FROM THERMODATA TO MODELS: A CASE STUDY FOR ARSENIC IN GEOTHERMAL SYSTEMS

The Uzon Caldera in Kamchatka, Russia is a geothermal system with actively precipitating arsenic sulphide mineralization.  The general mineral paragenesis for a down temperature section includes: pyrite – pyrite + realgar – realgar – realgar + orpiment – orpiment – amorphous As2S3.  This study used equilibrium geochemical modelling to understand the processes of arsenic transport and precipitation, and attempt to simulate the observed arsenic sulphide zonation.  A critical review of published experimental and theoretical thermodynamic data for As and S phases was required to compile and update existing and missing data in the extended Geochemists Workbench (GWB) data file (thermo.com.v8.r6+).  The data were updated in SUPCRT92 before generating and updating the required log K parameters in the GWB data file.  A comparison of thermodynamic data from different sources showed close similarities to ~125^oC, but at higher temperatures the calculated log K data from different sources diverge significantly (up to 3 log units).  Using the best data set available a series of speciation and reaction path models were completed using the Uzon fluids.  These results predict that arsenic is predominantly transported as H3AsO3(aq) and to a lesser extent as arsenic sulphide complexes As2S3(aq), Has2S4- and As2S4--.  Reaction path modelling with GWB predicts the observed mineral paragenesis by cooling of As-rich (15 mg/kg) fluids from 125 to 25^oC with slight oxygenation, but while still maintaining reducing conditions.  The oxygenation process is likely related to mixing with a surface fluid.  Amorphous orpiment was not stable in the models until the initial arsenic content of the fluid is doubled.  This study illustrates the importance of understanding the thermodynamic data relationships and sources of error, as well as fallibilities in the equilibrium model approach at these conditions.  However, despite many assumptions and deficiencies in the thermodynamic data and the use of equilibrium modelling, this study shows that acceptable analogues and predictive capabilities for aqueous arsenic systems can be developed.