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Sulfur in New Zealand geothermal systems: insights from stable isotope and trace element analyses of anhydrite from Rotokawa and Ngatamariki geothermal fields, Taupo Volcanic Zone

ABSTRACT Every day in the Taupō Volcanic Zone (TVZ) a large amount of sulfur is released into the atmosphere or precipitated on the ground. Although geothermal systems like Rotorua are well-known for their rotten egg smell characteristic of hydrogen sulphide, where the sulfur originates from is less clear. Trace element analyses and sulfur (δ34S) and oxygen (δ18O) isotopes of deep anhydrite and pyrite that formed in the Rotokawa and Ngatamariki geothermal reservoirs are reported in this study. Mineral analyses are compared with present-day fluid compositions of the Rotokawa field and fossil hydrothermal anhydrite from porphyry and epithermal systems. Sulfate in the central TVZ originates from multiple sources: (1) sulfate in equilibrium with deep H2S, (2) deep recycling of surface steam heated and quantitative H2S oxidation or supergene pyrite oxidation, and (3) sulfate reduction or disproportionation of magmatic SO2. No SO2 disproportionation is currently occurring in the central Taupō Volcanic Zone. Sulfur in the geothermal reservoirs is dissolved H2S. Today, the reduced sulfur is likely to have originate from the leaching of the pyrite-bearing greywacke basement rocks with no or minimal input of magmatic sulfur.