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Kawerau Geothermal field, Kawerau District, Bay of Plenty Region, New Zealandi

Regional Level Types
Kawerau Geothermal field	- not defined -
Kawerau District	District
Bay of Plenty Region	Region
New Zealand	Country

The Kawerau geothermal field is located immediately east of Kawerau township. Most of the surface thermal features are believed to have been in a natural state of decline prior to development, which arose from the energy requirements of the large Tasman pulp and paper processing plant built in the 1950s. Until the commissioning of the Ohaaki power station at Broadlands-Ohaaki, Kawerau was the second largest producing field in New Zealand and its steam supply for industrial use equates to half of the world's total steam supply for industrial applications.

The town of Kawerau was established in the early 1950's to service the Tasman pulp and paper mill, and its associated geothermal power station. All the Onepu hot springs ceased flowing between 1952 to 1988. Unlike Rotorua, there is little geothermal activity to see on the surface, with the mill constructed next to the spring, and sludge from it, destroying what was left. Most of the voluminous writings about the geothermal field focus on the rock units, and aspects relating to power generation. Specimens like wairakite occur from hydrothermal alteration of the basement greywacke several hundred metres underground. Specimens seen for the location may have come from the numerous drill cores in the area, but is uncertain.

Mount Edgecumbe (Putauaki) (821) is a prominent 5000 year old dacite-andesite multi vent volcano immediately east of the town. The last substantial eruption was in 300 BCE, with no activity since 1850, although there was a major earthquake in the area in 1987.

The geothermal field lies near the north-west axis, and at the southern end of the Whakatane complex regional graben. This has been infilled with various rock units including rhyolites, andesite lava, ignimbrite, and more recent sediments. The area is a series of stepped blocks created by north-east trending faults, and cross-cutting north-west faults. The faults are not often observable on the surface due to alluvium. The Kawerau geothermal field sits on the eastern edge of the Taupo Volcanic Zone, and unlike other fields in the area is not part of a caldera.

The units from the surface are:
1) Recent alluvium- peat, sand, gravel, pyroclastics (10-50 metres thick).
2) Hydrothermal eruption breccia (up to 10 metres thick).
3) Unconsolidated pyroclastics- unwelded pumiceous pyroclastic flows and airfall tuffs (up to 10 metres thick).
4) Onepu Formation- twin surficial domes of rhyodacite with plagioclase and minor quartz, amphibolite, pyroxene, biotite, and magnetite (up to 200 metres thick).
5) Matachina Formation- ignimbrite, partly welded, grey, and vitric tuff, with obsidian, plagioclase, quartz, hornblende, pyroxene (up to 410 metres thick).
6) Tahuna Formation- crystal rich fine sandstone, siltstone, muddy breccia, and unwelded pumice rhyolite lapilli tuff (up to 360 metres thick).
7) Coxton Formation- buried domes of spherulitic and banded rhyolite and intrusive, with quartz, plagioclase, biotite, apatite, and amphibolite (up to 450 metres thick).
8) Karaponga Formation- partly welded crystal lithic tuffs (up to 180 metres thick).
9) Onerahi Formation- tuffaceous to muddy breccias and coarse tuffaceous sandstone (up to 85 metres thick).
10) Kawerau Andesite- augite and plagioclase andesite flows, breccias and tuff (up to 300 metres thick).
11) Raepaphu Formation- partly welded crystal lithic tuffs (up to 165 metres thick).
12) Tasman Formation- muddy breccia, sandstone, and siltstone (up to 25 metres thick).
13) Te Teko Formation- partly welded grey crystal vitric tuffs, with quartz, plagioclase, minor biotite and ferromagnesian minerals (up to 225 metres thick).
14) Rotoroa Formation- tuffaceous sandstone, poorly sorted crystal and lithic siltstone (up to 200 metres thick).
15) Waikora Formation- greywacke pebble conglomerate, and minor intercalated tuff and siltstone (up to 450 metres thick).
16) Basal greywacke.

Hydrothermal alteration in the Mesozoic greywackes has produced three assemblages 1) wairakite-prehnite, 2)calcite 3) quartz-calcite-adularia-calc silicates with illite, abundant epidote and clinozoisite, and some laumonite. The earliest hydrothermal activity produced (1), followed by high gas pressure via a hydraulic fracturing event precipitating (2), and finally more recent hydrothermal activity (3).

Calcite is said to be common near the top of the greywacke, while wairakite is locally abundant. (Tulloch) states calcite comes in uncommon platy forms, as well as prismatic and scalenohedron, associated with sulphides pyrrhotite, with trace sphalerite, galena, chalcopyrite, and covellite. (Bignall et. al., 2012) states alteration minerals are quartz, adularia, albite, clay, calcite, chlorite, titanite, pyrite, and epidote. (Milicich, 2013) adds alunite, halloysite, cristobalite, smectite, kaolinite, tridymite, pyrophyllite, chalcedony, diaspore, dickite, anhydrite, sericite, rutile, and apatite, depending on acidity, temperature, and salinity of the fluids.

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References

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