THE FORMATION AND DISTRIBUTION OF CO2-ENRICHED FLUID INCLUSIONS IN EPITHERMAL ENVIRONMENTS

Fluid inclusions from two geothermal systems associated with volcanic activity were studied to document the distribution of CO2 in modern epithermal environments. The fluid inclusion data, combined with mineral distributions and chemical analyses of the production fluids from both systems, provide a record of steam and gas flux to depths in excess of 1 to 2 km and of transient variations in the gas contents of the reservoirs. The liquid-rich fluid inclusions can be grouped into two types on the basis of their CO2 contents. Inclusions with CO2 contents of less than about 4 wt% typically have calculated gas contents that are higher than the present-day reservoir fluids. However, the calculated pressures and temperatures of these inclusions are consistent with their depth of formation, indicating that they may have formed in response to boiling and mixing processes in the reservoir. Liquid-rich fluid inclusions with CO2 contents between approximately 4 and 6 wt% are characterized by CO2 clathrate dissociation temperatures greater than 0.0-degrees-C. These inclusions occur on the margins of the thermal systems where they define umbrella-shaped caps around the main zones of upwelling. The CO2 contents of the inclusions require that they formed at pressures several tens of bars above hydrostatic. Elevated pressures and gas contents may have developed through compression and condensation of CO2-enriched steam by tectonic stress.