Evolution of the Ohakuri Geothermal System

Abstract

Mapping and petrographic study of hydrothermal alteration at the now-extinct Ohakuri geothermal field has revealed a long history of activity which varied in nature lacation. neutral to alkaline chloride-type water through rocks now exposed was followed by extensive nearsurface flow of acid-sulphate water, with minor hot -spring discharges, which fdrmed scattered sinters, comprising the last a c tivity. High permeability of the porous host rocks facilitated fluid movement, and regional hydrology may have significantly controlled flow patterns, whereas control by tectonic fractures appears to have been minor. in rock sealed by silicification was sustained by hydrofracturing which resulted in veining and local brecciation. temperatures probably did not exceed and were largely less than at the levels observed. The distribution of hydrothermal features suggests that upflow zones migrated northward and westward with time, and that the now-active Atiamuri geothermal system is probably descended from the fossil Ohakuri field. Early flow of Fluid