Hydrothermal Epidote - An Indicator of Temperature and Fluid Composition

Epidote tends to form under a rather restricted temperature range, generally above 250° C. The thermal regime inferred by contouring the depths of the first occurrence of epidote in an area may be compared with fluid inclusion temperatures measured in associated quartz and calcite veins to work out temperature fluctuations during a mineral-depositing event. In geothermal areas, however, thermal regime depicted by the epidote-temperature map may also be compared with the present-day temperature distribution. A case-study is presented here from the Ohaaki-Broadlands geothermal field, New Zealand.

As epidote is deposited by high pH, CO₂-deficient fluids, its association with minerals forming under relatively Low pH and high CO₂ partial pressure conditions, i.e., clay minerals and calcite (only when there is a large scale precipitation) suggests disequilibrium and a possibility of more than one hydrothermal events.