Page 108 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 85

zones of fine-grained sediments. As such, gases below the water table are likely to be
dispersed by groundwater and are not expected to form tight, well-centred anomalies
immediately over mineralisation nor slightly offset rabbit-ear anomalies, which often
show a depletion over mineralisation. Carbon dioxide and elevated carbonate content in
soils have been used to account for rabbit-ear anomalies in arid environments but have
been attributed to direct oxidation of sulphide (Smee, 1998) in what is presumably an
unsaturated environment. These anomalies are discussed further below.

Electrochemical transport

Characteristic element dispersion patterns have long been recognised over
mineralisation at certain sulphide deposits and other electronic conductors in bedrock.
These patterns include apical anomalies overlying mineralisation or rabbit-ear anomalies
which flank mineralisation. Anomalous parameters reported include H § organic carbon,
electrical conductivity and metals. Many authors (e.g., Govett, 1973, 1976; Govett and
Chork, 1977; Bolviken and Logn, 1975; Smee, 1983; Govett et al., 1984; Clark, 1996;
Jackson, 1995; Bajc, 1998; Hamilton, 1998; Hamilton and McClenaghan, 1998) have
attributed these patterns to electrochemical processes.
Although the electrochemical transport models presented in the literature are
sometimes conflicting, most are based on the same underlying idea. The conduction of
electrons upward, along mineralisation, from deep, more chemically-reducing areas
results in anomalous electrochemical gradients in the surrounding country rock and
overlying overburden. The movement of mass and charge in the form of ions results in
the development of geochemical anomalies in the overlying overburden. Different names
have been attached to this process, including geobattery, oxygen concentration cell,
natural galvanic corrosion, oxidation cell and natural voltaic cell.
The primary appeal of using electrochemical transport to explain selective leach
anomalies is that it can account for their development in almost any environment,
including thick, young, exotic drift. The rate of movement of charged species in an
electrochemical field can far exceed that of chemical diffusion, provided the gradient is
high enough. Hamilton (1998) determined theoretical transport times through 30 m of
clay along a realistic electrochemical gradient to be less than 2000 years for most
species. The presence of clay confining units, groundwater flow or saturated/unsaturated
conditions should not impede the movement of charge provided an electrochemical
gradient exists through overburden as a whole.
All four of the mechanisms described above probably operate to some degree at
every site where mineralisation is buried and it is possible that any one of them could
dominate in selected environments. However, the first three are precluded as major
contributors to transport in thick, water-saturated Quaternary glacial environments. Since
selective leach anomalies are now commonly reported in glacial terrain, electrochemical
processes are likely to dominate in at least this environment.