Page 135 - Geochemical Remote Sensing of The Sub-Surface

112 S.M. Hamilton

Field evidence for the presence of cells

The four models of SP cells described above were developed independently in
different disciplines of Earth science for different reasons. However, they all address
field evidence that is best explained using electrochemical processes. The model of Sato
and Mooney (1960) was developed to resolve many conflicting theories in the
geophysical community as to the origin of spontaneous potential phenomena occurring
over conductive sulphide deposits and graphite. The model of Thornber (1975a, 1975b)
was developed partly on mineralogical evidence from supergene-enriched nickel
deposits to: (1) better understand the process of supergene enrichment; and (2) improve
exploration techniques for this type of deposit. The model of Hamilton (1998) was
developed to account for selective leach geochemical anomalies that occur over gold and
base metal deposits in thick (>30m), young (-8 Ka), water-saturated glacial deposits and
that can not be explained by any other transport mechanism. The model of Pirson (1981)
and Tomkins (1990) was developed to unify observed geochemical and geophysical
phenomena that had been known for many years to occur over petroleum reservoirs. All
four of these models involve a redox cell localised over a geological source of reducing
agents.
For many years, the formation of "rabbit-ear" anomalies has been attributed to
electrochemical processes. Rabbit-ear anomalies were first recognised over conductive
mineralisation in shallow glacial overburden environments (Govett 1973, 1976;
Bolviken and Logn, 1975; Govett and Chork, 1977; Nuutilainen and Peuraniemi, 1977;
Smee, 1983). They have since been recognised over much thicker glacial overburden
(Jackson, 1995; Bajc, 1998). They also occur in temperate non-glacial and tropical areas
and are very commonly reported in arid to semi-arid environments (e.g., Govett et al.,
1984, Govett and Atherden, 1987; Clark, 1996, 1997; Smee, 1998). Where rabbit-ear
anomalies are reported, usually apical anomalies of other elements are noted also.
Anomalies in metals are most commonly reported because it is usual to determine metal
concentrations in mineral exploration surveys. However, where H +, electrical
conductivity and anions have been measured, these parameters have often been found to
be anomalous. Apical and rabbit-ear patterns are almost invariably centred directly over
mineralisation or other geological features such as faults (Clark, 1997; Bajc, 1998).
Where corresponding SP data are available at sites, rabbit-ear anomalies usually
correlate with central SP lows over mineralisation (Bolviken and Logn, 1975; Govett,
1976; Govett and Chork, 1977; Smee, 1983; Hamilton and McClenaghan, 1998). Until
recently, the third-dimensional shape of such SP anomalies was not known and this led
to various interpretations of current flow-patterns and ion movement. A recent cross-
sectional study (Hamilton and McClenaghan, 1998) involved down-hole SP in thick (30
m) glacial overburden overlain by a peat-bog and underlain by graphitic argillite hosting
gold mineralisation. The results show equipotential lines and inferred ion flow patterns
that closely follow the model of Hamilton (1998) and that correlate with apical and
rabbit-ear geochemical anomalies in shallow groundwater and peat. It was concluded