Oxygen isotope and hydrothermal alteration studies at Kawerau and Ohaaki-Broadlands geothermal fields, New Zealand

Abstract

This thesis describes the oxygen isotope geochemistry, whole rock major and trace element chemistry and hydrothermal alteration studies of the Kawerau and Ohaaki-Broadlands geothermal fields located in the Taupo Volcanic Zone, North Island, New Zealand. The Kawerau geothermal field has a well preserved record of its history in the form of successive hydrothermal mineral assemblages which are distinguishable both petrologically and isotopically. The field has experienced at least three hydrothermal regimes; alteration assemblages belonging to the earlier events are mainly confined to the basement rocks consisting of Mesozoic greywacke with occasional interbeds of argillite. The spatial distribution of mineral assemblages formed under different regimes allows their demarcation into zones. The earliest hydrothermal regime, evidently associated with the earliest faulting in the area, was responsible for the formation of prehnite and wairakite, possibly during two distinct phases of activity. The Kawerau basement rocks with veins of prehnite and wairakite (wairakite-prehnite zone) are characterised by the paucity of epidote. Addition of CO2 to a fluid in equilibrium with prehnite may have pushed its composition past the epidote stability field to lower aCa++/a2H+values where wairakite was stable; lower pH of the fluid, due to CO2 addition, is suggested by the fact that almost all wairakite bearing cores are characterised by the alteration of primary K-feldspar to illite. The basement rocks belonging to the wairakite-prehnite zone have δO18 values of 5-7 ‰ and have exchanged isotopically with a meteorically derived hydrothermal fluid with a δO18 value of -1 to +1 ‰ in the temperature range of 280 to 300°C. Wairakite, apparently precipitated from this fluid, at about the same temperatures, has a δO18 of about 3.5 ‰. The formation of the wairakite-prehnite assemblage was followed by a period of hydraulic fracturing as a result of large additions of CO2 and consequent increase in total pressure of the system (hydrostatic + CO2 partial pressure) in excess of the local lithostatic pressure. This was followed by the development of a mineral assemblage with abundant calcite in the southern part of the field. These calcite bearing (calcite zone) rocks are typically less altered and hydrothermal phases (mainly calcite and quartz) generally occur within veins. The isotope data from here indicate that marked changes in fluid composition resulted in an abrupt termination of the older meteoric-hydrothermal regime and caused massive precipitation of calcite from boiling and degassing solutions with δO18 values of 4.4 to 8 ‰. The basement rocks belonging to this calcite zone, therefore, have little exchanged δO18 values of 9.2 to 12.64 ‰ and host calcite with similarly high δOl8 of >9 ‰. The isotopic evidence indicate a trend of increasing δO18 of fluid during this event signifying an increasing contribution of magmatic water derived from deeper levels. The hydrothermal events responsible for the formation of wairakite-prehnite and calcite zones are mainly confined to the basement rocks in the southern part of the field. The whole rock geochemistry of cores from here shows addition of Ca, immobility or depletion of K and immobility of Na. The addition of Ca corresponds petrographically to the formation of wairakite and calcite, the behaviour of K is attributable to alteration of K-feldspar to illite and the immobility of Na may be a result of albitisation accompanying the formation of wairakite. Precipitation of minerals during these two early regimes resulted in sealing of fractures in the southern part. The latest mineral assemblage comprising calcite, quartz, calc silicates, illite and adularia is characterised by the absence of prehnite and greater abundance of epidote and clinozoisite. The basement rocks hosting this modern alteration assemblage have δO18 of 3 to 5 ‰. Calcite and wairakite occurring in these rocks have δO18 values of about 3 and <2 ‰, respectively, and are thus isotopically distinguishable from their relict counterparts. Alteration is related to the youngest (Quaternary?) faulting which enabled fluids to move northeastwards after flow was precluded in the southern part due to diminished permeability. There are isotopic indications that uniformly high temperatures, 280 to 290°C, once covered a much larger area than now but sealing of permeable zones and cooling by dilution have reduced the area of high basement temperatures to a small northeastern part of the field. The northern part of the field is, in general, distinguishable from the southern part on the basis of whole rock geochemistry. Cores from the former are depleted in Na, show addition of K and variable trends in Ca. The behaviour of these elements corresponds petrographically to the formation of a calcite-calc silicate-illite assemblage in this part of the field, in equilibrium with the present-day fluid. In this alteration assemblage, plagioclase is invariably altered to illite, K-feldspar is relatively unaltered and the amount of