Contrasting Alteration Stylesof Andesitic and Rhyolitic Rocks in Geothermal Fields of the Taupo Volcanic Zone, New Zealand

Abstract

Deep (1-2.25 km) drillholes at several geothermal fields in the Taupo Volcanic Zone, New Zealand, encountered thick sequences of Quaternary rhyolitic and andesitic rocks resting upon Mesozoic greywackes and argillites. The occurrence of andesites is uncommon in New Zealand geothermal fields but here allowscomparison of their alteration with that of more silicic tuffs and in a hot environment in which circulate dilute alkaline chloride fluids of nearneutral Fluid inclusion measurements show that the observed hydrothermal minerals formed under the present field conditions.

The andesites are of medium K,calc-alkaline, orogenic type which, where fresh, contain calcic-plagioclase, augite, hypersthene, titanomagnetite and occasional hornblende. These phases typically alter to chlorite, calcite, epidote, adularia, illite, pyrite and quartz, but veins mainly consist of different proportions of calcite, epidote, wairakite, adularia, chlorite and pyrite. The silicic volcanic rocks commonly alter to the same assemblages but the intensity of alteration of the two rock types differs considerably depending mainly upon how fluids move through them. Alteration intensity of the andesites changes sharply even over very short distances (order of because thermal fluids in them via joints; by contrast, the silicic volcanic rocks commonly have a much more pervasive and homogenous alteration style consistent with thermal fluids penetrating them along grain boundaries and through interconnected pores.