Abstract
Deep drilling in the Ohaki-Broadlands geothermal area with mineralogical examination of hydrothermally altered volcanic rocks at levels down to 7000 ft (2134 m) and the chemical analysis of coexisting fluids, shows that hydrothermal mineral alteration at temperatures of 230 to 290 degrees at deep levels in permeable rocks tends to an equilibrium assemblage of K-mica, K-feldspar, albite, chlorite, calcite, and quartz (and occasionally wairakite). The deep waters are dilute (ca. 1200 ppm Cl) chloride-bicarbonate solutions containing appreciable carbon dioxide (m <sub>CO2</sub> = 0-15). Individual minerals and their occurrences are discussed briefly, and stability diagrams are developed to correlate observed rock alteration mineralogy with water chemistry. Changes in water composition caused by boiling off steam and CO <sub>2</sub> can be related to changes in mineral assemblages, such as calcite precipitation and adularia and epidote formation. Variations in mineralogy are discussed in terms of temperature, permeability, water composition, and carbon dioxide concentrations, and a comparison is made with alteration at Wairakei where the carbon dioxide concentration is lower. Pyrite, pyrrhotite, chalcopyrite, sphalerite, and galena occur at depth in some Broadlands altered rocks and are mentioned briefly.
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