Rapid fluid inclusion data
for exploration (decrepitation)
A sample analysed as J276, from a gold deposit in Victoria, Australia, has very intense decrepitation at low temperature (200-300 C) which is typical of fluid inclusions with high CO2 or gas content. Fitting gaussian distributions to the decrepitation result showed the presence of 4 populations of fluid inclusions in the sample. This study attempted to identify the inclusions associated with each population by decrepitating the same sample material to 3 different temperatures (based on the fitted gaussian population temperatures) and microscopically observing the used sample material from each temperature step and comparing these with fresh, un-decrepitated sample.
The 4 analyses J276, J310, J311 & J312 are of
the same sample, but heated to 4 different maximum
The skew-gaussian fit to analysis J276 shows 4 fluid inclusion populations.
Based on this fit, an analytical cut-off
temperature of 350 C was selected to selectively remove
inclusions from population 1, and a cut-off of 490 C to
selectively remove the inclusions of populations 1 & 2.
Complete decrepitation to 640 C removed all 4 populations of
inclusion. Note that population 4 is a function of the quartz
weakening as the quartz changes from alpha to beta phase at 573
here) and is not an actual fluid population event.
The observations were made on crushed grains
immersed in refractive-index (RI) oil (tritolyl phosphate, RI
1.555, the RI of quartz is 1.553) to conceal grain boundary
effects, using a Nikon petrographic microscope with 40x
objective and 10x eyepiece with a graduated scale. All
observations were made at room temperature of 30 to 32 C which
is close to or above the critical temperature of CO2
(31 C) and so liquid CO2 could not occur or be
The fresh sample, labelled J276, but not
decrepitated, had numerous fluid inclusions, typically 2-5
microns in size, equant, subhedral with 2 phases visible
and 50% vapour or gas. There were less numerous but still
abundant large 15-20 micron irregular inclusions, the
majority of which were 2 phase with large vapour %, but
some are empty or gas filled. No damage fractures were present.