Page 107 - Geochemical Remote Sensing of The Sub-Surface

84 S.M. Hamilton

1. In extremely sluggish groundwater flow environments, e.g., as methane pockets in
shale where the gas is in pressure-equilibrium with surrounding groundwater and
where the only dispersal mechanism is diffusion.
2. In environments where the fluid pressure is decreasing, i.e., where groundwater is
moving up, such as at discharge zones.
3. In environments where the vapour pressure is increasing or at least being maintained
by ongoing processes at a continuously high level, i.e., where gas is being actively
generated or being supplied from below.

Scenarios 1 and 2 can be ruled out because, as mentioned, neither diffusion nor
groundwater discharge can account for the majority of selective leach anomalies. If
scenario 3 were applicable as a transport mechanism for metals and gases below the
water table, a large source of gas would be necessary. If it were applicable as a cause of
rabbit-ear anomalies in all environments, a large source of gas would be necessary at
every mineral deposit that produces such anomalies. It follows, therefore, that the source
of gas must be genetically associated with the weathering of the mineral deposit. Finally,
if a separate gas phase were present, it would require that gas be generated at
mineralisation in high enough concentrations to partition into a vapour phase. If it did
not, the rate of diffusion of elements from mineralisation would be in a similar range to
that of other dissolved species, which is far too slow to account for many of the
anomalies noted in thick, young, glacial terrain.
Elevated CO2 has been shown to be coincident with rabbit-ear metal anomalies over
mineralisation in arid terrain and will be used as an example. For any gas in equilibrium
with water, the concentration of that gas in solution is proportional to total pressure. At a
depth of 30 m below the water table and assuming CO2 is the dominant dissolved gas
phase, the partial pressure required to exsolve CO2 would be about 4 atmospheres, which
is an extremely high concentration (exceeding the pCO2 of soft drinks). Concentrations
exceeding 1 atmosphere are very rare in natural meteoric groundwater and usually occur
only in thermal springs and spring discharges along seismically active faults (Barnes et
al., 1978; Hamilton et al., 1990). Concentrations this high in areas of abundant carbonate
would dissolve vast quantities of carbonate, which would re-precipitate when the CO2
degasses from near-surface groundwater. The general lack of such deposits in
association with rabbit-ear anomalies over conductors suggests that transport of elements
by CO2 or CO2-saturated groundwater cannot be responsible for many of the reported
selective leach anomalies.
It is difficult to conceive of any naturally-occurring gas other than CO2 that could be
called upon to transport metals or other species from sulphide mineralisation as a
widespread process. Similarly, other gases would also require high concentrations to
exsolve as depth and hydrostatic pressure increase, and are therefore unlikely to be
responsible for transport of metals below the water table. Furthermore, in stratified
geological environments, gases do not typically move straight up but are trapped by