Page 192 - Introduction to Mineral Exploration

8: EXPLORATION GEOCHEMISTRY 175

have been used of which mercury has been the
24° 30'S most successful. Mercury is the only metallic
WIND
DK element which forms a vapor at room tempera-
N 2 ture and it is widely present in sulfide deposits,
DK particularly volcanic-associated base metal
7 deposits. The gas radon is generated during the
decay of uranium and has been widely used
with some success. More recently the enrich-
DK DK ment of carbon dioxide and depletion of oxygen
6 DK caused by weathering of sulfide deposits has
DK 5 DK 9 been tested, particularly in the western USA
4 3 DK
1 (Lovell & Reid 1989).
Ilmenite counts In general results have been disappointing
> 5 because of the large variations in gas concen-
> 10 tration (partial pressure) caused by changes in
> 20 0 10 km
> 40 environmental conditions, particularly changes
DK2 Kimberlite in barometric pressure and rainfall. The more
24° 45'E
successful radon and mercury surveys have
sought to overcome these variations by inte-
FIG. 8.16 Dispersion of ilmenite around the Jwaneng grating measurements over weeks or months.
kimberlites, Botswana. Ilmenite is one of the heavy
minerals used to locate kimberlites. (From Lock Gaseous methods work best in arid areas as
1985.) diffusion in temperate climates tends to be
overshadowed by movement in groundwater.
The need to “see through” thick overburden
and easily contaminated by human activity. coupled with improvements in analytical and
In general where surface waters are present sampling methodologies has prompted a re-
it is far more reliable and easier to take a stream assessment of gas geochemistry and the tech-
sediment sample. The possible exception is nique is likely to be used more in the future.
when exploring for fluorite as fluoride is
easily measured in the field using a portable
single ion electrode. Where hydrogeochemistry 8.4.6 Vegetation
becomes useful is in exploration of arid areas Vegetation is used in two ways in exploration
with poor outcrop. In this terrain water wells geochemistry. Firstly the presence, absence or
are often drilled for irrigation and these wells condition of a particular plant or species can
tap deep aquifers that can be used to explore in indicate the presence of mineralisation or a par-
the subsurface. This approach has been used in ticular rock type, and is known as geobotany.
uranium exploration although it has not been Secondly the elemental content of a particular
very successful as the uranium concentration plant has been measured, this is known as
is dependent on the age of groundwater, the biogeochemistry. Biogeochemistry has been
amount of evaporation, and its source, which used more widely than geobotany and has
are extremely difficult to determine. For good found particular application in the forest re-
discussions with case studies see Taufen (1997) gions of northern Canada and Siberia where
and Cameron et al. (2004). surface sampling is difficult, but it should only
be used with caution. A comprehensive treat-
ment of the subject can be found in Brooks
8.4.5 Gases
(1983) and the biogeochemical part has been
Gases are potentially an attractive medium to revised in Dunn (2001).
sample as gases can diffuse through thick over-
burden; in practice most surveys have met with
discouraging results, although there has been Geobotany
a recent resurgence in interest due to more One of the pleasurable, although all too
robust sampling methods. A number of gases infrequent, parts of exploration is to identify