Development of the MultiGAS for determining fumarole gas chemistry in geothermal systems

Geothermal exploration relies in part on the gas geochemis- try of fumaroles, bubbling springs and steaming ground to offer insight into the nature of the fluids at depth, processes affecting them when rising to the surface, and provide estimates of the temperature of last equilibration of the gases within the reservoir. Traditional measurements involve direct sampling of the gases in pre-evacuated Giggenbach glass bottles before laboratory chemical analysis. Gas component analysis, combined with the isotope ratios of certain components (e.g., CO2, He), provides insight into the proportion of magmatic, crustal, meteoric and atmospheric components in the fluid, and the state of equilibrium and temperature of these fluids at depth. The Multi-component Gas Analyzing System (MultiGAS) was developed by the volcanological community over 10 years ago as a field-portable instrument for in-situ analysis of the major volcanic gas components in diffuse and dilute gas emissions. No two instruments are identical but all consist of various sensor types now capable of simultaneously analyzing for H2O, CO2, CO, SO2 and H2S. In geothermal systems, surface manifestations are often com- prised of gas emissions. In some cases, low temperatures and/or low flow rates make traditional sampling of fumaroles difficult, due to rapid vapor condensation (in water-rich fumaroles) and atmospheric contamination. The MultiGAS is best suited to these types of manifestations, providing a tool that can be used at a wide variety of locations with differing gas emission styles. The MultiGAS has been field-tested in two very different geothermal prospect areas: one characterized by cold, CO2-rich gas seeps and bubbling springs, and the other characterized by steaming ground and fumaroles with temperatures near the boiling point of water. Results of the MultiGAS analysis are compared with the equivalent ratios obtained from traditional sampling and analytical procedures (where possible), in order to identify the ad- vantages and disadvantages of this new technique. The MultiGAS allows for the rapid characterization of the gas geochemistry in the field, aids in mapping/targeting of fumaroles in a large field, and allows for the selection of the most ideal fumaroles to sample using the traditional Giggenbach method.