| Abstract |
Twelve wells drilled by the Ministry of Works and Development at the Ngawha geothermal field, Northland, penetrated a 445-730 m thick sequence of deformed, sheared and brecciated Cretaceous-Tertiary tstones, sandstones, claystones, limestones, and argillaceous limestones; overlain by thin 40 m) basalt flows and lake beds. Fine-grained sandstones and argil1ites belonging to the Waipapa Group of Permian to Jurassic age, underlie the younger sediments and are of great, but unknown, thickness. The two groups of sediments contrast in their hydrology. 'The Tertiary rocks are generally soft, incompetent and contain abundant montmorillonite, illite and minor, but widespread, kaol in and chlorite. These rocks do not allow the deep, near neutral, alkali chloride water to reach the surface, only steam and gas, mainly CO. Consequently, heat moves through these rocks by conduction. The Waipapa Group rocks are of very low inherent porosity but can act as the reservoir because of locally high permeabi1ty due to the presence of numerous closely spaced and faults. These controls on hydrology are reflected in the nature of the resultant hydrothermal teration; the Cretaceous-Tertiary rocks record movement of CO by the presence of recrystallised calcite; loca? steam condensation produced minor kaolin and heat transfer by changes in the hydration characteristics and crystal1i n ity of the other clay minerals. These teration features are best seen by X-Ray Diffraction and Differential Thermal Analysis. abundance in the Waipapa Group greywackes and argillite s ; clearly predated the onset of the present geothermal activity, but the youngest calcite, quartz, pyrrhotite and pyrite ) are probably 'modern However, other secondary minerals present in the rocks, both in the matrix and forming veins, probably determine, in some measure, the composition of the circulating reservoir fluid. include i11ite, epidote, orite, c l inozoi site, l y ite, prehni t e and hematite. these rocks are locally Veins of different ages occur in great the formation of many of the veins These minerals |