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The mineralogy, texture and significance of silica derived from alteration by steam condensate in three New Zealand geothermal fields

Published online by Cambridge University Press:  09 July 2018

K. A. Rodgers ,
K. L. Cook ,
P. R. L. Browne  and
K. A. Campbell
K. A. Rodgers*
Affiliation:
Department of Geology, University of Auckland, Private Bag 92019, AucklandNew Zealand
K. L. Cook
Affiliation:
California Institute of Technology, 1201 E. California, Pasedena, CA 91126, USA
P. R. L. Browne*
Affiliation:
Geothermal Institute, University of Auckland, Private Bag 92019, AucklandNew Zealand
K. A. Campbell*
Affiliation:
Department of Geology, University of Auckland, Private Bag 92019, AucklandNew Zealand
*
†Australian Museum, SydneyNSW 2000, Australia
*E-mail:, prl.browne@auckland.ac.nz
‡California Academy of Sciences, San Francisco, CA 94118, USA
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Abstract

Opaline silica residue accumulates on the surface and in the near surface of the Te Kopia, Tikitere and Rotokawa geothermal fields, where rhyolitic tuffs are attacked by steam condensate, made acid (pH 2–3) by sulphuric acid produced by oxidation of H2S that accompanies steam discharge. Silica residue is one product of this alteration process that also yields kaolinite, sulphur, sulphide and aluminous sulphates, including alunite and alunogen, as pH, Eh and available moisture fluctuate across the field surface. Coagulation of colloidal polymeric silica or, possibly, direct deposition of monomeric silica can occur from the acid solutions of the digested country rock, depending on pH, concentration, temperature and the presence and concentration of other species. As with silica sinter, the first-formed silica phase consists of disordered opal-A microspheroids, commonly 0.1–5 μm in diameter. These coalesce and become overgrown by further opaline silica to yield a mass resembling gelatinous ‘frog spawn’ that lines cavities and envelops surfaces. This mass is the principle component of botryoidal, transparent to translucent hyalite that comprises much residue. Following deposition, this juvenile residue may crystallize to opal-CT lepispheres, 1–3 μm across and, subsequently, to chalcedonic quartz. Both the opal-A and opal-CT of the New Zealand residues are more disordered than those occurring in typical moderate- to low-temperature sinters. The opaline silica of silica residues enjoys a reaction relationship with both kaolinite and aluminium sulphates, including alunite and alunogen. These phases and the silica precipitate continuously and undergo dissolution at the surface of all three localities. The precise pathway followed depends upon the prevailing surface conditions, including humidity, pH, Eh, and Al and K activities. As Al is flushed from the system, the ultimate stage of alteration that may result is the dissolution of the silica itself in acidified rainwater, fogdrip or further steam condensate.

Keywords

opaline silicaalterationgeothermal fieldsteam condensateNew Zealand
Type
Research Article
Information
Clay Minerals , Volume 37 , Issue 2 , June 2002 , pp. 299 - 322
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2002

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