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178 C.J. MOON

electromagnetic methods (see section 15.3.8). in residual areas suffering active weathering is
In particular rock geochemistry has detected stream sediment sampling which is a relatively
the cone-shaped alteration zones beneath simple method and allows the rapid evaluation
them, which are enriched in Mg and depleted of large areas at low cost. Panning to collect
in Na, Ca and sometimes in K, and recognized heavy mineral concentrates can be carried out
distal exhalite horizons which are enriched in at the same time, thus enhancing the value
more mobile elements such as arsenic, anti- of this method. In glaciated areas of difficult
mony, and barium (for a good case history see access lake sediment rather than stream sedi-
Severin et al. 1989). Another major application ment surveys may be used.
is in the exploration of sulfide-rich epithermal Overburden geochemistry is often a follow-
gold deposits, which often show element up to stream sediment surveys in areas of re-
zonation (Heitt et al. 2003). The more volatile sidual and transported overburden. The method
elements such as As, Sb and Hg are concen- of sampling is very varied and dictated by the
trated above the major gold zones whereas any nature of the overburden. When the overbur-
copper is at depth. The successful application den is transported (glaciated terrains or sandy
of these methods on a prospect demands a deserts) sampling problems are severe and solu-
detailed knowledge of the subsurface geology. tions to them expensive.
The use of hydrogeochemistry, gases, and
vegetation is restricted but rock geochemistry
8.6 SUMMARY is increasing in importance both in surface and
underground work as well as by utilizing drill
Exploration geologists are more likely to be core samples.
directly involved in geochemical surveys than
in geophysical ones and they should therefore
have considerable knowledge of this technique 8.7 FURTHER READING
and its application in mineral exploration. A
geochemical program naturally commences The literature on exploration geochemistry
with a planning stage which involves choice is large and reasonably accessible. Geoche-
of the appropriate field survey and analytical mistry in Mineral Exploration by Rose et al.
methods suitable for use for a particular area (1979) remains the best starting point. A good
and the commodity sought. Individual areas general update on soil sampling is provided
will present very different problems, e.g. areas by Fletcher et al. (1987) with reviews of
with deep or shallow overburdens, and there- most other geochemical techniques in Gubins
fore orientation surveys (section 8.1.1) are very (1997). The Handbook of Exploration Geo-
desirable. If possible such a survey should be chemistry series, although expensive, provides
one in which a variety of sampling methods are comprehensive coverage in the volumes so far
tested over a deposit of similar geology to that published: chemical analysis (Fletcher 1981),
of the target and in similar topographical con- statistics and data analysis (Howarth 1982),
ditions to determine which produces the best rock geochemistry (Govett 1983), lateritic areas
results. This done, the logistics of the major (Butt & Zeegers 1992), arctic areas (Kauranne
survey can be planned. et al. 1992), and stream sediments (Hale &
The analysis of samples collected during Plant 1994). Readers are strongly advised to try
geochemical surveys is generally carried out by the practical problems in Levinson et al. (1987).
contract companies and the quality of the data Discussion of techniques above can be found
they produce must be monitored, and for this in Explore, the newsletter of the Association
reason the geologist in charge must be familiar of Exploration (now Applied) Geochemists
with sample collection and preparation, ana- which also publishes the more formal Geo-
lytical methods, and the statistical interpreta- chemistry: Exploration, Environment, Analy-
tion of the data obtained. sis. The reader will also find articles of interest
The material which will be sampled depends in Journal of Geochemical Exploration,
upon whether the overburden is residual or Applied Geochemistry, and Exploration and
transported. The most widely used technique Mining Geology.