Comparing Gold Values - Acme Labs, Bureau Veritas

Bureau Veritas Minerals
Comparing Gold Values
Determined by Fire Asssay and Aqua Regia
MINERALS
Comparing
Gold Values
Determined by Fire Assay and Aqua Regia
The complexity of gold bearing systems means that geochemistry must often do more than determine how
much gold is in a rock. Fire assay remains the benchmark technique for quantifying gold, but minor and trace
element data are often used to vector within a mineralised system with Aqua Regia as the leach of choice.
Aqua Regia is very good at dissolving gold, and as a consequence geochemists and geologist are often faced
with reconciling two gold numbers for each sample, and sometimes they do not match. This article examines
how the distribution of gold in geological samples can affect the representivity of a sub sample and the
challenges this creates for data integration.
1 n It can sometimes be difficult to compare data from different analytical techniques, especially when they utilize very
different sample weights. Fire assay is always done on a large subsample to ensure it is representative and to mitigate
against the common situation where gold occurs as relatively large particles. Comparatively, aqua regia digestion for
trace and pathfinder metals (including gold) is usually determined on much smaller subsamples to reduce costs, and
because most elements have a relatively homogeneous distribution within a sample.
Au ppm: Aqua Regia Digestion
Figure u
2 n Figure 1 shows data for a set of samples that
were analyzed for Au using both a 50g fire assay
2.5
charge, and a 0.5g subsample leached with Aqua
2
Regia. The results show relatively good agreement
1.5
between the two methods. It suggests that a
1
substantial amount of the Au is present as fine
0.5
particles, and that a sufficient number of particles
0
are captured by the small test portion used for the
00.5 1 1.5 22.5
Aqua Regia leach to produce a representative result
Au ppm: 50g Fire Assay
(compared to the 50 g Fire assay). There is a small
*Au_PPM
Linear (Au_PPM)
positive bias towards the 50 g fire assay. This could
be attributed to the fire assay method being a total digestion, whereas, Aqua Regia is a partial digest. There may be
a few very fine particles of Au being entirely encapsulated by refractory minerals (i.e. quartz) that are not available to
leaching.
3 n Agreement between small sample weight aqua regia and large sample weight fire assay is not always the case.
The data in figure 2 is from a suite of samples that was subsequently determined to contain coarse gold by follow up
metallics fire assay. It shows a bias in Aqua Regia results that becomes increasingly pronounced at higher grades.
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Compared to figure 1, however, figure 2 demonstrates that for some samples, the Aqua Regia results are inconsistently
higher than the fire assay. The difference is not systematic.
Au Assay Comparison -­‐ All v
Assays greater than Figure
0.2ppm
2.5
2.5
4 n Figure 2 illustrates that at low Au grade, there
is good agreement between FA and Aqua Regia
suggesting that there is a relatively consistent
y = 1.2053x
R² = 0.8737
component of fine gold particles distributed. As the
Au ppm: Aqua regia
Au ppm: Aqua Regia
22
grade increases, there is an ever higher probability
1.5
1.5
that larger particles of Au are sampled, and is the
underlying cause for the observed bias. The coarse
11
grains appear to be less abundant (suggested as
most of the data agree well). However, when one (or
0.5
0.5
a few) of the grains are captured in a test portion they
effect the results of the two tests in very different ways.
00
0
0.5
1
1.5
2
2.5
00.5
1
1.5
22.5
Take, for example, a single grain of native Au that is
Au ppm: 50g Fire Assay
Au ppm: 50g
Fire(Au_PPM)
Assay
Au_PPM
Linear *Au_PPM
Linear (Au_PPM)
captured in each of the analytical methods (aqua
regia at 0.5 g and fire assay at 50 g). One grain in a
50g sample will have much less of an effect on the reported concentration than would one in a 0.5g sample. In fact, that
nugget would have an effect 100 times larger on the smaller sample. Aqua regia is very effective at dissolving native gold.
5 n To further illustrate, error envelopes have been added to Figure 2 showing the expected error for a 0.5 g analysis
versus a 50 g analysis for Au. The dashed blue line marks the bottom limit on the negative error and is to be expected
due to fine gold present, which is consistently sampled in both test portion weights. The black line is the positive
corollary of the blue line and is the expected positive error (if error is random and has an even distribution about the
mean). From the figure, we observe that there are many more points above the positive error envelope than below the
negative error envelope. The positive bias is much larger due to the impact that a single large nugget of Au has on a
0.5 g analysis (there is really no upper limit or “ceiling” on this). The concept has been drawn stylistically (not to scale)
using the black error bars. They show not only that the expected error is larger for the low sample weight test, but that
the expected positive error is larger than the expected negative error.
6 n In conclusion, the comparison of data from different analytical techniques needs to be performed carefully,
particularly for gold. Depending on the mineral species in which an element resides, each test will be variably effective
at extraction - in many cases the results may not be exactly the same. We can use the differences, along with our
understanding of the sample mineralogy, to gain additional insight into the geological systems.
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