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

Every day in the Taupo 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 Taupo 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. (author)