Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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   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.

Brusson mine, Italy    Favona vein, Waihi, NZ

Geol Soc NZ conference, Oamaru, Nov. 2009

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