Successive Hydrothermal Events, as Indicated by Oxygen Isotope Composition and Petrography of Greywacke Basement Rocks, Kawerau Geothermal Field, NZ

Abstract

Fifteen drill holes at the Kawerau geothermal field penetrated a sequence of Quaternary volcanic rocks overlying Hesozoic greywackes and argillites in the depth range of 650 to 1220m below sea level. Maximum temperature in the basement is 250 to 303?C. Twelve greywacke cores were modally analysed in order to determine the i r intensity of alteration, which in turn was compared with the i r oxygen isotope composition. Mineralogical ly, these greywackes show vertical as well aslateral variations, with primary quartz forming from 10 to 34% and feldspars from 22 to 49% of the rocks. On the basis of their hydrothermal mineral assemblages the samples can be divided into three distinct groups characterised by the abundance of (a) wairakite- prehnite ( b ) calcite, ( c ) quartz-calcite-adularia-calcsilicates. In terms of a relative scale (0 to loo), the intensity of their hydrothermal alteration varies from 26 t o 84, with most lying in the range of 69 t o 77.