Proceedings of the 7th New Zealand Geothermal Workshop 1985. Successive hydrothermal events, as indicated by oxygen isotope composition and petrography of greywacke basement rocks, Kawerau geothermal field, New Zealand. New Zealand Geothermal Workshop [7, 6-8 Nov. 1985, New Zealand].

Absar, A. Blattner, P.

1985

1067
CIENA

0-86869-065-1

Fifteen drillholes at the Kawerau geothermal field penetrated a sequence of Quaternary volcanic rocks overlying Mesozoic 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 their intensity of alteration, which in turn was compared with their oxygen isotope composition. Mineralogically, these greywackes show vertical as well as lateral variations, with primary quartz forming from 10 to 34 percent and feldspars form 22 to 49 percent 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-calc silicates. In terms of a relative scale (0 to 100), the intensity of their hydrothermal alteration varies from 26 to 84, with most lying in the range of 69 to 77. The measured 180-composition of 25 whole rock grey-wacke cores ranges from 2.90 to 12.64 percent 18o SMOW with 10, having values between 4.32 to 5.01 percent. The calculate pre-alteration 180 value for these rocks is 9.9 to 13.0 percent. The 18o values of geothermal calcites fron greywackes have a range of 3.0 to 10.2 percent. The high 18o calcite values are hosted by little exchanged rocks from wells KA 25 and 29 in the Southeast part of the field. These calcites have precipitated from a high 18o fluid (+3.65 percent), such a fluid would not be capable of shifting the host greywacke isotopic composition down to 5 percent at measured at 1420 m in KA29.. ABSTRACT. INTRODUCTION. Quantitative mineralogy. HYDROTHERMAL ALTERATION. Intensity of alteration. Alteration of feldsparsChronology of mineral veins. Relative abundances of calcite and calc-silicates. OXIGEN ISOTOPE COMPOSITION OF GREYWACKE. Pre-alteration 180 composition. 180 composition of hydrothermally altered greywacke. Oxygen isotope composition of minerals. DISCUSSION. CONCLUSIONS. ACKNOWLEDGEMENTS. REFERENCES.

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NUEVA ZELANDIA; ENERGIA GEOTERMICA; GEOTERMIA; CAMPOS GEOTERMICOS; GEOVULCANOLOGIA; GEOQUIMICA;