Acidified steam condensates that are enriched in SO4 but poor in Cl are found in the upper parts of many geothermal systems occurring in steep terrains. In this paper, we characterize the chemical evolution of these acid-sulfate fluids by combining mineral and fluid inclusion relationships from Karaha-Telaga Bodas with numerical simulations of water-rock interactions.

These interactions were modeled at temperatures up to 250oC, using the compositions of the lake water from Telaga Bodas and a typical andesite. The simulations predict mineral distributions consistent with the observed alteration patterns and a decrease in the freezing point depression of the fluid with increasing temperature. Fluids trapped in anhydrite, calcite, and fluorite at temperatures of 160o-205oC display a similar decrease in their freezing point depressions. This change is due mainly to the incorporation of SO4 into the newly formed hydrothermal minerals. Between 205o and 235oC, the freezing point depressions remain at their minimum values. These values represent the residual salinity of the fluid, which is controlled primarily by Cl and unreacted CO2.

The salinities of fluid inclusions containing Cl poor steam condensate are better expressed in terms of H2SO4 equivalents than the commonly used NaCl equivalents. At solute concentrations >1.5 molal, freezing point depressions represented as NaCl equivalents overestimate the salinity of Cl poor waters. At lower concentrations, differences between apparent salinities calculated as NaCl and H2SO4 equivalents are negligible

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