Water-rock interaction at the boundary of Wairakei geothermal field

Drilling through the sharp NE boundary of Wairakei geothermal field has provided evidence that stratigraphy and structure constrain fluid movement, that hydrothermal alteration has not produced an extensive zone of self-sealing, and that the position of the boundary has remained essentially static with time. Interlayered permeable tuffs and impermeable siltstones channel hot sind cold flows through the boundary, and a postulated, buried, listric caldera fault may control deep hot fluid movement. The distribution of temperature-sensitive hydrothermal minerals such as epidote and clays suggests the boundary has remained in its present position throughout much of the life of the field, but production-induced pressure decline is now allowing cold water to invade, and the boundary to retreat inwards.