Page 115 - Geochemical Remote Sensing of The Sub-Surface

92 S.M. Hamilton

currents in Earth materials. They arise largely because of differences in the oxidation-
reduction (redox) potential of Earth materials.
Spontaneous potentials are useful in mineral exploration because SP anomalies are
often associated with electronically-conductive mineralisation. The vast majority of
these anomalies are negative over mineralisation relative to surrounding terrain, which
suggests that mineralisation acts as a source or conduit of electrons (Sato and Mooney,
1960). Burr (1982) reports that sulphide mineralisation in Canada produces negative
anomalies of up to 350 mV, whilst anomalies of over 450 mV are usually due to the
presence of graphite, which is a far better conductor of electrons. These ranges are in
contrast to typical background variations of only a few millivolts to a few tens of
millivolts where mineralisation is absent (Parasnis, 1973). In thick overburden the
contrast between anomalies and background readings decreases to the extent that 100 m
or more of overburden can render the SP response due to bedrock features too weak to
interpret (Lang, 1956).

Measurement of spontaneous potential

Theoretically, SP can be measured in two ways. One way involves the use of an
oxidation-reduction potential (ORP) meter on samples of Earth materials taken from two
areas; the measurement should provide voltage differences that approximate the
spontaneous potential between the two areas. An ORP probe, in contact with a moist
sample, represents a reversible voltaic cell. The probe is connected to a millivolt meter
that can measure the voltage difference between the sample and a half-cell such as Ag-
AgCI, which is usually located inside the probe. An inert metal, usually platinum, on the
outside of the probe serves as an electrode in direct contact with the sample. A semi-
permeable junction of porous glass or ceramic maintains electrolytic contact between the
sample and the half-cell. If the sample is more reducing than the Ag-AgCI half-cell
(standard electrode potential = 222 mV) the platinum wire behaves as an anode and
accepts electrons from the sample. Inside the probe, electrons are simultaneously
provided to Ag + to form Ag(s) on the Ag electrode. If the solution is more oxidising than
the Ag-AgCI half-cell, the platinum behaves as a cathode and the reverse reaction occurs
inside the probe. Thus, the ORP readings on the millivolt meter represent a relative
voltage difference between the half-cell and the sample. If desired, the Eh of the solution
can be obtained from these results by adding to the readings the voltage difference
between the reference cell and the standard hydrogen electrode.
In practice, ORP probes have inherent limitations that often render this approach to
SP measurement unworkable. Because platinum is effectively inert, the gain or loss of
electrons on the electrode surface necessitates the attenuation of ions from the sample by
their conversion to neutral species or to charged species of a higher or lower oxidation
state. For the readings to be reproducible, one or more reversible redox-couples, such as
Fe 2§ / Fe 3+, HS / $042 or Cu 2+ / Cu +, must be present in solution near the electrode.