Page 140 - Geochemical Remote Sensing of The Sub-Surface

Spontaneous potentials and electrochemical cells 117

result will be a general outward movement of ions that are capable of transporting
negative charge, such as Fe z+, Co z+, Cu +, HS and $2032-, and an inward movement of
ions that are capable of transporting positive charge, such as UO22+, MoO4 2-, VO43,
SeO42, AsO4 3", $042 and dissolved oxygen radicals. The final transfer of negative
charge at the top of the cells involves redox reactions that, in many cases, attenuate the
transported reduced species into the solid phase, thereby forming geochemical
anomalies.
One outcome of the migration of ions from one redox region into another should be a
zonation of elements in relation to the reduced column. Element zonation is a reported
feature of selective leach anomalies (Clark, 1996). Zonation could occur due to a variety
of processes, the most important of which would be progressive deposition of redox-
active species as they migrate into or out of the reduced column. The migration paths of
reduced and oxidised ions are predictable provided the current flow patterns can be
inferred and therefore, if the redox behaviour of a particular ion is known, the shape of
anomalies can be inferred.
Two of the factors that should control where mobile elements are deposited in
relation to the reduced column are: (1) the Eh at which a redox-active species converts to
a species of a different oxidation state; and (2) the mobility of the new species. For
example, inward migrating species that are highly oxidising and show low mobility in
reducing environments, such as UO22+, are expected to become reduced early and form
anomalies at the outermost edges of the reduced column (Fig. 3-12A). At the other
extreme, inward migrating, weakly oxidising species that can show high mobility in
reducing environments, such as SO42, do not become reduced until they reach the inner
part of the column, and even then might not form anomalies in soils because of their
high mobility. Similar but opposite processes occur with reduced species in the outward
direction.
One of the more important reduced species capable of forming anomalies in soils is
Fe 2+. Both the abundance of and the secondary processes associated with iron reactions
suggest that Fe 2+ will have a major impact on the geochemistry of surface soils. It is
moderately-to-strongly reducing and Fe 3+ has very low mobility. It is therefore expected
to form anomalies at the redox front near the inner edge of the reduced column. When
most reduced metals (and in particular, Fe 2+) oxidise, they hydrolyse water to form
insoluble metal hydroxides. In the process, large amounts of acid are generated (Fig. 3-
12B). Anomalies of H § are commonly reported in association with rabbit-ear anomalies
over mineralisation. In the phreatic zone H + is most likely to be generated, not by the
downward diffusion of oxygen and the oxidation of the sulphide itself, as in the past had
always been assumed, but by the upward transport of reduced iron along an
electrochemical gradient to oxidising agents at the redox front (i.e., the reducing agents
are brought to the oxidising agents rather than the other way around). Secondary process
related to iron oxidation may cause CO2 production in areas where carbonate is present
(Fig. 3-12B). Acid produced at the edge of the reduced column by Fe 2+ oxidation should
produce dissolved CO2 as H2CO3 (Fig. 3-12B) by dissolution of carbonate in rock or