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Grainsize studies and multiple analyses results
for Krivoy Rog samples
A number of samples from Krivoy Rog were studied in more detail
to confirm and understand the decrepitation results better.
Decrepitation analyses are done on crushed grains of the sample.
The grains are sieved to a consistent size fraction of <420
microns and >200 microns (the standard grainsize in use)
and either 0.5 or 1.0 grams of these grains are analysed.
Sample 2137A (vein quartz) was analysed on the standard grainsize
fraction and also on a finer grainsize fraction of <200 microns
and >100 microns. The standard grainsize fraction
(labeled coarse), shown here by the red graph, has much more
intense decrepitation than the fine grainsize fraction. This is
because crushing to a finer grainsize destroys many of the fluid
inclusions during the sample preparation and so the coarser
standard grainsize fraction is used for all routine analyses.
The magnetic fraction of sample 2137B was also analysed on
both coarse and fine grainsize samples. (The same grainsize
fractions as above.)
The results are unusual, because the coarse fraction, shown in red,
had less intense decrepitation than the fine fraction. This
unexpected result seems to be caused by the composite nature of the
magnetic fraction particles. Most particles were comprised of both
quartz and magnetite. Even a small quantity of magnetite in a
dominantly quartz particle caused that particle to separate into the
magnetic fraction. In the coarser grainsize magnetic fraction,
almost all particles were mostly of quartz with only a small
subgrain of magnetite. But in the finer grainsize fraction, the
crushing size more closely approached the crystal size within the
rock and these smaller particles were often composed solely of
magnetite. Because the quartz does not decrepitate at all (as seen
in the V3 chert plot below), the fine grainsize fraction shows
greater decrepitation because of the increased abundance of
completely magnetite grains. The cause is the interaction between
the crushing grainsize and the crystal grainsize in the rock sample
and the method of magnetic separation which allowed composite
quartz-magnetite grains into the magnetic fraction. A separation
based on density rather than magnetism would avoid this issue.
To confirm that the decrepitation counts observed are caused by
fluid inclusions, rather some unexplained other process, the used
material after analysis h2591 was re-analysed as analysis h2617.
Note that this is not a duplicate analysis of 2 aliquots of the
sample 2137B. The re-run of previously decrepitated material (the
green plot) has essentially no decrepitation which confirms that the
counts observed in analysis h2591 are caused by an irreversible
process, such as fluid inclusion decrepitation. Other potential
processes such as thermal expansion or crystal structure changes are
most likely reversible, so after cooling, a re-run of the used
sample would again give rise to counts. These plots confirm that an
irreversible process, such as gas release during fluid inclusion
decrepitation, is causing the observed decrepitation counts. The low
level counts near 800 C on analysis h2617 are due to incomplete
decrepitation of the sample at the end temperature, perhaps due to
thermal gradients within the sample. Other tests on supergene
magnetite within laterite nodules, which would not contain
fluid inclusions, show no decrepitation and confirm that the
observed counts are not merely inherent to the nature of magnetite
and are most likely caused by fluid inclusions within high
temperature or hydrothermal magnetite.
In addition to the magnetite fraction of sample 2137B, the
non-magnetic fraction was also analysed. This material is the cherty
quartz fraction of the banded iron formation (BIF). This quartz gave
no response, these counts being random background. Although this
quartz underwent the same metamorphism as the associated magnetite,
it has not incorporated large enough fluid inclusions during the
metamorphism to give a decrepitation response.
Summary
Analyses of multiple size and magnetic fractions of BIF and
crosscutting quartz vein within this one sample confirm that the
decrepitation responses are reliable. The tests on different
grainsizes can be explained by the effect of less intact
fluid inclusions available for decrepitation as the host
particle size is decreased during sample crushing and
separation. And the re-analysis of previously analysed material
confirms that decrepitation is caused by an irreversible
process, most probably gas release when inclusions decrepitate.
Although we cannot visually confirm the existence of fluid
inclusions in opaque magnetite, their presence is the most
likely explanation for these observed decrepitation responses.
Applied Mineral Exploration
Discussion and research relevant to mineral
exploration. 
