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Calculations and implications of fluid
inclusion abundances in quartz
Overview
Counts of the abundance of fluid inclusions in typical milky white
quartz give very large numbers , some 10 ^{9} inclusions
per gram. Estimates of the total volume of the inclusions show that
this is still only about 0.5% by volume of the quartz. Further
estimates of the effect of trace elements potentially within the liquid
in the fluid inclusions indicates that it would be very unlikely for
elements within inclusion fluids to affect a whole rock analysis by
even as much as 1 ppm.
Discussion
A sample of milky quartz from a gold producing area near Pine Creek, NT,
Australia, designated PCB4, has been subjected to decrepitation
analysis. A thin section of this sample was also examined
microscopically to determine the abundance of fluid inclusions and
their size distributions, as summarised here. Because of the large
number of inclusions counted, a calculation of the volume of these
inclusions was done.
For the counting, a microscope with a 40 times objective was used together with an eyepiece
magnification of 10 to give a total magnification of 400 times. Each
counted area was 50 microns by 50 microns and the depth of
field-of-view of the microscope was 5 microns, giving a counted volume
of 50 * 50 * 5 = 12500 cubic microns, or 1.25 x 10^{-8}
cc. The
inclusions were grouped by size during counting and inclusions less
than 1 micron across were too small to see and not considered. No
attempt was made to discriminate between primary and secondary
inclusions and these counts are the total of all visible fluid inclusions.
Using 2.6 g/cc as the density of quartz, we multiply the average counts
by 1.54 x 10^{7} to get the number of inclusions per HALF gram.
(Half a gram is used as this is the quantity used in the decrepitation
analysis.)
A total of 20 areas of the thin section of PCB4 were counted and the
number of inclusions for each count area are shown here, grouped by the
size of the inclusions in microns (u).
Area #
1-5 u
5-10 u
10-20 u
>20 u
1
30
3
1
1
2
35
4
1
0
3
28
1
1
1
4
50
5
2
1
5
33
3
1
2
6
32
2
3
0
7
48
3
3
0
8
60
3
2
1
9
44
2
1
1
10
42
3
1
1
11
45
1
3
1
12
30
2
2
1
13
66
5
1
0
14
51
2
1
0
15
54
1
0
1
16
63
2
1
0
17
56
3
1
1
18
45
2
1
0
19
35
3
1
0
20
58
1
1
1
Totals
905
51
28
13
Average
45.3
2.6
1.4
0.65
For sample PCB4, the number of inclusions per half gram were:
1-5 u
697 million
5-10 u
40 million
10-20 u
21 million
> 20 u
10 million
And these counts are shown on the decrepitation summary graph here.
As a cross check, an estimate of the volume of this many inclusions was
done. Inclusions were assumed to be spherical. A 1 micron diameter
inclusion would have a volume of pi*D^{3}/6*10^{-12}
cc, or approximately 0.5 * 10 ^{-12} cc. This volume is expected to be rather higher than actuality because the assumptions are quite generous.
Size range
average diameter
inclusions/gram
total volume cc
1-5 u
2 u
1394 million
1.4 * 10^{-3}
5-10 u
7 u
80 million
2.8 * 10^{-4}
10-20 u
15 u
42 million
3 * 10^{-4}
>20 u
25 u
20 million
2.5 * 10^{-4}
TOTAL
1536 million
2.23 * 10^{-3}
Adjusting for the density of quartz, the total volume of inclusions is
some 0.6% of the volume of the host quartz. So despite the enormous
number of inclusions per gram, they occupy only a tiny volume of the
quartz.
Using this inclusion volume we can estimate the effect on a whole rock
analysis if the fluids in the inclusions contained a trace element that
was not present in the quartz itself.
Assuming that all of the fluid inclusions contain 50 % water and 50 %
vapour, and that all inclusions contain liquid of the same composition
and that this liquid contains 0.1% (10,000 ppm) of trace element "X" then because the inclusion contents have a mass of about 0.2% of the quartz, the
contribution of the inclusion fluids to the whole rock analysis of
element X would be 0.6 * 0.2 = 0.12% , or about 12 ppm for the assumed concentration of 0.1%.
This is an upper estimate and is quite unrealistic as it ignores the fact that a
great many of the fluid inclusions are secondary in origin and would
have very different fluid compositions, lacking element X.
And a fluid composition of 10,000 ppm is also extremely high and most
unlikely. Realistically, the effects from the contribution of trace
elements within fluids in the fluid inclusions in a sample would be much
less than 1ppm. It is therefore quite improbable that fluid
inclusion fluids could significantly affect a whole rock analysis.
Three other samples from this location were also studied in this way, but the results are similar and not shown here.