The Tokaanu-Waihi geothermal system

Abstract

The Tokaanu-Waihi hydrothermal system is located on the flanks of the Kakaramea-Tihia andesite massif in difficult terrain within the Tongariro volcanic centre. TheTongariro volcanic centre is located at the southern end of the Taupo Volcanic Zone(TVZ) and extends southeast from the Taupo volcanic centre to Mt Ruapehu.The geological structure of the Tokaanu-Waihi area is complicated and poor exposures mask stratigraphic relationships. Gravity studies show, however, that the Tokaanu-Waihi hydrothermal system lies on the boundary between a gravity low,representing the unconsolidated pyroclastic material infilling the Taupo Caldera, and the Tertiary sediment-filled graben within the Waipapa greywacke basement. The wider Tokaanu-Waihi area is characterised by a short wavelength magnetic low that is elongate to the east and has a very steep boundary to the west and north of the field area. The amplitude of this magnetic low is caused by hydrothermal fluids causing a demagnetising of the primary minerals within the rocks.Apparent resistivity values of less than 20 Q m measured over the central parts of the Tokaanu-Waihi field indicate the presence of geothermal waters at approximately 250 m depth. In the east, the resistivity boundary of the field is well defined to a few hundred metres depth and lies close to the Tokaanu Tailrace canal. At greater depths,however, the deeper penetrating MT soundings show a low resistivity layer persevering further to the east. In Lake Taupo, a low resistivity zone (<15 Q m) extends under the lake for at least 1 km northeast in Waihi Bay. This area is offshore from Waihi Village where hot springs and seepages have been mapped up to 50 m into the lake.The low apparent resistivity area is coincidental with the area of demagnetised rock indicated by the negative magnetic anomaly. A band of intermediate resistivity values and the magnetic low anomalies on the northern side of Waihi Bay present a nelongate pattern along the Waihi fault trace. This pattern represents hydrothermalalteration from thermal fluids moving along the fault and discharging in the deeper regions of Waihi Bay.The hydrological model of the system shows that beneath the area west of the Hipaua,Waihi and Tokaanu thermal areas, hot water of approximately 240°C with a chloride content of 2200 mg/kg and an initial stable isotope composition of 518O=-4.0 and SD= -40 ascends within thermally altered rocks. The flow of hot water to the east of the upflow area is through channels associated with permeable basement fractures,which intersect younger fractures in the overlying volcanic rocks. There is a strong linear relationship between the B/Cl ratio of water discharges located within the resisitivity boundary which indicates that the waters at Tokaanu and Waihi have a common origin within the system.In the Tokaanu Domain the springs are neutral sodium-chloride waters that show evidence of chloride concentration from boiling resulting in values up to -3500mg/kg. The transitional chemistry from the features in the Tokaanu Domain through to the Waihi foreshore features indicate that there is mixing between high boron and chloride waters with less mineralised waters. Other variations in chloride and magnesium concentrations, temperature and flow rate were measured in the springs.These were related to the rise and fall in the controlled water level of Lake Taupo. The total heat loss from the Tokaanu-Waihi thermal areas has been calculated from the chloride flux at a minimum of 72 MW ± 14 MW. A second value of 64 MW 17MW was also calculated using an energy balance model.The major constituents in the Tokaanu-Waihi steam discharges are carbon dioxide,methane, nitrogen and hydrogen sulphide. Gas-geothermometers based on H2/Ar and CO2/Ar ratios indicate that deep equilibration temperatures lie between 200-300°C.This is consistent with the gas equilibration temperatures based on carbon dioxide,hydrogen, methane and argon, which suggest temperatures between 150 and 300°C.The lower temperature is interpreted as being a result of sampling secondary steam with lower H2/Ar or re-equilibration by hydrogen to cooler conditions as the fluid reaches shallower parts in the system.Relative N2/Ar and CO2/He ratios in steam discharges vary in a systematic manner with tectonic setting. N2/Ar ratios with values of >100 are typical of gases associated with andesitic magmatism reflecting the addition of nitrogen from subducted sediments. Discharges associated with this type of magmatism are found at Kawerau,Broadlands-Ohaaki and Rotokawa along the eastern boundary of the TVZ. Fields located over the western areas of the TVZ, discharge fluids with much lower CO2/Heand N2/Ar ratios as seen at Wairakei and Mokai. This group of discharges originate from high-alumina basalts, depleted in subduction volatiles and are understood to pond beneath the zone of crustal extension. Ratios of CO2/He and N2/Ar in the Tokaanu-Waihi discharges are high and similar in value to those found discharging at Kawerau and Rotokawa. The tectonic environment from which the Tokaanu-Waihi discharges originate has been interpreted from chemical constituents in both the water and gas phases. The B/Cl ratios for the Tokaanu-Waihi waters range from 0.03-0.04. These values are high and represent parent waters typically deriving from andesitic arc typemagmatism. Sharp increases in B/Cl ratios to 0.06 along the eastern margin of the TVZ can be explained by the mobilisation of boron from the greywacke due to andesitic-dacitic magmatic intrusions. The location of the rhyo-dacitic domes of Maunganamu, within the resistivity boundary, and Kuharua, close to northwestern shallow resistivity boundary, supports this model.The geochemical data strongly indicates derivation of the Tokaanu-Waihi source fluid from a tectonic environment similar to those of the Kawerau, Rotokawa and OhaakiBroadlands fields. The origin of the volatiles in the Tokaanu-Waihi discharges are considered in terms of the interaction of volatiles released due to the dehydration of subducted sediments and melting of the mantle wedge. These processes lead to the formation of a volatile charged magma beneath the Tokaanu-Waihi hydrothermal system. During the convective rise of this magma away from the subduction slab it undergoes fractionation, crystallisation and assimilation forming andesite melts. At the surface these melts have been extruded within the Tongariro volcanic centre to form Kakaramea-Tihia, Pihanga, Tongariro and Ruapehu volcanoes