The Mineralogy, Texture and Significance of Silica Residue from the Te Kopia Geothermal Field, Taupo Volcanic Zone

Abstract

In the Te Kopia geothermal field, steam, accompanied by HzS, rises through landslide debris at the foot of the Paeroa Fault. The resulting acid-sulfate fluids corrode the variably altered rhyolitic country rocks. Silica precipitates during this alteration process as rinds, botryoidal loaves, and recemented crusts. The initial silica is strongly disordered 0pal-A~ commonly as transparent to translucent hyalite. Depending upon conditions, including humidity, pH, and aluminium activity, this opaline silica can (1) react with aluminous solutions to yield kaolinite, (2) crystallise to microcrystalline quartz plus moganite, or, (3) wholly dissolve in acidified rainwater, fogdrip or additional steam condensate - the ultimate stage in alteration of the country rock. No silica phase intermediate between opal-A and quartz was conclusively identified at Te Kopia, but the presence of small amounts of poorly ordered opal-CT are suspected from skewed x-ray diffraction traces.