Applied mineral exploration methods, hydrothermal fluids, baro-acoustic decrepitation, CO2 rich fluids
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Microscopic observations of crushed grain samples, Nova Scotia

Grains of representative samples were mounted in refractive index oil and observed as a control on the decrepitation data and to try and observe features in the fluid inclusion populations related to the decrepitation patterns. All observations were done at room temperature of 30 C, preventing the formation and observation of any liquid CO2 phase. The grains observed were the same size fraction (-420 +200 microns) as the grainsize used in the decrepitation analysis.

Interpretation:

Dufferin samples with / without low temperature decrepitation, and with very low overall decrepitation.

In sample 2008C, a significant number of quite large, irregularly shaped, vapour rich inclusions were seen and these are likely to be CO2 rich and responsible for the prominent low temperature decrepitation peak. In contrast, in sample 2008B the larger inclusions were smaller (maximum 12 instead of 20 microns) and were much less abundant. It is these larger inclusions which cause the prominent low temperature decrepitation in 2008C and  very minimal low temperature decrepitation in sample 2008C.  The almost complete lack of decrepitation in sample 2008D is likely to be due to a recrystallisation event which has eliminated almost all inclusions greater than 2.5 microns and resulted in complex crystallinity even within the <420 micron maximum grainsize observed.  Although such small inclusions  can  decrepitate, their lack of decrepitation in this sample is presumed to be because they are vapour, rather than liquid dominated. Or alternatively, all the primary inclusions were eliminated during recrystallisation and the inclusions observed are low temperature secondaries, which do not register on the decrepigram.

2008B, Dufferin mine, lacking low temperature (CO2 rich inclusion) decrepitation.
Moderately intense fracturing and very abundant fluid inclusions. Most inclusions are only 2-5 microns across, with very rare inclusions up to 12 microns. The smaller inclusions are equant and about half of these contain a 2 phase fluid with small vapour bubbles (usually 20% but up to 40% vapour), the remainder being single phase.  Very rarely these inclusions contain moving bubbles. The larger inclusions comprise less than 1% of the population and are of irregular shape. They may contain  large bubbles of 40-50% vapour, suspected to be due to CO2.
2008C, Dufferin mine, showing low temperature (CO2 rich inclusion) decrepitation.
Moderately intense fracturing and very abundant fluid inclusions. Most common inclusions are 2-5 microns across with many up to 20 microns. The smaller inclusions are equant, often showing negative quartz crystal shape and contain vapour bubbles of 20-50% volume which are often moving.  The large inclusions comprise about 5% of the inclusion population and have very irregular shapes with  vapour bubbles typically 20-40% but occasionally up to 80% volume and are probably CO2 rich.
2008D, Dufferin mine, with very little overall decrepitation.
Intensely fractured quartz with numerous tiny inclusions <2.5 microns across and only very rare larger inclusions of maximum size 5 microns. The small inclusions are equant and sub-rounded and seem to contain 2 phases, but it is difficult to see into them.  The rare larger inclusions are of irregular shape with vapour bubbles of about 25% volume.  No moving bubbles are present. The fracture planes are not a  dominant control on fluid inclusion occurrences. Many of the grains are complex aggregates of quartz growths and show multiple extinction angles in polarized light. This quartz has undergone recrystallisation and extensive fracturing and possible overgrowths, resulting in destruction of original inclusions and leading to the very low decrepitation intensity observed.


Interpretation:

Dufferin samples with / without decrepitation near 550 C.

Despite most inclusions in both these samples being quite small (2 microns), there is abundant decrepitation. In sample 2009D a wide range of vapour bubble size from 10% to 30% was observed  and this was not seen in sample 2009E which had a more uniform vapour bubble size of about 10%. This wide range of vapour bubble size is interpreted to be the cause of the broad decrepitation in sample 2009D. Often such variations in degree-of-fill are due to boiling of the fluids during deposition, but these observations on grains in oil are not precise enough to draw a firm conclusion on this, just a suspicion.
 

2009D, Dufferin mine, broad decrepitation from 450 C to 570 C, no decrepitation decrease near 550 C.
Strongly fractured quartz with abundant small inclusions, mostly less than 2 microns across. The largest inclusions are only 5 microns across. The small inclusions are equant and seem to contain only a single phase, but it is difficult to see into them. The larger inclusions are usually of irregular shape and contain 2 phases with a vapour bubble volume of 10-15% in many but with some having up to 30% vapour bubble volumes. There is an unusually wide range of vapour bubble sizes in these inclusions.
 
2009E, Dufferin mine, prominent decrease in decrepitation near 550 C.
Moderately fractured quartz with abundant small inclusions up to 2 microns across and only occasional larger inclusions up to 5 microns across. About half of the small inclusions contain 2 phases with a vapour bubble of 10 % volume, the remaining small inclusions containing only a single phase. There are very rare larger inclusions with large bubbles of 40% vapour.

 

Interpretation:

Cochrane Hill, Upper Seal harbour and Forest Hill comparison.

Samples 2017C and 2020B both contain medium sized inclusions up to 10 and 13 microns across, but they are more abundant in 2020B. These larger inclusions in sample 2020B have a wider range of vapour bubble sizes (up to 80%) than in sample 2017C  (less than 40%) and this broad range of degree-of-fill also results in a broad decrepitation pattern. The vapour rich inclusions will not develop high internal pressures until they are heated well beyond their homogenization temperatures and are likely responsible for the broad decrepitation extending to high temperatures at 550 C.  Sample 2023F has a much more uniform suite of inclusions with smaller (20%) vapour bubbles, resulting in a narrower range of decrepitation starting much later near 400 C and also having little decrepitation at 550 C.
These 3 samples have been plotted together for comparison.

