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

V3 quartz reanalyses

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.

V3 magnetics size fractions

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.

V3 rerun of used sample

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.

V3 chert


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.

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