| Abstract |
Four exploration wells at the Mangakino geothermal field reveal a thick sequence of flat-lying ignimbrites and an alteration pattern similar to other active geothermal fields in the Taupo Volcanic Zone (TVZ). Rocks encountered in the deepest well, MA2 (3192m), are interpreted as thick caldera infill with five separate ignimbrites identified, the thickest being >800m thick. Surficial ignimbrites exposed nearby have distinctive mineralogies and crystal contents, which have enabled correlations to be made with down-hole lithologies. The alteration mineralogy of cuttings and core samples is similar to other geothermal fields in TVZ. Ca-silicate minerals (epidote, wairakite) occur in MA2 below 2200m, and adularia replaces, wholly or in part, primary andesine. Other hydrothermal minerals present include chlorite, quartz, calcite, titanite and pyrite. Measured downhole temperatures reach 250?C at 3000m in MA2 and are consistent with the mineralogical assemblages represented in the youngest phase of alteration. Fluid inclusions in secondary calcite show high temperatures (300 & 315?C) while inclusions in primary quartz show ~165?C (the current temperture at the depth of the core), recording current conditions. This evidence for two different temperatures in the fluid inclusion data and two episodes of alteration may indicate a previous thermal event which is possibly related to dike intrusion on the periphery of the Mangakino field |