Skip to Main Content
Skip Nav Destination
GEOREF RECORD

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

K. A. Rodgers, K. L. Cook, P. R. L. Browne and K. A. Campbell
The mineralogy, texture and significance of silica derived from alteration by steam condensate in three New Zealand geothermal fields
Clay Minerals (June 2002) 37 (2): 299-322

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 H (sub 2) S 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 mu m 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 mu m 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.


ISSN: 0009-8558
EISSN: 1471-8030
Coden: CLMIAF
Serial Title: Clay Minerals
Serial Volume: 37
Serial Issue: 2
Title: The mineralogy, texture and significance of silica derived from alteration by steam condensate in three New Zealand geothermal fields
Affiliation: University of Auckland, Department of Geology, Auckland, New Zealand
Pages: 299-322
Published: 200206
Text Language: English
Publisher: Mineralogical Society, London, United Kingdom
References: 36
Accession Number: 2003-085230
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
S47°30'00" - S34°30'00", E166°30'00" - E178°30'00"
Secondary Affiliation: California Institute of Technology, USA, United States
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200324
Close Modal

or Create an Account

Close Modal
Close Modal