Page 141 - Geochemical Remote Sensing of The Sub-Surface

1 18 S.M. Hamilton

overburden material. In adjacent areas near or above the water table, dissolved CO 2 can
degas causing carbonate supersaturation and deposition. From here, CO2 and carbonate-
charged soil moisture could also disperse upward through the unsaturated zone by
capillary action. As moisture evaporates, near-surface CO2 would degas and carbonate
would precipitate, forming both soil carbonate and CO2 gas anomalies (Smee, 1998).
Smee (1998) attributed rabbit-ear anomalies of Ca, Mg, Sr and, possibly, Au and As to
their transport in bicarbonate complexes and their precipitation in shallow soils due to
processes such as these occurring in the unsaturated zone.
The thousands of possible redox reactions that facilitate charge transfer away from a
reduced source could result in a net loss of cations or anions from solution in the vicinity
of some reactions. As such, the movement of redox-inert species is likely to be
continuously occurring from one part of the overburden electrolyte to another in order to
prevent local charge imbalances. However, it is difficult to predict the transport paths of
these ions because their movement depends on the specific nature of the reactions
occurring at a given site. Empirical observation is probably the most viable method for
determining the behaviour of a redox-inert species. Clark (1996) reports that an
empirically-observed oxidation suite of elements that often forms rabbit-ear anomalies
flanking ore deposits includes C1, Br, I, As, Sb, Mo, W, Re, Se, Te, V, U and Th. Most
of the elements in this suite are far more mobile in their oxidised forms than in their
reduced forms and as such probably migrate inward to form reduction anomalies
(notwithstanding the name applied to the suite). However, several elements are likely to
be mobile as redox-inert species (e.g., CI, Br, I) or perhaps as ligands in metallic
complexes, and therefore their movement may be a secondary result of other species
migrating and subsequent reactions occurring inside and outside the column.

CONCLUSIONS

Despite the different origins of the four SP cell models described in this chapter, their
underlying principles are similar and can be summarised as follows. Within the redox
field of the Earth, Ohm's Law suggests that a cell will form if either the redox gradient
or electrical conductivity is anomalously high with respect to that of surrounding Earth
materials. An Earth cell is defined as an area of spontaneously elevated electrical current
between two separated sources of oxidising and reducing agents. Resulting similarities in
the geochemical surface expression of the cells would include: a reduced SP centre
above the cell; the outward movement of negatively charged ions; the inward movement
of positively charged ions; and an element dispersal pattern that is characteristic of a
redox cell. The most typical anomaly morphologies are the centred rabbit-ear and apical
patterns which are often coincident in different elements.
Thus, the primary criterion for development of a cell is the existence of a buried
reduced feature. Such features of potential economic interest include the following.