Page 184 - Introduction to Mineral Exploration

8: EXPLORATION GEOCHEMISTRY 167

Heavy mineral concentrates have been similar way to that from stream sediments and
widely used as a means of combating excessive corrections should be made for organic matter.
dilution and of enhancing weak signals. The Lake sediment is only useful if glacial material
method is essentially a quantification of the is locally derived and is ineffective in areas
gold panning method, which separates grains of glaciolacustrine material such as parts of
on the basis of density differences. Panning Manitoba. Further details are contained in the
in water usually separates discrete minerals reviews by Coker et al. (1979) and Davenport
with a density of greater than 3. Besides pre- et al. (1997).
cious metals, panning will detect gossanous
fragments enriched in metals, secondary ore
minerals such as anglesite, and insoluble 8.4.3 Overburden geochemistry
minerals such as cassiterite, zircon, cinnabar, In areas of residual overburden, overburden
baryte, and most gemstones, including dia- sampling is generally employed as a follow-up
mond. The mobility of each heavy mineral will to stream sediment surveys but may be used
depend on its stability in water, for example as a primary survey for small licence blocks
sulfides can only be panned close to their or more particularly in areas of exotic over-
source in temperate environments whereas burden. Perhaps the key feature is that it is
diamonds will survive transport for thousands only employed when land has been acquired.
of kilometers. The results are generally plotted at scales from
The samples collected are usually analyzed 1:10,000 to 1:1000.
or the number of heavy mineral grains are The method of sampling depends on the
counted. The examination of the concentrate nature of the overburden; if the chemistry of
can prove very useful in remote areas where the near-surface soils reflects that at depth then
laboratory turnaround is slow and the cost of it is safe to use the cheap option of sampling
revisiting the area high, as it will be possible soil. If not, then samples of deep overburden
to locate areas for immediate follow-up. The must be taken. The main areas where surface
major problem is that panning is still some- soils do not reflect the chemistry at depth are
thing of an art and must be practised for a few glaciated areas, where the overburden has been
days before the sampler is proficient. The dia- transported from another area, in areas of wind-
gram and photograph in Figs 8.9 and 8.10 show blown sand, and in areas of lateritic weathering
panning procedures. Useful checks on panning where most trace metals have been removed
technique can be made by trial runs in areas from the near-surface layers.
of known gold or by adding a known number
of lead shot to the pan and checking their re-
covery. It is usual to start from a known sample Surface soil sampling
size and finish with a concentrate of known The simplest sampling scheme is to take near-
mass. However, the differences in panning surface soil samples. The major problem is to
technique mean that comparisons between decide which layer of the soil to sample, as
different surveys are usually impossible. the differences between the layers are often
greater than that between sites. The type of
soil reflects the surface processes but in general
8.4.2 Lake sediments
the most effective samples are from a zone at
In the glaciated areas of northern Canada and around 30 cm depth formed by the downward
Scandinavia access to rivers is difficult on foot movement of clays, organic material, and iron
but the numerous small lakes provide an ideal oxides, the B horizon indicated in Fig. 8.11, or
reconnaissance sampling medium as they are the near-surface organic material (A horizon).
accessible from the air. A sample is taken by This downward movement of material is re-
dropping a heavy sampler into the lake sedi- sponsible for the depletion and concentration
ment and retrieving it. The sampling density is of trace elements causing variations that may
highly varied and similar to stream sediments. be greater than that over mineralisation. It is
Productivity is of the order of ten samples per essential that the characteristics of the soil
flying hour. Essentially data are interpreted in a sampled are recorded and that an attempt is