Hydrothermal alteration and thermal evolution of the Wairakei-Tauhara geothermal area
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. The Wairakei-Tauhara geothermal system is situated at the northern end of Lake Taupo, and extends for about 10km in a northwesterly direction from the base of Mt. Tauhara. It is located near the eastern edge of the Taupo Fault Belt, and one major regional fault. The Kaiapo Fault, and several subordinate faults; Te Mihi, Waiora, Wairakei, and Karapiti pass through the active area of the Wairakei section of the field. Cores recovered during drilling for geothermal steam in this field form the basis of this study. The petrographic, fluid inclusion, and geochemical analysis of these samples, in conjunction with a re-interpretation of the fluid chemistry and hydrology, have lead to a number of conclusions. An analysis of the early well discharges indicated that a fluid of 255-263°C containing 1595-1550 mg/kg chloride and about 25-29mMoles of CO2 per 100mMoles of total discharge, ascended along the Kaiapo, Wairakei, and Waiora Faults at Wairakei. Fluid, about a kilometre to the east was both cooler and about 100 mg/kg more dilute. To the west, along the Te Mihi fault, fluid was 10°C hotter and 100mg/kg more concentrated. This, it is inferred, is the result of mixing between the fault controlled hot upflow and cool lateral inflows. The fluid inclusion study here was unusual in that it was almost entirely based upon secondary inclusions that had formed in primary igneous quartz crystals. These inclusions form as a result of the annealing of fractures within the host quartz and can occur at any temperature above about 200°C and below the modern boiling point for depth temperature. Several inclusions indicate that a small additional hydrodynamic component to the fluid pressure of hydrostatic plus 10% exists, although, in one well this appears to have reached 35% at some time. There is, however, no evidence for the large overpressures that are inferred by some for the initiation of hydrothermal eruptions. An alternative hypothesis is put forward that explains these phenomenon as a process of the normal hydrostatic pressures that occur within this system. Namely, that small shallow eruptions can occur under hydrostatic pressure conditions, but propagate in size and depth to match those found as eruption deposits in most systems. Fluid inclusions also indicate that the deep upflow has been both hotter in the past and has contained CO2 contents of up to an order of magnitude greater than that of the pre-exploitation system. This variation in temperature may indicate variation in deep mixing between the hot upflow and the regional groundwater flux. Other aspects of the hydrology are illustrated by clay petrography, which has been used here for the first time to indicate the presence of cool lateral inflows into the pre-exploitation system at Wairakei-Tauhara. Cool water enters at the periphery of the system and flows towards the centre of the upflow, warming and ascending as it does so. The presence of these inflows is further evidence of the fault control that exists on the hot upflow. The hydrothermal alteration mineralogy at Wairakei-Tauhara consists of the assemblage; quartz, chlorite, albite, adularia, smectite-illite, wairakite, epidote, sphene, pyrite, and calcite; they are in thermal equilibrium with the present system. The presence of epidote and adularia in two cold wells, both to the east and west of the present thermal activity, is evidence that either the size, and/or, the location of activity has been different in the past. Epidote-wairakite co-existence can be explained by CO2 contents in the fluid of about twice that of the pre-exploitation system, while variation in the iron content of epidote, suggests that CO2 contents could have been as low as two orders of magnitude less than this at some stages. However, this is consistent with gas loss due to boiling over the temperature and CO2 ranges inferred for the field. Mass transfer within the system can be subdivided into two groups; (a) Constituents that are immobile; SiO2, MnO, MgO, TiO2, Al2O3, total iron, Zr, P2O5, V, Y, Nb, Pb, and Ba. (b) Constituents that are mobile; Na2O, K2O, CaO, Cs, Rb, Fe3+, Fe2+, NH3, H2O+, CO2, H2S, S(O3), Zn, and As. The mass transfers that occur in the second group are the result of five factors; (a) Primary phenocryst alteration (b) Recrystallization of the tuffaceous matrix (c) Proximity to permeable channels (d) Boiling and gas separation (e) Redox reactions At Wairakei-Tauhara, the fluid chemistry, hydrology, mineralogy, and mass transfer, are all interrelated aspects of the system that are a product of the hydrothermal processes at work there.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1988