Page 131 - Geochemical Remote Sensing of The Sub-Surface

108 S.M. Hamilton

negative redox anisotropy in groundwater-saturated basal overburden over
mineralisation relative to the surrounding overburden over country rock. Whilst
electronic current continues, there must be continual outward dissipation of negative
charge into surrounding overburden; otherwise the build-up of reducing conditions
around the top of the conductor would eliminate the voltage differential and current
would cease. Once solid-phase oxidising agents are consumed, this dissipation of
negative charge must take place by the outward migration of reduced ions away from, or
the inward migration of oxidised ionic species toward, the cathode. This process is
necessarily coupled with oxidation-reduction reactions, occurring between the top
surface of the conductor and the water table, that involve both reduced and oxidised
species. The dissipation of negative charge away from the conductor is accomplished as
the oxidation states of reduced species change during this process, resulting in dissolved,
gaseous or precipitated products that have a higher redox potential than the reduced
reactants.
Figure 3-9 provides a hypothetical example of the possible outcome of this process in
young, exotic sediments. Figure 3-9A depicts a fine-grained glacial material, shortly
after deposition, in which a background redox differential of 150 mV exists between
groundwater at the water table and in basal overburden units. An electronically-
conductive, steeply-dipping mineral deposit occurs in bedrock. More reducing
conditions immediately above the conductor result in a redox differential of 300 mV
between the top of mineralisation and the water table. Spontaneous potential contrasts
exceeding 150 mV between conductive mineralisation and adjacent rock have often been
reported (e.g., Pflug et al., 1996; Bolviken and Logn, 1975).
At the time of overburden deposition, a very strong vertical redox gradient exists just
above the bedrock conductor along which ions have a tendency to move. The outward
movement of reduced ions such as HS, Fe 2+ or $2032- results in the migration of a
reduced front away from mineralisation. At the front, reduced ions come into contact
with oxidising agents and redox reactions take place, thereby dissipating negative charge
away from the conductor. Once the front reaches the water table, a reduced column will
have developed in the groundwater-saturated overburden above mineralisation.
If the capacity for electrical current in the conductor is high, the production of
reduced species might exceed the capacity of the unsaturated zone above the column to
provide oxygen across the water-table phase boundary. In this case, the column would
probably widen out. As it widens, the surface area exposed to oxidising agents along the
entire reducing front increases. Once the column diameter is sufficiently large, the
capability of the water-table phase boundary and surrounding overburden to provide
oxidising agents equals that of the conductor to provide negative charge. The supply of
oxidising and reducing agents is therefore balanced and a steady-state kinetic
equilibrium is established between the two processes within the cell. The equipotential
lines around the column above the conductor cease to move outward and their
previously-horizontal configuration becomes nearly vertical in the vicinity of the
reduced column. The end result is a permanent Eh anisotropy in overburden between the