Geoscience Reference
In-Depth Information
Fig. 5.7
Accuracy and precision.
Left
is precise but inaccurate;
center
is accurate but imprecise; and
right
is both precise and accurate
re-assay them. In this case, failed check samples trigger cor-
rective measures for future assaying.
The sampling QA/QC program should cover (a) sampling
conditions in the field; (b) sample preparation; (c) analytical
accuracy and precision; and (d) correctness of the laboratory
reports and transfer of the information to the database(s).
The materials to be used in the QA/QC program include (a)
standards, or reference material; (b) blanks, which are samples
with no grade; (c) field duplicates, taken at the drill hole site or
core box; (d) coarse duplicates, taken as the first reject at the
sample preparation stage, typically −10 mesh in size; and (e)
pulp duplicates, which are taken from the last size-reduction
and splitting at the end of the sample preparation process.
There are generally two or more laboratories involved that
would include a primary or principal laboratory for routine
work, and a secondary or check laboratory. Occasionally, a
referee laboratory is needed when discrepancies between the
primary and secondary laboratories cannot to be resolved.
Sampling and assaying protocols are established prior to
processing the samples from the field. These protocols should
cover all aspects of sample processing and handling including
chain of custody. The sampling theory originally developed
by P. Gy (
1982
) can be used to determine optimum sample
preparation protocol, such that errors introduced in the prepa-
ration and assaying procedures are minimized (see above).
The sample preparation and assaying protocols should
be identical for the primary and secondary laboratory. The
mining company should have a staff person in charge of the
overall QA/QC program whose duties include ensuring that
the protocols used at the different labs are consistent. That
person should inspect the facilities on a regular basis.
handling of the samples. There should be both pulp and
coarse blanks prepared and inserted into the sample prepara-
tion stream. In the case of core samples, the coarse blank
is introduced after the first crushing stage, while the pulp
blank should be inserted as a separate envelope in the sample
batch.
It is advisable that the blank have the same matrix (min-
eralogy) and result in a pulp with the same characteristics
as the main samples so it is not obvious to the laboratory
that the sample is different in any way. This is sometimes
difficult to achieve, although at least the main characteris-
tics such as color should be as similar as possible. Very low
grade samples should not be used as proxies for blanks.
5.4.2.2 Standards
Standards are samples for which its grade is known within a
certain precision. They are used to check the accuracy of the
analytical laboratories, by comparison of the re-assays to the
reference value.
There are commercial standards that can be purchased.
They provide samples with known grades for some types of
ore deposits. This material can be purchased from labora-
tories and institutions around the world. The standards are
delivered with certificates stating the accepted value and its
precision, in addition to a full description of the procedures
used to analyze them.
Alternatively, the mining company has the option of
developing its own standards. The material used to obtain
the standard is typically from the same deposit as the main
sample stream, which ensures that differences in the sample
matrices will be minimal. The certification of standards re-
quires major analytical work, which can be done through a
round robin analysis using no less than 6 laboratories, and
more commonly 8. This work can be managed by the mining
company or outsourced to an external analytical laboratory,
which would also be part of the round robin laboratories,
and would provide the final certification for the samples and
their corresponding tolerance limits.
5.4.2
Elements of a QA/QC Program
5.4.2.1 Blanks
Blanks are samples with no grade of interest whose purpose
is to check laboratory contamination and to verify correct
