Page 186 - Introduction to Mineral Exploration

8: EXPLORATION GEOCHEMISTRY 169

FIG. 8.10 Panned concentrate
sampling in northern Pakistan. The
water is glacial so edge of river
sampling is essential.

made to sample a consistent horizon. If differ- occur across the target and the type of over-
ent horizons are sampled anomalies will reflect burden, but a rule of thumb is to have at least
this as shown in Fig. 8.12. two anomalous samples per line if a target
The usual method of soil collection in tem- is cut. Spacing in the search for veins may be
perate terrains is to use a soil hand auger, as as little as 5 m between samples but 300 m
shown in Fig. 8.13. This allows sampling to a between lines, but for more regularly dis-
depth of the order of 1 m, although normally tributed disseminated deposits may be as much
samples are taken from around 30 cm and as 100 m by 100 m. Sample spacing may also be
masses of around 100–200 g collected for base dictated by topography: in flat areas or where
metal exploration. In other climatic terrains, the topography is subdued then rectilinear
particularly where the surface is hard or where grids are the ideal choice, but in mountainous
large samples (500 g to 2 kg) are required for areas ridge and spur sampling may be the only
gold analysis, then small pits can be dug. The reasonable choice.
area of influence of a soil sample is relatively A typical example of overburden sampling
small and should be determined during an is shown in Figs 8.14a–c. The area is on the
orientation survey. The spacing is dependent eastern side of the Leinster Granite in Ireland
on the size of the primary halo expected to and the target was tin–tungsten mineralisation