Te Kopia geothermal area, Rotorua District, Bay of Plenty Region, New Zealandi

Regional Level Types
Te Kopia geothermal area	Area
Rotorua District	District
Bay of Plenty Region	Region
New Zealand	Country

A large and vigorously active geothermal area located in the Taupo Volcanic Zone.

Alkali-chloride waters discharged from hot pools and geysers recently as 3,000 years ago. Today surface thermal manifestations consist of boiling acid pools, small steam vents, mudpots and steaming ground, all typical of acid-sulfate thermal activity. The latest surface alteration event, now in progress, involves interaction of all the previously altered rocks by steam, gases and steam condensate to yield an assemblage initially dominated by kaolinite ± opal-A ± alunite ± alunogen (Martin et al., 2000)

The geothermal area is 39 kilometres south of Rotorua. Turn off State Highway 5 onto the Waikite Valley Road, then after 9 kilometres turn south onto Te Kopia Road, and travel another 8 kilometres.

The area is a conservation reserve, and collecting specimens is illegal.

The country rock is the Te Kopia Ignimbrite containing 1) lower hard dark grey rock, with much crystal fragments; 2) middle 50 foot thick member of chaotic tuff breccia, containing rhyolite and andesite blocks up to 5 feet across; 3) upper 90 foot thick member of black to brown shiny vitrophyre (volcanic rock with large crystals embedded in glassy groundmass), with a thin cover of dark grey tuff.

The geothermal area has been active for 120,000 years. Much of the literature is about how steam and acid sulphate water is altering further the ignimbrite rocks, as noted above. The site has been extensively written about with some describing alunogen, meta-alunogen, and alunite, all closely related, as well as some silica, all with similar physical location descriptions. Silica, opaline silica, quartz, opal-A, opal-C, hyalite, and chalcedony described in several references probably needs pictures for the reader to gain a clear understanding of the differences of each at the location.

(Martin, et. al., 1999) Alunogen and meta-alunogen form white fibrous tangled masses, consisting of thin micro platy crystals. Meanwhile steam condensate forms alunogen and tamarugite efflorescence. Minor constituents include potash alum, and halotrichite-pickeringite (and several other 'possible' species).

(Bignall, 1994) Surface alteration to kaolin, cristobalite, alunite, hematite, and jarosite. In the 1960's some deep drill holes were sunk at the geothermal area, and under 500 metres depth the alteration was quartz-albite-adularia, with variable amounts of chlorite, pyrite, calcite, wairakite, epidote, pyrrhotite, titanate, leucoxene, sericite, clinozoisite, illite, and rare almandine. (Noted for interest, but species not added to Mindat due to depth). Mordenite is noted in cooler areas (?).

(Cook, et. al., 2000) describes silica alteration as rinds, botryoidal loaves, and recemented crusts, caused by steam with acid sulphate fluids corroding the rocks. The silica forms opal-A and hyalite (and morganite?). The rind silica is described as a 3 mm layer of white glistening silica coating ignimbrite boulders, often discoloured from iron oxides and biological matter. Some is also viewed as water clear hyalite. Crustose silica is described unflattering as scab-like, grey to off white opaline crusts up to 5 mms thick, sometime crudely layered, and containing sometimes quartz crystals below the layers. Loaf silica is masses of dense white translucent botryoidal silica up to 20 mms thick, with a thin hyalite layer, resembling 'frog spawn', below the ground surface, and immediately above clay red hematite stained ignimbrite. Also noted is spheroid chalcedony 0.5-2 mms, and under 1 mm framboidal pockets of pyrite clusters. Leucoxene is added to the expanding list of alteration minerals in ignimbrite.

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