Page 194 - Introduction to Mineral Exploration

8: EXPLORATION GEOCHEMISTRY 177

recent growth. Unfortunately for the geochem- connaissance, it is really in detailed work,
ist the rate of uptake and concentration of ele- where there is good outcrop or where there is
ments is highly dependent on the species and drill core, that this technique becomes most
the season. In general sampling is conducted effective.
on one plant species and one part of the plant, On a regional basis the most successful
usually first and second year leaves or twigs. applications have been in the delineation of
The variation in concentration with season mineralized felsic plutons and of exhalative
is so severe that sampling must be restricted horizons. Full details are provided in reviews
in time. In general around 500 g of sample are by Govett (1983, 1989) and Franklin (1997).
collected and ashed prior to analysis. Plutons mineralized in copper and tungsten
The main advantage of biogeochemistry is are usually enriched in these elements but in-
that tree roots often tap relatively deep water variably show high variability within a pluton.
tables which in glacial areas can be below Tin mineralisation is associated with highly
the transported material and representative evolved and altered intrusives and these are
of bedrock. A similar effect can be obtained easily delineated by plotting on a K–Rb versus
by sampling the near-surface humic horizons Rb–Sr and Mg–Li diagram or examing the
which largely reflect decayed leaves and twigs geochemistry of minerals, such as micas. It is
from the surrounding trees (Curtin et al. 1974). recommended that 30–40 samples are taken
from each pluton to eliminate local effects.
Studies in the modern oceans indicate haloes of
8.5 FOLLOW-UP SAMPLING the order of kilometers around black smoker
fields and these have been detected around a
Once an anomaly has been found during re- number of terrestrial massive sulfide deposits,
connaissance sampling and a possible source notably by the manganese content in the
identified, it is necessary to define that source exhalative horizon.
by more detailed sampling, by highlighting Some care needs to be taken in the collec-
areas of elemental enrichment, and eliminat- tion of rock samples. In general 1 kg samples
ing background areas until the anomaly is are sufficient for base metal exploration but
explained and a bedrock source, hopefully a precise precious metal determination requires
drilling target, proven. If the reconnaissance larger samples, perhaps as large as 5 kg, if
phase was stream or lake sampling, this will the gold is present as discrete grains. Surface
probably involve overburden sampling of the weathering products should be removed with
catchments. In the case of soil or overburden a steel brush. Rock geochemistry depends on
sampling it will mean increasing the density of multi-element interpretation and computer-
sampling until the source of an anomaly is based interpretation with careful subdivision
found, proving the source by deep overburden of samples on the basis of lithology.
sampling, and sampling rocks at depth or at
surface when there is outcrop. A typical source
can be seen in Fig. 8.14. The regional tungsten Mine scale uses
anomaly was followed up by deep overburden One of the major applications of rock geo-
sampling such as that shown in Fig. 8.14b and chemistry is in determining the sense of top
the suboutcrop of the dyke defined. The dip of and bottom of prospects and detecting altera-
the dyke could be estimated from the occur- tion that is not obvious from visual exam-
rence of disseminated sulfides, which would ination. Geochemists from the former Soviet
also respond to induced polarization, and a Union have been particularly active in develop-
drilling target outlined. ing an overall zonation sequence which they
have used to construct multiplicative indica-
tions of younging for a variety of deposit types
8.5.1 Rock geochemistry
(Govett 1983, 1989). Rock geochemistry has
Rock sampling is included in the techniques been widely applied in the search for volcanic-
for follow-up because although it has been associated massive sulfide deposits in Canada,
applied with some success in regional re- especially in combination with downhole