decrepitation)

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Fluid inclusions in mineral exploration

The baro-acoustic decrepitation method

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.  The reason for this is explained here.

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.

Almost all quartz vein samples contain abundant fluid inclusions and give intense decrepitation responses. Even in zones of pervasive silicification where the inclusions are typically very small, good decrepitation responses are usually obtained. In contrast, chert and jasperoid (low temperature silicification of carbonate rocks) give negligible decrepitation and this distinction can be useful in discriminating between the hydrothermal or supergene origin of quartz samples.

Decrepitation results from the Cosmo Howley mine, NT, Australia show intense decrepitation and the presence of abundant CO₂ rich fluid inclusions. This fluid inclusion assemblage only occurs in hydrothermal quartz and NOT in cherts, which have wrongly been assumed to be the host rock.The decrepitation data shows that genetic models of this mine which assume a low temperature synsedimentary chert origin are wrong and that this deposit is in fact a mesothermal deposit formed from CO₂ rich, high pressure fluids.

At the Enterprise Au mine at Pine creek, NT, Australia, decrepitation again shows the abundance of CO₂ rich fluid inclusions. In early microthermometric studies, the mine operators failed to notice this important fluid feature as they were not careful enough with their microscope work. Decrepitation reliably and quickly measures these fluids as it is computer controlled and not subject to errors committed by bored, tired or overworked microscopists!

CO₂ is also a major fluid component at the Getchell and Twin Creeks mines in Nevada, USA.

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.

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