The Implication of 34S and 18O Isotope Systematics on the Sulfur Cycle in Mahanagdong Hydrothermal System (Philippiness)

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.