Applied Mineral Exploration
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Rapid fluid inclusion data
for exploration (decrepitation) |
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Acoustic
decrepitation of quartz samples containing CO2-rich fluid
inclusions
gives a distinctive peak at low temperatures from 150o to 300oC,
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.
