Fluid inclusion evidence for geothermal structure beneath the Southern Alps, New Zealand

Abstract

Fissure veins containing adularia, bladed calcite, quartz, and chlorite occur in fractures in schist immediately west of the mountain crest in the Southern Alps, an active collisional mountain range. The vein minerals contain primary fluid inclusions which homogenise between 240 and 260°C. The fluids have low dissolved salt content (<2 wt% NaCl equivalent) and low CO₂ content (<1 wt%). Fluid inclusions in adularia show physical (co‐existing liquid and vapour) and chemical (variable CO₂ contents) evidence for boiling during entrapment. The mineral assemblage is similar to that seen in boiling zones of modern geothermal systems. Boiling occurred at 500 ± 150 m below topographic surface, or c. 1 km above sea level, and fluid temperature was higher than rock temperature. In contrast, fluids trapped in the same rock sequence at 300–350°C at 6–10 km under lithostatic and hydrostatic fluid pressure were approximately the same temperature as host rock and define part of a conductive thermal anomaly. The boiling zone developed due to topography‐driven two‐dimensional circulation of meteoric water into the uplift‐induced conductive anomaly, followed by rapid buoyant rise of heated and partially isotopically exchanged water to shallow levels under hydrostatic fluid pressure.

Farther west, near the Alpine Fault, the conductive thermal anomaly has resulted in fluid and rock temperatures of 300–350°C at <5–8 km under lithostatic and hydrostatic fluid pressure. The fluid is mainly meteoric in origin, but has partially exchanged isotopically with the host rock. Minor buoyant rise of fluid has resulted in penetration of hot (200°C) fluid into relatively cool rock at shallow levels (<2 km). Hot springs emanate from the surface above this portion of the hydrothermal system, but these springs are fed by topographically driven meteoric water, which penetrates to only shallow levels in the crust and is isotopically distinct from the deeper fluids.

Keywords:

• fluid inclusions
• boiling
• immiscibility
• fluid pressure
• heat flow
• Southern Alps
• tectonics
• mountains