Page 128 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 105

gradient that exists in the Earth's crust. The conductor provides a less resistive route to
upward flow of negative current from depth and concentrates the background current,
thereby short-circuiting the redox field. The conductor and redox field combined
represent the voltaic cell: without the redox field, there would be no reactants; and
without the conductor there would be no focusing of current and hence no cell.
The consumption of oxidising agents around the upper part of the conductor results
in more reducing conditions immediately around the top of the conductor than in
adjacent areas (Hamilton, 1998). Likewise, locally-anomalous oxidised conditions
develop around the bottom of the conductor because of the consumption of reducing
agents. However, conditions can never become as reducing at the top of the conductor as
they are at the lower end or all current would cease. The result of the process is to
modify the shape of the redox field around the conductor (Fig. 3-7). This also modifies
the lines of current flux since, in isotropic media, current moves perpendicular to lines of
equal potential.
The specific redox half-reactions that result in current flow through the conductor are
of less importance than their aggregate effect on voltage. The stability field of water
restricts the overall voltage of the cell to less than 1500 mV. Redox reactions involving
the reduction or oxidation of water, such as occur in most electrolytic cells, are unlikely
to be part of the SP charge-transfer mechanism despite contrary statements by some
authors (Bolviken and Logn, 1975; Sivenas and Beales, 1982; Clark, 1997).
Electrochemical cells after the model of Sato and Mooney (1960) develop due to
zones of anomalously-high electrical conductivity in Earth materials in what would
otherwise be a roughly uniform vertical redox gradient. The issue of the conductor being
electronically conductive is, perhaps, a red herring. A zone of fault gouge made up of
water-saturated rock flour and phylosilicates could conceivably have a very high
electrolytic electrical conductivity, especially relative to surrounding poorly-fractured
rock. This should also develop a significant electrical current within it, provided an
upward-increasing redox gradient also exists.
Two hidden assumptions implicit in the model of Sato and Mooney (1960) are: (1)
that the conductor consists of a single phase, such as graphite or pyrite; and (2) that
oxidation of the conductor would result in its conversion to a non-conductive phase.
Thornber (1975a, 1975b) presents a reactive conductor model in which the conductor
itself is the reducing agent, which is in apparent contrast to the model of Sato and
Mooney. However, the reactive conductor model is based on the presence of one
oxidised phase and at least one reduced phase relative to the first phase, all of which are
electronically conductive. Such scenarios have been noted in terrain with deep
weathering profiles due to the phase conversion of reduced sulphide minerals to more
oxidised forms.
A sulphide body containing pyrrhotite in its lower part and pyrite in its upper part is
an example of such a cell (Fig. 3-8). Pyrrhotite at the boundary between the two phases
oxidises to form pyrite, which remains electronically conductive. Electrons liberated into
the sulphide mineralisation move up toward the more oxidising environment and allow