Application of Chlorite Geothermometry in Hydrothermally Altered Rotokawa Andesite, Rotokawa Geothermal Field, New Zealand

Abstract

Chlorite is a common hydrothermal alteration product in rocks hosting Taupo Volcanic Zone geothermal systems. It typically replaces ferromagnesian minerals (pyroxene, biotite, hornblende) and volcanic glass, or occurs via direct precipitation in open spaces (vugs, veins, etc). Its composition is dependent on formation temperature, fO2 and pH of the hydrothermal solution and bulk composition of the host rock, in particular the Fe/(Fe + Mg) ratio. Previous workers have used variations in chlorite chemistry as an empirical and thermodynamic geothermometer to determine temperature gradients in hydrothermal systems.

The Rotokawa Andesite is widespread across the Rotokawa Geothermal Field, between 800 m and 1800 m depth, and up to 2 km thick. It is typically moderately to strongly altered to a hydrothermal mineral assemblage dominated by chlorite, but also calcite, illite, albite, quartz, pyrite and epidote. We investigate the use of the chlorite geothermometer on Rotokawa Andesite drillcore, focusing only on this rock unit to minimise the influence of variable host-rock chemistry on chlorite composition. Independent estimates of reservoir temperature were obtained from chlorite geothermometry, chlorite crystallinity (from XRD analyses) and fluid inclusion microthermometry (from quartz and/or calcite vein samples in chlorite-altered host rock) and compared with measured downhole temperatures from well completion tests.

There is reasonable correlation between temperatures derived from chlorite geothermometry and chlorite crystallinity and, in many cases, between chlorite geothermometry and the measured reservoir temperatures. In many samples, “open-space chlorite” has a distinct composition and hence estimated temperature, compared to “replacement chlorite”. This is likely due to the chemistry of “open-space chlorite” being buffered by fluid chemistry, whereas replacement chlorite is buffered by host-rock composition. The combination of chlorite composition, XRD analysis and fluid inclusion microthermometry provides valuable insights into the Rotokawa geohydrology, highlighting fluid pathways within the geothermal system.