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.

摘要

在陶波火山区 (TVZ)，每天都有大量硫磺释放到大气中或沉淀在地面上。尽管像罗托鲁瓦这样的地热系统以其硫化氢特有的臭鸡蛋味而闻名，但硫的来源尚不清楚。本研究报告了在 Rotokawa 和 Ngatamariki 地热储层中形成的深层硬石膏和黄铁矿的微量元素分析和硫 (δ ³⁴ S) 和氧 (δ ¹⁸ O) 同位素。矿物分析与现在 Rotokawa 油田的流体成分和来自斑岩和超热液系统的化石热液硬石膏进行了比较。中央 TVZ 中的硫酸盐有多种来源：(1) 与深 H ₂平衡的硫酸盐S，(2) 地表蒸汽加热的深度循环和定量 H ₂ S 氧化或表生黄铁矿氧化，以及 (3) 硫酸盐还原或岩浆 SO ₂歧化。目前在陶波火山带中部没有发生 SO ₂歧化。地热储层中的硫溶解了 H ₂ S。今天，还原的硫很可能源自含黄铁矿的灰岩基岩的浸出，而没有或很少输入岩浆硫。