Page 138 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 115

Sharply-defined Eh zones within the overburden stratigraphy tend to promote the
replacement of charge-carrying ions with others. An actively-flowing and oxygenated
aquifer, for example, might force a change in the charge carrier to a less reduced species,
or might short-circuit the cell altogether so that the cell operates between mineralisation
and the base of the aquifer instead of the ground surface. The water table is also a sharp
redox boundary and almost certainly forces changes in the species that carry charge.
A series of processes that involve the electrical double layer on clay and oxide
surfaces could limit the rate of mass-transport of ions to the surface. Electro-filtration is
the exclusion of some ions during their transport through double-layer media. Electro-
filtration may limit the movement of certain ions though thick overburden thereby
slowing the whole charge transfer process. Ion exchange and sorption are also double-
layer processes that could reduce the rate of mass and charge transfer. Ion exchange of a
redox-active ion in solution for a redox-inert ion will effectively stop the transfer of
charge at that exchange site and could exchange migrating species that originated from
mineralisation with others that originated in overburden. This could slow down both
mass and charge transfer until a significant proportion of the sorption or exchange sites
are occupied by redox-active species. It should be noted, however, that double-layer
processes limiting mass transfer are also likely to limit charge transfer. The commonly-
reported surface selective leach anomalies in thick overburden above mineralisation are
evidence that charge transfer does occur, even in young deposits. Therefore it appears
that double-layer processes are not major inhibitors of the development of SP cells in
overburden.
Not all geochemical processes in overburden necessarily slow down mass transport
and charge transfer. Groundwater in overburden above mineralisation may have higher
ionic strength than that in surrounding areas due to the attraction of ions to, and/or
generation of ions in, the reduced column. As such, there may be an area of increased
electrical conductivity within the cell that should enhance current flow. Furthermore,
clay surfaces impart a significant electrical conductivity to groundwater in overburden
(which, incidentally, is also a double-layer process) and this might also enhance current
relative to that which would occur in a pure electrolyte.

Geochemical anomalies

The issue of replacement of the charge carrier as just described is extremely
important to the use of selective leach procedures for geochemical exploration,
particularly in younger terrain. In older geological terrain, the process may have operated
for long enough to inundate the overburden with elements and species transported from
depth and to give the overburden an elemental signature that is partially reflective of that
of bedrock. In younger terrain, however, the charge could just as easily have been
transported by exotic species liberated from overburden materials and, as such, surface
anomalies would be partially or wholly reflective of the geochemistry of overburden. In