Abstract
The stability field of andradite in H2O-CO2 mixtures has been calculated on the basis of previous work in the systems CaO-SiO2-CO2 and CaO-FeOX-SiO2-O2, and the thermo-chemlcai data. Under relatively high oxygen fugacities where hedenbergite is unstable, the andradite field is restricted by the following reactions: (1) andradite+CO2=calcite+magnetite+quartz+O2 (2) andradite+CO2=calcite+hematite+quartz. Compared with the grossular field, the andradite field extends towards the low-temperature and CO2-rich side in the T-XCO2 diagram. For example, where the oxygen fugacity is represented by the NNO buffer, the equilibrium boundary for Reaction (1) at 2000 bars passes approximately through the points: XCO2=13mole%, T=500°C, and XCO2=0.44mole%, T=300°C. Since grandite garnet occurs commonly in skarn-type ore deposits, the upper limit of the amount of CO2 in fluids under which ore-bearing skarn can be formed is inferred to be 3 mole % at 400°C. This suggestion agrees with the fact that CO2-rich inclusions are not common in ore deposits of this type.
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