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Cosmo Howley mine - misidentification of
"chert" host rock
At the Cosmo Howley mine, Pine Creek area, NT,
Australia,
the Au occurs within extensive stratiform quartz units. These were
claimed
to be stratiform cherts within the sequence. However, decrepitation
shows
that all these quartz samples contain abundant high temperature fluid
inclusions,
with high CO2 contents and high internal pressures. These
types
of fluid inclusions are inconsistent with a chert host material
and
it is clear that the auriferous horizon is in fact an epigenetic quartz
related to the intrusion of the nearby granites. The decrepitation
results
show that this deposit is in fact a stratabound replacement type
deposit,
and not a "syn-sedimentary exhalative gold in sedimentary cherts" type,
as has been stated elsewhere.
Enterprise Au mine, Pine Creek, NT
: CO2-rich fluids
An initial microthermometric fluid inclusion study of
samples
from the Enterprise mine, came to the conclusion that the deposit
formed
from fluids which lacked and CO2 content. A subsequent
decrepitation
study showed that over 50% of the samples collected had a prominent low
temperature decrepitation peak, indicating that CO2-rich
fluids
were in fact a major component of the mineralising fluids. Samples from
the eastern limb of the fold tended to lack CO2, but samples
from the ore bearing sheeted and stockwork vein areas always contained
CO2-rich fluid inclusions. The Au mineralisation is
closely
related to the CO2-rich fluids.
The initial study was compromised by the inability
to
collect an extensive suite of relevant samples and the choice of the
operator
to use transparent quartz, rather than the more common milky quartz. In
this case decrepitation analyses were a much more reliable indicator of
the mineralisation fluids than were the microthermometric data.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
