Page 176 - Introduction to Mineral Exploration

8: EXPLORATION GEOCHEMISTRY 159

After the orientation survey has been con-
ducted the logistics of the major survey need
to be planned. The reader should devise a
checklist similar to that of Thomson (1987).
1 Hire field crew with appropriate experience n n
and training;
2 Obtain base maps and devise simple sample Concn
numbering scheme; Precise, accurate Imprecise, accurate
3 Designate personnel for communication
with the laboratory;
4 Arrange quality control of laboratory;
5 Arrange for data handling and interpretation; n n
6 Organize archive of samples and data;
7 Liaise with other project staff (e.g. geophysi-
cists) and arrange reporting to management.
Reporting of geochemical surveys is import- Precise, inaccurate Imprecise, inaccurate
ant and readers should consult the short book- FIG. 8.2 Schematic representation of precision and
let by Bloom (2001). This provides a wealth of accuracy assuming normal distribution of analytical
information as well as the basis for fulfilling error.
legal requirements in Canada.
A number of schemes have been devised but
the most comprehensive is that of Thompson
8.2 ANALYSIS (1982). Precision is measured by analyzing
samples in duplicate whereas accuracy requires
As the geologist generally sees little of the the analysis of a sample of known composi-
process of analysis, which is usually done at tion, a reference material. The use of duplicate
some distance from the exploration project, samples means that precision is monitored
analytical data tend to be used uncritically. across the whole range of sample composi-
While most laboratories provide good quality tions. Reference materials can be acquired
data they are usually in business to make a commercially but they are exceedingly expen-
profit and it is up to the geologist to monitor sive (>US$100 per 250 g) and the usual practice
the quality of data produced and investigate is to develop in-house materials which are
the appropriateness of the analytical methods then calibrated against international reference
used. materials. The in-house reference materials
can be made by thoroughly mixing and grind-
ing weakly anomalous soils from a variety of
8.2.1 Accuracy and precision
sites. The contents should be high enough to
The critical question for the geologist is how give some indication of accuracy in the anti-
reproducible the analysis is and how repres- cipated range but not so high as to require
entative of the “correct” concentration the special treatment (for copper, materials in the
concentration is, as shown in Fig. 8.2. The range 30–100 ppm are recommended). If com-
reproducibility of an analysis is termed the mercial laboratories are used then the reference
precision and its relation to the expected or materials and duplicates should be included
consensus value the accuracy. For most pur- at random; suggested frequencies are 10% for
poses in exploration geochemistry it is vitally duplicates and 4% for reference materials. If
important that an analysis is precise but the the analyses are in-house then checks can be
accuracy is generally not so crucial, although made on the purity of reagents by running
some indication of the accuracy is needed. At blank samples, i.e. chemicals with no sample.
the evaluation stage the analyses must be pre- Samples should if possible be run in a different
cise and accurate. The measurement of accur- and random order to that in which they are
acy and precision requires careful planning and collected. This enables the monitoring of sys-
an understanding of the theory involved. tematic drift. In practice this is often not easy