Geoscience Reference
In-Depth Information
If the standards are done using a commercial laboratory,
it should be involved in the assaying of the project samples.
The most probable value for the standard should be reported
with the ± 2
σ
(2 standard deviations of the distribution of all
assayed values). These or alternative upper and lower lim-
its should be used as acceptance criteria for the re-assayed
standard.
samples. A batch should contain sufficient samples to allow
the insertion of control samples. At the same time it cannot
be too large to become too difficult to manage, evaluate, or
re-assay. For drill hole samples, it is generally recommend-
ed that a batch be no less than 20 samples, and preferably
40 samples. For blast holes and crusher samples, the batch
is generally larger and composed of samples from a fixed
timeframe (work shift or day).
For each 40-sample batch, and assuming that the mining
company has full control of the sample preparation stages,
the following are the suggested control samples to be sent to
the primary laboratory:
1. For drill hole samples:
a. Two blanks (5 % of total number of samples). Of these,
insert one coarse blank for every 4th blank inserted
(25 % of the total number of blanks inserted).
b. Two pulp duplicates (5 % of total number of samples).
c. Two coarse duplicates (5 % of total number of sam-
ples).
d. Two standards appropriate to the expected grade of the
batch samples (5 % of total).
2. For blast hole and crusher samples:
a. One blank (2.5 % of total number of samples).
b. One pulp duplicate (2.5 % of total number of samples).
c. One coarse duplicate (2.5 % of total number of sam-
ples).
d. One standard appropriate to the expected grade of the
batch samples (2.5 % of total).
This implies that there will be 20 % check samples for ex-
ploration data, and 10 % additional control samples for pro-
duction data. A second check laboratory should be used for
the drill hole samples, but it is not necessary for production
samples. Since production samples are normally processed
in-house, an additional 2.5 % of pulp control samples should
be sent out for re-assaying at a different laboratory as routine
check.
For drill hole samples, the control samples sent to the sec-
ond (check) laboratory should be from pulp duplicates in all
cases and should include one blank, two sample pulps, and
one standard for every 40-sample batch. This implies an ad-
ditional 10 % sent to a second laboratory.
There are cases where the mining company or project de-
velopment team does not have a sample preparation facility,
and is not able to control the sample preparation process. For
these cases when the sample preparation is done by the labo-
ratory itself, coarse duplicates should be sent for preparation
and assaying by the second laboratory.
All control samples should have a pre-defined logical se-
quence of numbers, such that the flow of samples is easy to
control, and the control samples are inserted into the stream
in a disguised fashion.
5.4.2.3 Coarse and Field Duplicates
The purpose of the coarse duplicates is to quantify the vari-
ances introduced into the assayed grade by errors at different
sample preparation stages. They provide a measure of the
sample precision. There will commonly be more than one
size reduction and splitting steps in the preparation stage.
These coarse duplicates should be inserted into the primary
laboratory stream, providing an estimate of the sum of the
assay variance plus the sample preparation variance, up to
the first crushing stage.
An alternative is to obtain a field duplicate. In the case of
diamond drilling, a duplicate from the core box (i.e., a quarter
core or the other half core) is sent to the laboratory, most com-
monly with the intention of replacing the coarse duplicate.
The advantage is that the variance observed in field duplicates
includes the actual sampling and the first size reduction step.
The price of leaving the interval without core may be too high.
Also, a quarter core may be too small a volume for the du-
plicate to be representative. In the case of reverse circulation
drilling, it is more likely that field duplicates do not exhaust
the sample as generally there are abundant chips available.
In the case of blast hole sampling, it is also possible to
take a duplicate sample in the field from the cuttings pile or
the reject from the hydrocyclone if an automatic sampler is
used. These field duplicates can be used to check the first
stage crushing and sampling process.
5.4.2.4 Pulp Duplicates
Pulp duplicates provide a measure of precision of the an-
alytical procedures used. They are taken at the final stage
of sample preparation, and generally are a second enve-
lope with the 100 or 200 g final sample sent for assaying,
inserted blindly into the sample batch. Pulp duplicates sent
to the same primary laboratory provide an estimate of the
analytical variance of that laboratory. When sent to the
second, check laboratory, the pulp duplicates quantify the
precision (analytical variance) between the two laboratories.
5.4.3
Insertion Procedures and Handling
of Check Material
The basic unit is a batch. This can be defined for drill hole
samples, blast hole samples, or any other type of production
