| Abstract |
The study of 18O and 34S stable isotopes in fluid and minerals of the Mahanagdong hydrothermal system aims to identify the origin of the acidic SO4-rich waters in the deep reservoir. The results of the study suggest that the Mahanagdong neutral-pH and acidic fluids are hydrothermal in origin, as these ratios depart from the primary magmatic fluid composition of ‰34S in dissolved S and solid sulfur, and ‰18O in water +10o/oo. The ‰34SSO4-H2S geothermometer appears to be applicable only in acidic wells, possibly because of enhanced isotope fractionation in low pH and high total dissolved sulfur environment. The ‰18OSO4-H2O geothermometer failed to give meaningful results, suggesting that 18O in Mahanagdong is not primarily dependent on the fractionation between water and SO4. Similarly, the ‰34SAH-PY mineral isotope geothermometers, when applied to suspected acidic feed zones, predicted higher than measured temperatures, possibly because anhydrite and pyrite formed from different, and not a single, reaction. Unaltered volcanic rocks in the Mahanagdong geothermal system has total sulfur of 560 mg/kgr, the expected composition of granodiorites. Addition of about 4,000-8,000 mg/kgr sulfur took place in the neutral-pH reservoir possibly through the continuous circulation of mineralising fluids. In the acidic reservoir, about 10,000-43,000 mg/kgr total sulfur has been added. |