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 die 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). 

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 HjS (Chapter 9). 

Just as trace constituents of mineral deposits can act as conventional geochemical pathfinders, trace volatile constituents are potentially gaseous pathfinders. Some sulphide minerals, in particular sphalerite, accommodate trace quantities of Hg. When hberated into 

Finally, the very process of sulphide oxidation at depth can provide geochemical signals at surface. Sulphide oxidation consumes O2 that is ultimately drawn from the aerated rock and soil voids in the immediate vicinity. The chemical reactions of oxidation create a low pH environment in which any carbonate minerals break down with the liberation of CO2, some of which finds its way into the neighbouring rock and soil voids. Thus anomalous concentrations of O2 and CO2 in the near-surface soil air provide an indication of oxidising mineralisation at depth (Chapter 14). 

Diffusion is the most fundamental mechanism of gas migration in that it requires only a partial pressure (concentration) gradient. The rate of diffusion of a gas is then determined by the medium in which diffusion takes place, its temperature and absolute pressure, and the diffusion coefficient of the gas. The diffusion coeffiecient is a function of molecular weight, the shape of molecules, and their intermolecular attraction. Every gas thus has a different diffusion coefficient. The influence of the medium in which gas diffusion occurs is related to the density of the medium gases diffuse less quickly through a solid than through another gas. The rate at which a gas diffuses in a specified medium is sometimes termed its diffusivity. 

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