Page 190 - Introduction to Mineral Exploration

8: EXPLORATION GEOCHEMISTRY 173

(c) Lightweight drills are cheaper and easier to
400 operate but results are often ambiguous, as it is
not easy to differentiate the base of overburden
As ppm 300 from striking a boulder. The use of heavy
equipment in most glaciated areas is restricted
200
100 to the winter when the ground is frozen. The
difficulties of finding the source of glacial dis-
persion trains for small targets at Lac de Gras,
0
Canada, are discussed in section 17.2.
1.25
In sandy deserts water is scarce, most move-
1.00 ment is mechanical, and most fine material
Bi ppm 0.75 is windblown. Thus the −80 mesh fraction of
overburden is enriched in windblown material
0.50
and is of little use. In such areas either a coarse
0.25
fraction (e.g. 2–6 mm), reflecting locally derived
0.00 material, or the clay fraction reflecting ele-
ments moved in solution is used (Carver et al.
50 1987). One of the most successful uses of
Cu ppm 25 this approach has been in the exploration for
kimberlites in central Botswana. Figure 8.16
shows the result of a regional sampling pro-
0 gram which discovered kimberlite pipes in
0 100 200 300 400 500 600 700 800 900 1000 1100 the Jwaneng area. Samples were taken on a
Distance (m)
0.5 km grid, heavy minerals separated from
the +0.42 mm fraction and the number of
kimberlite indicator minerals, such as picro-
ilmenite, counted. The anomalies shown are
displaced from the suboutcrop probably due
FIG. 8.14 (c) Soil traverse across the mineralized to transport by the prevailing northeasterly
dyke. Note the sharp cut-off upslope and dispersion winds.
downslope. Recent studies in Chile and Canada have
examined the use of various weak extractions
to maximize the signature of deposits covered
In glaciated terrains overburden rarely by gravel and till. Figure 8.17 shows the result
reflects the underlying bedrock and seepages of of a survey over the Gaby Sur porphyry cop-
elements are only present where the over- per deposit. The weak extractions, including
burden is less than about 5 m thick. In addi- deionized water, show high contrast anomalies
tion the overburden can be stratified with at the edge of the deposit, probably generated
material of differing origins at different depths. by the pumping of groundwater leaching the
If the mineralisation is distinctive, it is often deposit (Cameron et al. 2004).
possible to use boulder tracing to follow the
boulders back to the apex of the boulder fan,
as in the case of boulders with visible gold, 8.4.4 Hydrogeochemistry
sulfides, or radioactive material. Generally Hydrogeochemistry uses water as a sampling
however the chemistry of the tills must be medium. Although water is the most widely
examined and basal tills which are usually of available material for geochemistry, its use
local origin sampled. Figure 8.15 shows a typ- is restricted to very specific circumstances.
ical glacial fan in Nova Scotia. Usually basal The reasons for this are that not all elements
till sampling can only be accomplished by show equal dissolution rates, indeed many are
drilling; the most common methods are light insoluble, contents of trace elements are very
percussion drills with flow-through samplers low and have been difficult to measure until re-
or heavier reverse circulation or sonic drills. cently, being highly dependent on the weather