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

Abstract

AbstractOpaline 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.