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Rapid fluid inclusion data for exploration (decrepitation)

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Gold exploration using baro-acoustic decrepitation

K. Burlinson

Burlinson Geochemical Services P/L

Early attempts to use fluid inclusion decrepitation methods for mineral exploration were compromised by a failure to understand the importance of the presence of gas rich inclusions and their thermodynamic behaviour when heated, leading to misinterpretation and the premature demise of the technique.

We know that many gold deposits form from CO₂ rich fluids and these fluids can be readily detected using baro-acoustic decrepitation. Consequently we can use decrepitation as a mineral exploration method to locate CO₂ rich, potentially auriferous hydrothermal quartz. Using a computerised decrepitation instrument provides quick, cheap, reproducible and objective measurements of CO₂ in contrast to the slow, subjective and labour intensive microthermometric techniques. Although the method is best applied to mesothermal deposits formed at high pressures, it does also work with many epithermal deposits. Gas-rich fluid inclusions give a distinctive low temperature decrepitation peak because these inclusions have high internal pressures at room temperature and when heated, the pressure increases linearly with temperature in accordance with the gas law. In contrast, aqueous fluid inclusions have a condensed liquid phase and do not generate high internal pressures until temperatures above their homogenisation temperatures.

Samples from the Waihi epithermal gold deposit, NZ, have been analysed and they show low overall decrepitation intensities, as expected from epithermally formed fluid inclusions, but they clearly show low temperature decrepitation indicating the presence of CO₂ rich fluid inclusions. Samples from or near the active workings show low temperature decrepitation caused by CO₂ rich inclusions whereas distal samples either from the same vein or from other veins nearby lack this CO₂ caused decrepitation peak. This provides a means of evaluating potentially auriferous quartz veins based on their CO₂ fluid contents.

At the Brusson mine in northern Italy, alpine quartz veins were mined for their gold content. These mesothermal veins give intense decrepitation with large and prominent low temperature peaks caused by CO₂ rich inclusions. The decrepitation patterns can be used to distinguish between quartz samples which otherwise appear to be identical.

Baro-acoustic decrepitation can give valuable fluid inclusion data to use in mineral exploration. Many mineral deposits are “fossilized” fluid systems and we can surely benefit by using fluid inclusion information when exploring for them, not merely for forensic analysis of the deposits we have already found.

International Mineralogical Association meeting, Budapest, July 2010