Page 30 - Geochemical Remote Sensing of The Sub-Surface

Genesis, behaviour and detection of gases in the crust 7

Gases contemporaneous with resource emplacement

Some gases are physically trapped in mineral deposits and petroleum accumulations at
depth but escape in trace quantities and migrate to the surface. The emplacement of many
hydrothermal mineral deposits is accompanied by the introduction of large quantities of CO2
into the surrounding host rocks. Much of this CO2 is either trapped in fluid inclusions or
incorporated into carbonate minerals. Its detection may act as a guide to the presence of the
mineral deposit with which its introduction was associated (Chapter 4).
Petroleum and natural gas accumulations require a physical trap for their preservation.
Such traps are rarely gas-tight and the more volatile hydrocarbons (indeed, m some cases,
heavier hydrocarbons) may escape to the surface, producing microseeps. Attempts to detect
light hydrocarbon microseeps in the 1930s mark the origins of gas geochemistry.
Progressive sophistication has yielded techniques to chartacterise effectively microseeps
both onshore and offshore (Chapter 5). Regional surveys involving the determination of
light hydrocarbons adsorbed onto soil have contributed to successful petroleum prospecting
(Chapter 6). These light hydrocarbons are the near-surface expression of a flux of gas
leaking from the reservoir and creating towards the surface a reduction chimney in an
otherwise aerated and oxidising environment. The effects induced m rocks and vegetation
can sometimes be detected from satellites (Chapter 7).

Gases of post-mineralization provenance

Many metalliferous mineral deposits formed at depth are in the reduced state. Where
they interface with the near-surface oxidising environment, there is considerable chemical
reactivity. This typically takes the form of sulphide oxidation, which includes the generation
of several meta-stable sulphur gases that have been shown to be useful in mineral
exploration (Chapter 8). Incompletely oxidised sulphide anions and compounds are
transported away from mineral deposits at depth by the groundwater, and can be mapped at
surface as dispersion patterns of H2S (Chapter 9).
Uranium deposits, by virtue of their radiogenic constituents, present a special case in
mineral exploration. Many of the disintegrations in the radiodecay chains of U and Th
liberate alpha particles (He nuclei). These are quickly stabilised as atoms of He gas,
making He a potential guide to U (Chapter 10). Amongst the daughter elements in the
radiodecay chains of U and Th, the only gas is Rn. Owing to a combination of its
conveniently limited half-life and relative ease of measurement, Rn has been used
extensively as a guide to U (Chapter 11). Strictly speaking it is a guide only to its
immediate parent, Ra, which may have become geochemically separated from earlier
members of its radioactive decay series, including U.
Just as trace constituents of mineral deposits can act as conventional geochemical
pathf'mders, trace volatile constituents are potentially gaseous pathfinders. Some sulphide
minerals, in particular sphalerite, accommodate trace quantities of Hg. When liberated into