Ca lost due to the alteration of plagioclase is compensated for by the deposition of calcite and calc silicates. The study of alteration mineralogy at Ohaaki-Broadlands reveals two major hydrothermal events. The earlier was characterised by the formation of an epidote-prehnite-adularia assemblage from an alkaline fluid. Subsequent addition of CO2 to this fluid has resulted in a calcite-dominated assemblage which is overprinted onto and overlies the earlier assemblage. The abundance of CO2 in the present-day fluid has in general suppressed the formation of calc silicates; minerals such as quartz, adularia, illite and chlorite are commonly associated with calcite in both the basement and overlying volcanic/sedimentary rocks. However, the situation is completely different in and around major upflow zones, particularly those at shallower levels. The availability of CO2 + steam separated from liquid at deeper levels, and the lithology of the field, have played a dominant role in developing perched zones occupied by CO2 –rich fluid. These zones are characterised by the presence of a wairakite-illite assemblage - a feature typical to Ohaaki-Broadlands. The mineralogical indications for the presence of a relatively low pH fluid in these zones are corroborated by the observed casing corrosions at levels of wairakite occurrence in reservoir rocks. The presence of wairakite in cores and cuttings at Ohaaki-Broadlands may therefore be used in identifying potential zones of casing corrosion. Moreover, its presence suggests relatively less permeable conditions since none of the wairakite-bearing zones at Ohaaki-Broadlands are potential aquifers. An epidote-isograd map shows the lateral and vertical temperature distribution in the field during an earlier thermal regime and comparison with the present-day temperatures suggests that some significant changes have occurred. These are consistent with fluid inclusion homogenisation temperature data. The whole rock geochemisty of basement rocks underlying the Ohaaki-Broadlands geothermal field indicate that Si and Al are either isochemically redistributed or else have been added. The addition of these elements corresponds to the precipitation of quartz and chlorite/illite, respectively. In agreement with the presence of chlorite and/or pyrite, Fe and Mg show a general trend of addition or isochemical redistribution. As primary plagioclase in these rocks is predominantly albite, the depletion and addition of Ca corresponds petrographically to dissolution or alteration of calcic ferromagnesian minerals and deposition of calcite, respectively. The behaviour of Na and K is consistent with the observed alteration of plagioclase to illite. An inferred major upflow zone, encompassing most of the northern part of the field is characterised by the lowest range of whole rock δO18 values, from -0.5 ‰ at 2600 m to about 2.5 ‰ at 1350 m, and a decrease in calculated δO18 water (in equilibrium with quartz and calcite) with depth at a rate of 1 ‰ per km. This behaviour of δOl8 in basement rocks and water is attributed to O18-exchange during dominantly vertical movement of fluid along permeable "flow lines" in the upflow zone. Large cumulative fluid throughput and temperature in excess of 290°C in this upflow zone has apparently facilitated partial O18-exchange in primary quartz, which is here reported for the first time from an active geothermal field. Hydrothermal quartz, in general, trace an evolutionary drop in δOl8 of water from -1.6 to -3.5 ‰. However, some quartz samples have apparently grown in isotopic equilibrium with the present-day conditions. Most of the analysed quartz samples from drillhole BR-7 have relict δOl8 as evidenced by adaptive O18-exchange in co-precipitated calcite and adularia. The latter, due to a fast rate of exchange is in equilibrium under the present-day conditions. Most of the calcites indicate equilibrium with the modern fluid. This is consistent with the petrographic evidence of calcite generally being the youngest mineral deposited during the current hydrothermal regime. The oxygen isotope data indicate that no significant changes in fluid isotopic composition have occurred since the formation of the relict epidote-prehnite-adularia assemblage. It is possible that addition of carbon dioxide to the fluid to reach its present-day concentrations and inception of the current phase of activity is a relatively recent event at Ohaaki-Broadlands. Oxygen isotope data from mineralogically unaltered to intensely altered rocks indicate that substantial oxygen isotope exchange precedes the visible effects of hydrothermal alteration. A comparison of intensity of alteration and extent of oxygen isotope exchange indicates that at least for feldspars, constituting the bulk of the rocks, O18-exchange is a faster process than the accompanying hydrothermal alteration. The application of one-dimensional model of oxygen isotope exchange indicates that the time taken for the evolution of Kawerau and Ohaaki-Broadlands systems to attain close to present-day δO18 composition is of the order of tens of thousands of years. A volumetric water to rock ratio of about 2 is calculated for the both fields.