The Recognition of Quenched Magmatic Gases in Fumaroles as a Geothermal Exploration Tool

Abstract

Magmatic and quenched magmatic gases in fumaroles can be identiped by using a H2S-H2-CH, ternary diagram even where SO2, HCl and HF have been scrubbed @om the gases or have not been analysed for. Stundard gas geothermometry is not applicable to such gases although high temperatures at depth are implied by their presence. However, fluids ut depth in the immediate vicinity of fumaroles with this chemistry are likely to be acid or produce acid condensates and are not currently exploitable - although outlying convective parts of the same system may be exploitable. The diagram is also useful for identiaing neutral reservoirs where overlying firmaroles are superheated, a feature occasionally mistaken as magmatic.

Whilst most quenched magmatic gases have an ôandesitic compositionö, often represented by their position on NTHe-Ar plots, they are not exclusive to it. Gases @om neutral reservoirs can also have ôandesiticö compositions, hence the N2-He-Ar diagram should not be misused as an indicator of magmatic acidity at depth.