SILICA FROM STEAM CONDENSATE ALTERATION AT TIKITERE, NEW ZEALAND

Abstract

Silica residue is the main product of the alteration of a rhyolitic, tuff-rich, volcanic substrate by acid sulfate condensate formed, in a major steam field at Tikitere, near Rotorua. Here steam and accompanying H2S discharge in an area characterized by large standing pools of acid water and mud pots. As in silica sinters, that precipitate from hot, near-neutral, alkali chloride waters, the first formed silica phase in the residue consists of non-crystalline opal-A that subsequently crystallizes to paracrystalline opal-CT. Residue and sinter can superficially resemble each other and there is a need to clearly differentiate both types of deposit in order to correctly interpret the hydrology or paleohydrology of a geothermal field. Both the opal-AN and opal-CT in the Tikitere residues are more disordered than those in typical sinters. Tikitere residue also lacks mesoscopically-recognizable, biologically mediated textures, vascular plant remains, and unaltered tuffaceous clasts, commonly present in silica sinters. However, once crystallized to opal-CT lepispheres (1-3 mm across) and, subsequently, to quartz, both residue and sinter are less readily distinguished as earlier textures become overprinted.