Electrical and electromagnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 254

Ohm s Law, potential differences form across segments of

the pathways which decay with dispersal of the charges. It is
this decaying voltage that is measured by induced polarisa-
tion (IP) surveys (see Section 5.6.3 ).
There are two mechanisms by which the capacitor-like
behaviour occurs in the natural environment: membrane
polarisation and grain polarisation. The latter may be
associated with potentially economic minerals, the former
is not.

' and involves a chemical reaction between the mineral and

the solution (the pore fluid).
The exchange of electrons results in the conductive grain
becoming polarised, i.e. opposite charges accumulate on
opposite sides of the grain. Ions of opposite polarity (sign)
will tend to concentrate next to the grain boundary. If its
ability to exchange electrons does not match that of the
ions in the electrolyte, or there is insuf cient energy for the
exchange to take place, there is a build-up of ions adjacent
to the grain. While the potential is applied, charges of like
polarity are forced into proximity, again analogous to
forming a charged capacitor.
Grain polarisation requires conductive mineral grains,
so it is expected to form when conductive sulphide or oxide
minerals, or graphite, are present.

5.3.2.1 Membrane (electrolytic) polarisation
As described in Section 5.3.1.3 , there are negative surface
charges on most mineral grains. When these grains form
the walls of pores, negative ions are repelled and positive
ions in the pore fluids are attracted to the grains creating a
surface layer, or membrane, of ions (about 100 μ min

thickness). When a pore narrows suf ciently, the positive

ions create a barrier across its entire width ( Fig. 5.19b )
preventing the movement of negative ions through the
pore fluid. This impedes the current flow and results in a
local concentration of these ions on one side of the barrier
and a de ciency on the other side, i.e. there is an electrical
polarisation. While the potential is applied, charges of like
polarity are forced into proximity, analogous to forming a
charged capacitor.
Clay and fibrous minerals have particularly strong
surface charges so when these are in contact with the pore
space they may attract a cloud of positive ions. Again the
movement of negative ions is hindered ( Fig. 5.19c ). The
polarisation effect depends on the clay mineral species
present, with maximum effect occurring with typically
~10% volume of clay.
Membrane polarisation most successfully develops
where the grain surface charge is greatest, so it is predom-
inantly associated with clay minerals and where pores are
small. It also increases with pore

5.3.2.3 Geological controls on polarisation
For both membrane and grain polarisation to occur, there
must be signi cant porosity and permeability and a suit-
able electrolyte within the pore space. So as with conduct-
ivity, the rock

is texture is an important in uence. Many
factors are known to in uence electrical polarisation effects
and they may interact to produce unexpected responses.
Some of these in uences are summarised as follows:

'

• The type of electronic conducting minerals. Figure 5.20

illustrates the wide variation obtained from samples
containing the same amounts of various conductive
minerals.

The amount and distribution of the conductive material.
For disseminated material

•

the polarisation response

Chargeability (ms)
02468 0 2 4

Pyrite
Chalcocite

fluid salinity.

Copper
Graphite
Chalcopyrite
Bornite
Galena
Magnetite
Malachite
Haematite

5.3.2.2 Grain (electrode) polarisation
When ions are flowing through pore fluids in response to
an applied potential they may encounter electronically
conductive grains, for example sulphides, that may form
a barrier with which the ions can electrically interact
( Fig. 5.19d ). For the current to flow, the ions on one side
of the barrier must exchange electrons with the conductive
grain, which in turn exchanges electrons with ions on the
other side of the barrier. The electrical circuit locally con-
sists of a combination of ionic and electronic conduction

Figure 5.20 Chargeability of 1% volume concentration of a variety of
conductive ore minerals measured using a square-wave pulse of 3 s
with the decay integrated over a period of 1 s. See Section 5.6.3 for
explanation of the measurement of induced polarisation. Based on
data in Telford et al.( 1990 ).