NZ gold deposits, Waihi

GOLD EXPLORATION USING FLUID INCLUSIONS

K. Burlinson

Burlinson Geochemical Services, PO Box 37134, Winnellie, NT, Aust.

Despite our extensive understanding of fluid inclusions and the importance of fluid systems in the formation of many economic mineral deposits, there is negligible use of this important data as an exploration tool. The traditional methods of fluid inclusion study are tedious, slow and expensive and are unsuitable for routine field data collection. However, by using baro-acoustic decrepitation we can rapidly and economically collect very useful inclusion data, albeit not as pedantically precise as is used in research work. This decrepitation data is particularly useful in identifying the presence of CO2 rich inclusion fluids, which are well known to be commonly associated with Au mineralisation. 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 point.

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. There is a useful correlation between the Au content of the samples and the observed low temperature CO2 decrepitation measurements.

The Brusson mine in Italy is an alpine quartz vein which has been mined since Roman times. Samples from this mesothermal deposit have much more intense decrepitation and a prominent low temperature CO2 peak, as well as multiple other inclusion populations.

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.