| Abstract |
Sulphur isotope geothermometers for 42-H2S fluid pairs, and oxygen isotope geothermometry for dissolved and H20, gave temperatures similar to those measured in wells (within 1 @C) at the Mt. dpo geothermal field. The geothermometers also gave parent fluid temperature estimates identical to those calculated from cation and silica geothermometers. In contrast, temperature estimates made using SO42-H2S fluid and anhydrite-pyrite mineral pairs of Palinpinon wells are higher than actual bore temperatures. Sulphur isotope geothermometers used in this field may reflect isotopic equilibrium in the deeper portions of the geothermal system, unreached by drilled wells. Dissolved SO42 is believed to be a product of either (aj SO2 disproportionation to H2S and SO42- at a temperature below 400oC, (b) H2S oxidation at boiling depths, or (c) both. The first process accounts for the total dissolved SO42- but <15% of H2S, present in well discharges. To accoirnt for the remaining H2S, a direct magmatic input is invoked. The second process cannot be isotopicallv distinguished, but may be inferred because of the scatter of d4S values of sulphate. There is no evidence of major sulphate contribution from surface steam-heated S042- waters. Anhydrite and pyrite have dJS values similar to fluid SO42 and H2S, respectively. These suggest that sulphur in anhydrite is derived from dissolved sulphate, while pyrite sulphur is supplied by H2S. |