Aspects of the Chronology, Structure and Thermal History of the Kawerau Geothermal Field
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Aspects of the Chronology, Structure and Thermal History of the Kawerau Geothermal Field By Sarah Dawn Milicich A thesis submitted to Victoria University of Wellington in fulfilment of the requirements for the degree of Doctor of Philosophy in Geology Victoria University of Wellington School of Geography, Environment and Earth Sciences Wellington, New Zealand 2013 ii „It is what it is‟ id est quod est J.L. Wooden (pers. comm. 2010) iii iv ABSTRACT The development and management of high-temperature geothermal resources for electrical power generation requires accurate knowledge of the local geological conditions, particularly where they impact on the hydrology of the resource. This study is an integrated programme of work designed to develop new perspectives on the geological and structural framework of the Kawerau geothermal resource as a sound basis for field management. Although the geological approaches and techniques utilised in this study have previously been used, their application to an integrated study of a geothermal system in New Zealand has not been previously undertaken. Correlating volcanic and sedimentary stratigraphy in geothermal areas in New Zealand can be challenging due to similarities in lithology and the destruction of distinctive chemical, mineralogical and textural characteristic by hydrothermal alteration. A means to overcoming these issues is to utilise dating to correlate the stratigraphy. Zircons are resistant to the effects of typical hydrothermal conditions and were dated using SIMS techniques (SHRIMP-RG) to retrieve U–Pb ages on zircons. These age data were then used to correlate units across the field, in part aided by correlations to material that had previously been dated from fresh rock by 40Ar/39Ar techniques, and used to redefine the stratigraphic framework for the area. To aid correlation using U–Pb ages and petrographic descriptions, a 3–D visualisation of the geology was built using Leapfrog Geothermal modelling software. The ability to model geology in a 3–D interface has provided new insights regarding the geological framework, controls on deep–seated permeability, volcanic and basement stratigraphic correlations, and geothermal system evolution in the Kawerau area. Some key insights into the stratigraphy in the area include: Numerous coherent bodies of rhyolite, previously described as individual eruptive units, have been shown to be a series of extrusive domes and their associated intrusive equivalents emplaced around 0.36 ± 0.03 Ma (domes and sills) and 0.138 ± 0.007 Ma (dikes and domes still exposed at surface). Below the regional marker plane of the 0.32 Ma Matahina ignimbrite, three main groups of ignimbrites occur, with ages around 1.45 Ma, 1.0 Ma and 0.6 – 0.5 Ma, which are separated by sediment–dominated intervals (accumulated at average rates of 0.06 mm/yr) and one section of andesitic lavas. All of these ignimbrites represent marker horizons from other volcanic centres and do not reflect the presence of local magmatic heat sources. v The earliest dated ignimbrites yield age estimates of c. 2.4 and 2.1 Ma, consistent with these units being distal southern Coromandel Volcanic Zone deposits, pre–dating TVZ activity. The correlation of regional stratigraphic markers allowed inferences into the structural development of the area. Prior to 1.5 Ma, faulting beneath Kawerau was dominantly on northwest–southeast strike–slip structures, with associated development of half grabens in which accumulated sediment–rich intervals (greywacke gravels, and fine sediments with interbedded tuffs). Post–1.5 Ma normal faulting has accompanied episodic subsidence of the Kawerau area, with fault movement along northeast–southwest structures (associated with the geometry of the modern TVZ) and the reactivated northwest– southeast structures associated with most displacement in the area prior to 1.5 Ma. Net subsidence rates inferred from depths to key units do not reflect the present–day situation. Modern rates of tectonic subsidence (2 ± 1 mm/yr) associated with TVZ rifting processes can only have been active for no more than only ~50,000 years, based on elevation differences of the Matahina ignimbrite top surface. The disparity between emplacement of coherent rhyolite as sills at 0.36 Ma and dikes at 0.138 Ma reflects a shift in orientation of the principal stress axes in response to initiation of the modern TVZ rifting regime and the onset of the modern Whakatane Graben. Although previously inferred to be a long–lived system, the modern Kawerau Geothermal Field is a Holocene entity reflecting the rejuvenation of magmatic heat flux associated with Putauaki volcano superimposed on an area of multiple reactivated