Page 174 - Introduction to Mineral Exploration

8: EXPLORATION GEOCHEMISTRY 157

Residual largely be determined by the overburden con-
ditions but the density and exact nature of
the sample will be based either on previous
Overburden 2–5 m experience in the area or, if possible, on an
orientation survey.
Rock
8.1.1 Orientation surveys
Glacial One of the key aspects of planning is to evalu-
ate which techniques are effective for the com-
modity sought and in the area of search. This
Till 2 is known as an orientation survey. The best
Till 1 orientation survey is that in which a variety of
Ice sampling methods is tested over a prospect or
Rock
deposit of similar geology to the target and in
similar topographical conditions to determine
Deep Tropical the method which yields the best results. A
checklist for an orientation study is given
below (Closs & Nichol 1989):
1 Clear understanding of target deposit type;
Overburden >50 m 2 Understanding of surficial environment of
the search area;
Rock 3 Nature of primary and secondary dispersion
from the mineralisation;
FIG. 8.1 Sketch showing dispersion through major 4 Sample types available;
types of overburden. 5 Sample collection procedures;
6 Sample size requirements;
7 Sample interval, orientation, and areal
process forming the soil. Indications of back- density;
ground levels of elements in soils are given 8 Field observations required;
in Table 8.2, as are known lithologies with 9 Sample preparation procedures;
elevated concentrations which might provide 10 Sample fraction for analysis;
spurious or non-significant anomalies during 11 Analytical method required;
a survey. These background levels can be of 12 Elemental suite to be analyzed;
use in preparing geochemical maps which 13 Data format for interpretation.
can be used to infer lithology in areas of poor The relatively small cost involved in under-
outcrop. The reader is advised to get some idea taking an orientation survey compared with
of the background variation over ordinary rock that of a major geochemical survey is always
formations from a geochemical atlas, such as justified and whenever possible this approach
those for England and Wales (Webb et al. 1978), should be used. However, if a physical orienta-
Alaska (Weaver et al. 1983), the former West tion survey is not possible, then a thorough
Germany (Fauth et al. 1985), and Europe review of the available literature and discus-
(Salmimen et al. 2004). sion with geochemical experts is a reasonable
The basis of a geochemical program is a sys- alternative option. Of particular use in orien-
tematic sampling program (Thomson 1987) and tation studies are the models of dispersion pro-
thus decisions must be made in a cost-effective duced for various parts of the world; Canadian
manner as to the material to be sampled, the Shield and Canadian Cordillera (Bradshaw
density of sampling, and the analytical method 1975), Western USA (Lovering & McCarthy
to be employed. Cost/benefit ratios should be 1977), and Australia (Butt & Smith, 1980).
considered carefully as it may be that a slightly Particularly useful discussion on orientation
more expensive method will be the only effect- procedures is provided by Thomson (1987) and
ive technique. The material to be sampled will Closs and Nichol (1989).