2017C, Cochrane Hill pit, prominent decrease in decrepitation at 540 C.
This sample shows a complex mixture of grains of different quartz types and some 10% of the grains lack uniform extinction and are of composite origin.  The sample has undergone only minor fracturing. Inclusions are abundant, mostly small (2 microns across) but with about 1% being larger. These larger inclusions are usually about 10 microns across with some as large as 13 microns across. The small (2 micron) inclusions are equant and seem to be empty or single phase. Inclusions of about 5 microns across are 2 phase with vapour bubbles usually about 25% by volume, but occasionally up to 80% volume. The largest inclusions are usually of irregular shape, typically with small, moving vapour bubbles about 10%, but up to 40% by volume. There may be some daughter crystals in a few of the large inclusions although it is hard to be certain.
 
2020B, Upper Seal Harbour, broad decrepitation from 450 C to 570 C, pronounced peak at 520 C.
Moderately fractured quartz with abundant inclusions. Inclusions are typically small and <2 microns across with some large inclusions up to 10 microns across. The small inclusions are equant with vapour bubbles of 20% by volume and occasionally up to 40% volume. The larger inclusions comprise about 5% of the population and are of irregular shape with 2 phases, the vapour bubble ranging from 10% to 80% by volume. A few daughter crystals were seen in some of the larger inclusions. (Interpretative comments above)
 
2023F, Forest Hill, prominent decrease in decrepitation at 540 C.
Relatively unfractured quartz. Moderate inclusion abundance with some grains being almost inclusion free. Most inclusions are small and <2 microns across, with rare inclusions up to 5 microns across. The inclusions are equant and contain 2 phases with vapour bubbles of 20% by volume and uniform vapour bubble sizes. Many (but not all) of these inclusions are aligned on planes and it is suggested they are pseudo-secondary inclusions on healed fractures.  There are additional planar groups of clearly secondary inclusions which are <2 microns across and which lack any vapour bubble. These two different types of inclusion are distinct, despite their planar alignments. Some 60% of the grains in this sample showed complex extinction behaviour and the quartz is often very fine grained with complex overgrowths. (Interpretative comments above)

 

Interpretation:

Upper Seal harbour samples with / without low temperature decrepitation

Sample 2019C contained larger inclusions (25 microns versus 8 microns) and these were also much more abundant and contained larger vapour bubbles than inclusions in sample 2019E. These larger inclusions in 2019C are the most likely hosts of a CO2 rich vapour phase which causes the low temperature decrepitation. In addition, the wide range of vapour bubble sizes results in broad decrepitation from 250 C to 550 C.  In contrast, inclusions in sample 2019E are less variable in both size and degree-of-fill, resulting in a much narrower range of decrepitation from 400 to 550 C.

2019C, Upper Seal Harbour, showing low temperature (CO2 rich inclusion) decrepitation.
Only slightly fractured quartz with abundant inclusions. Inclusions are typically 2-5 microns across but there are also many larger inclusions up to 25 microns. The small inclusions comprise about are equant, sometimes showing negative quartz crystal shapes, and usually contain 2 phases with vapour bubbles typically of 25%  volume, but occasionally up to 80% vapour bubble volume.  The larger inclusions are of irregular shape with 2 phases and vapour bubbles usually 50% by volume, but occasionally even larger. There may be rare daughter crystals present. There were noticeably more large inclusions than usual in this sample.
 
2019E, Upper Seal Harbour,  lacking low temperature (CO2 rich inclusion) decrepitation.
Slightly fractured quartz with abundant inclusions. Most inclusions are < 5 microns across, with a typical size of 2 microns. There are rarely inclusions up to 8 microns across. The smaller inclusions are equant and usually 2 phase with vapour bubbles of about 20% by volume. The large inclusions are of irregular shape and contain 2 phases with a vapour bubble of 20% size only. There are very rare inclusions with larger vapour bubbles up to 60% volume.



 

Interpretation:

Black Bull Silica,  very narrow decrepitation temperature range.

The decrepitation in this sample is probably due to the 2 to 5 micron sized inclusions with 20% vapour bubbles. The smaller 2 micron inclusions are probably secondary inclusions which do not contribute to the decrepitation. The very narrow decrepitation range reflects the uniformity of this single population of inclusions.  It is unclear if this could be a consequence of the inclusions being pseudo-secondary rather than primary. The complex crystallinity shown in many grains is surprising for a sample with such a uniform range of inclusions as it suggests multiple quartz growth episodes which is not apparent from the inclusion uniformity.
(Black Bull sample results)

2003F, Black Bull Silica deposit, narrow range of decrepitation at 450 C.
Moderately fractured quartz, with many grains showing replacement textures and some grains of micaceous materials.  About 50% of the grains show complex extinction due to multiple quartz growths. Moderate to low abundance of fluid inclusions with most inclusions about 2 microns across and only rare inclusions up to 5 microns. The small inclusions usually contain only a single phase and seem to be empty, but some have tiny (<5% by volume)  vapour bubbles. The rare larger inclusions are equant, often with negative quartz crystal shape and contain 2 phases with a vapour bubble of about 20% by volume. Some grains almost completely lack fluid inclusions. The apparent empty inclusions might indicate a vapour dominant phase of quartz growth.