The thermal and geochemical structure of geothermal and epithermal systems: A framework for interpreting fluid inclusion data

Abstract

Fluid inclusions are often formed during mineral growth in hydrothermal systems. They may be studied in geothermal systems, where comparing the results with directly measured temperatures and fluid compositions allows short time scale variations in the system to be assessed. Insight into the interpretation of fluid inclusions is obtained from such comparisons, which can be applied to the interpretation of fluid inclusion data from extinct systems, e.g., epidermal ore deposits. The interpretation of fluid inclusion data from extinct systems is often much more difficult than for active systems, due to constraints imposed by the mineral record, which only preserves a partial history of a system, and uncertainty of the water level during activity. A good paragenetic control on timing of inclusion formation is also necessary, particularly where there are several overprinting events. Fluid inclusion data need to be integrated with all relevant geochemical and geological constraints to develop an internally consistent model of the systems; internal consistency is sometimes the only assurance that our conclusions are realistic. The fluid inclusion data reviewed here include T h and T m measurements and the composition of extracted gases from liquid-dominated hydrothermal systems of neutral pH and relatively low salinity. The deep fluids in many systems boil in the uppermost 1 to 2 km of ascent, though mixing with cooler waters on the margins and very close to the surface will quench boiling. These processes can be recognized from the interpretation of paragenetically-controlled T h and T m data; furthermore, the temperatures and compositions of the end-member fluids can be estimated, and the total gas content of a fluid inferred. However, the extraction and analysis of gases provides a more quantitative estimate of the total concentration while determining the species and relative amounts of gases. Gases may be used in studies of mineral-fluid equilibria, and as independent geothermometers (CO 2 , CH 4 , H 2 S and H 2 ); combined with Th data, this information helps to estimate the temperature of first boiling as well as the initial gas content of a fluid, while variation in gas ratios between samples may be evidence for boiling and vapour loss. Comparison of gas composition with the host mineral assemblages allows post-entrapment changes and analytical artefacts to be recognized (e.g., H 2 ), which are topics for further study before gases can be confidently used to their full potential. Relatively non-reactive gases (N 2 , Ar, He, as well as gas isotopes such as 3 He/ 4 He) can be used as tracers of sources