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Hydrothermal alteration and evolution of the Ohakuri hydrothermal system, Taupo volcanic zone, New Zealand

Journal of Volcanology and Geothermal Research
Publication Date
DOI: 10.1016/0377-0273(88)90034-0
  • Chemistry
  • Earth Science
  • Geography


Abstract Erosion and excavations at Ohakuri in the Taupo Volcanic zone have exposed the upper portion (100–150 m) of a hydrothermal system that was active sometime between 700,000 and 160,000 years ago. Extensive hydrothermal alteration occurred within a host sequence of young, relatively undeformed, chemically and lithologically similar unwelded rhyolitic ignimbrite and air-fall tuffs. Mapping and petrologic work have identified six distinct alteration types. An early event formed a concentrically zoned suite of alteration through the pervasive movement of alkaline chloride type water. In the innermost zone, primary rock components were almost entirely converted to quartz + adularia ± illite ± hematite ± leucoxene. Mineralized veins and breccias of quartz ± pyrite ± adularia ± chlorite formed here in response to episodic hydraulic fracturing. This zone grades outward and upward into a zone of less intense, lower rank alteration with a mordenite + clinoptilolite + smectite + opal ± hematite assemblage, then a zone of weak clay alteration and into fresh rock. Calcite is conspicuously absent from the entire suite. Acid-sulphate type water, formed from steam-condensate, dominated the shallow activity in a second stage of alteration that followed local erosion. Widespread but discontinuous alteration converted the ignimbrite to kaolinite + opal ± hematite, with alunite occurring in the more intense zones. This alteration locally overprints the early alkali-chloride produced suite, but the focus of the second-stage activity was north of the focus of the older event. Scattered opaline sinters and silicified surficial deposits are products of either still later activity or the waning part of the second stage. Chemical analysis shows that the various alteration types have characteristic patterns of major element addition and removal; these reflect the key hydrothermal mineral reactions that formed the new assemblages. Quartz-adularia alteration involved mainly silicification, dehydration and cation exchange (K + for Na 2+, H +, Ca 2+, Mg 2+), whereas alteration in the mordenite zone was mostly a moderate hydration process. Kaolinite alteration involved strong hydration, hydrolysis and redistribution of silica. Trace elements show varying degrees of mobility and correlation with major elements. Alteration features identify the important upflow zones, zones of mixing between hydrothermal and shallow groundwater, and changes in alkali chloride water level. They also reflect a transition from diffuse to channel flow as sealing eliminated original rock porosity, and led to hydraulic fracturing which maintained fracture permeability in the system. Mineralogy and fluid inclusion studies indicate that the primary fluid at now-exposed levels was a high-pH (7–8), low-CO 2 and low-H 2S water cooler than 200°C, probably modified by boiling at depth.

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