Page 189 - Introduction to Mineral Exploration

172 C.J. MOON

(b)
0
60–70
50–60 1 Copper ppm
40–50 Depth (m) 2
30–40
20–30 3
10–20 4
0–10

0
300–350
250–300 1 Arsenic ppm
200–250 2
150–200 Depth (m)
100–150 3
50–100 4
0–50

0
45–55
35–45 1 Tungsten ppm
25–35 Depth (m) 2
15–25
10–15 3
5–10 4
0–5
300 400 500 600 700 800
FIG. 8.14 (b) Dispersion through overburden along the soil traverse shown in Fig. 8.14(a). Note the different
mobilities of tungsten, copper, and arsenic.

density of soil samples on the surface traverse of their nickel contents, as nickel is mobile,
is clearly greater than needed. The reader may but possible if their multi-element signatures
care to consider the maximum sample spacing are used (e.g. Ni, Cr, Cu, Zn, Mo, Mn) or the Pt,
necessary to detect two anomalous samples in Pd, Ir content (Travis et al. 1976, Moeskops
the traverse. 1977, Smith 1977).
A special form of residual overburden is Drilling to fresh rock, often 50–60 m, has
found in lateritic areas such as in Amazonian been widely used to obtain a reasonable signa-
Brazil or Western Australia where deep weath- ture despite the costs involved. Mazzucchelli
ering has removed trace elements from the (1989) suggests that costs are of the order of
−1
near-surface environment and geochemical $A10 m and could easily reach $A1M km −2
signatures are often very weak. One technique for detailed grids. He recommends surface
that has been widely used is the examination of sampling to outline areas for follow-up work.
gossans. As discussed in section 5.1.6, gossans
have relic textures which allow the geologist
to predict the primary sulfide textures present Transported overburden
at depth. In addition the trace element signa- In areas of transported overburden, such as
ture of the primary mineralisation is preserved glaciated terrains of the Canadian Shield, sandy
by immobile elements. For example, it is diffi- deserts, or gravel-covered areas in the Andes,
cult to discriminate gossans overlying nickel sampling problems are severe and solutions to
deposits from other iron-rich rocks on the basis them expensive.