fault structures, sporadic magmatism and variable rates of subsidence. This study documents past patterns of fluid flow, temperatures and chemistry, and inferred permeability within the field. Using textural relationships in selected samples, the relative timing and patterns of hydrothermal alteration, and fluid flows can be established. These textural relationships are then calibrated against fluid inclusion palaeotemperature measurements and isotope data and related to temperatures and compositions of past fluids. Short–lived heat sources beneath the field resulted from local magma intrusions, and are responsible for the 0.36 Ma and 0.138 Ma rhyolites and Holocene eruptive activity of Putauaki andesite–dacite volcano. The Putauaki activity is inferred to be responsible for the thermal and alteration characteristics of the modern system. vi ACKNOWLEDGEMENTS Firstly I would like to thank my primary supervisor, Colin Wilson for guiding me and sharing his knowledge, it has been a pleasure to work with you. I also thank my co–supervisor Greg Bignall for many random questions and discussions, I‟m sure you will be happy with me not knocking on your door so frequently. Thanks also go to my co–authors on papers published from this work, Ben Pezaro, Bruce Charlier, Joe Wooden, Trevor Ireland and Candice Bardsley, your comments and discussions were invaluable. I would like to acknowledge funding support from Mighty River Power Ltd. and a Doctoral Research Scholarship from Victoria University which made this work possible. Additional funding came from support to CJNW and GB via the GNS Science CSA (Core Science Area) Geothermal Research Programme. Additional funding for CJNW came from the FRST Programme “Deep Geothermal Resources” through subcontracts from The University of Auckland, by courtesy of Mike O‟Sullivan. I would like to thank Mighty River Power Ltd, along with that of Ngati Tuwharetoa Geothermal Assets Ltd, for permission to publish the data used in this thesis. Thanks also go to A8D Trust, Putauaki Trust, Te Tahuna Putakuaki Trust and other local Maori trusts for access to well core and cuttings, and Darren Gravley for the Onepu Dome sample. Simon Barker and Nick Burrows undertook the mineral separations. Peter Holden, Brad Ito and Jorge Vazquez are thanked for their help at various stages in obtaining the ion probe age determinations at the USGS–Stanford University and Australian National University facilities. Samantha Alcaraz for help with learning how to drive Leapfrog Geothermal and editorial comments on the paper submitted for the 3–D modelling. Thanks to Aranz Geo Ltd. for a Leapfrog Geothermal research license which allowed me to undertake 3–D modelling. Thanks also to Michael Rosenberg for valuable comments and discussion on Kawerau related things in general, Mark Simpson for discussions on fluid inclusion microthermometry and Isabelle Chambefort and Kevin Faure for discussion and editorial comments on stable isotope data. Thanks to Sally Potter for being a fantastic office mate and support and Nellie Olsen for many skiing and tramping adventures. Melissa Rotella, Simon Barker, Alexa Van Eaton, Katy Chamberlain, George Cooper, Aidan Allan and many others at the uni, thanks for you guidance (particularly as a remote student). Finally to Mum, Dad and Ross, thank you for your love, support and encouragement. I couldn‟t have done it without you. vii viii DEDICATION To Mum and Dad …endless support... ix x CONTENTS ABSTRACT ............................................................................................................................................................................ V ACKNOWLEDGEMENTS ...................................................................................................................................................... VII DEDICATION ........................................................................................................................................................................... IX PUBLICATIONS ARISING FROM THIS THESIS ......................................................................................................... XVII CHAPTER 1 THESIS INTRODUCTION ......................................................................................................................... 1 1.1 Thesis outline ................................................................................................................................... 3 1.1.1 Concept statement .......................................................................................................... 3 1.1.2 Background and motivation of study .......................................................................... 3 1.1.3 Scope of work .................................................................................................................