title: Alteration mineralogy and whole-rock geochemistry of the Tauhara Geothermal Field

authors: Mauriohooho, Kate

subjects: geothermal

abstract: The Tauhara Geothermal Field lies over a central zone of rhyolitic volcanism and caldera formation in the TVZ where extension allows magma to penetrate the crust and NE-SW oriented faults permit hot fluids to ascend and convect as individual hydrothermal systems.  The Jurassic greywacke basement hosts the heat source critical to geothermal systems, the Waiora Formation is the permeable reservoir hosting fluids and the Huka Falls Formation is the cap rock to the system. 304 drill cutting samples and six drill core samples were collected from three wells across the Tauhara field (TH9, TH10 and TH12) at 20 m intervals, and a variety of petrographic, mineralogical and geochemical techniques were applied to the samples.  New information on fluid flow pathways (feed zones), alteration types, alteration zones, hydrothermal conditions and mineral inferred temperatures were identified and interpreted from the data collected.

Using both alteration mineralogy data (from powder x-ray diffraction, short wave infrared or SWIR, and petrography) and geochemical data (from x-ray fluorescence and portable x-ray fluorescence), three main zones of alteration were identified across all wells:  A shallow zone of argillic to intermediate argillic alteration followed by a transition zone with increasing depth as propylitic alteration begins to dominate and an intense propylitic alteration zone at the deepest levels.

Mineral inferred temperatures are generally cooler than measured well temperatures in all wells except in TH10.  Here, mineral inferred temperatures agree with hotter measured well temperatures (> 280 °C) and strong potassic alteration indicates that TH10 is closest to the major upflow zone, making south Tauhara a prospective area for geothermal production.  In TH12, mineral inferred temperatures are inconsistent with measured well temperatures implying a recent increase of fluid temperature.  However petrographic evidence of an illite-calcite overprint suggests a cooler fluid at the bottom of well TH12 which is supported by petrography in this study showing pervasive illite alteration.  TH12 is perhaps more marginal to the upflow zone and the change in temperature regime may be related to blind faults at depth bringing the cooler fluid in.  An alteration halo based on geochemical data was identified in the vicinity of the dike in TH9 at ~1400 mRF by high alteration index values determined by pXRF data, which is useful because illite and chlorite detected by SWIR did not show any appreciable mineralogical alteration halo.

The pXRF technique on cuttings correlated well with lab XRF on powdered cutting samples crushed using a ring mill, showing excellent correlation (rs >0.8) for elements As, Ba, Ca, K, Nb, Rb, Sr, Y, and Zr but poorer correlation (rs <0.7) for others.  Samples analysed by XRF were classified as rhyolite/dacite or andesite/andesite-basaltic.  One sample was reclassified as andesite/andesite-basaltic rather than ignimbrite and this finding was corroborated by downhole pXRF geochemical data which show it belongs in the andesitic lithic crystal breccia unit. These results mean that downhole pXRF geochemical trends of immobile elements (Zr, Y, Nb) can be used to refine lithological boundaries, which cannot be identified from visual logging alone.  In addition, downhole immobile element trends are able to separate lithology packets and distinguish subunits or internal stratigraphic variations.  Conversely, variations in mobile element concentrations (Ba, K, Ca, As, Rb and Pb) indicated feed zones or permeable zones of fluid movement by depletions or elevations in element concentrations.  The (AlOH / H2O) ratio derived from SWIR data was plotted against depth downhole and changes in the ratio appear to coincide with some of the locations of feed zones (fluid pathways) in TH9 and TH12.

This study is the first to apply pXRF to geothermal drill cuttings and has proven that the data collected can be used to aid visual logging, and quantify alteration intensity.  Application of rapid geochemistry on existing and future wells will generate detailed geology models and enable correlation between wells and across fields.  This newly developed approach can be applied worldwide to any geothermal fields, provided that appropriate quality assurance techniques are used to ensure consistency of data.

text: Alteration mineralogy and whole-rock geochemistry of the Tauhara Geothermal Field The Tauhara Geothermal Field lies over a central zone of rhyolitic volcanism and caldera formation in the TVZ where extension allows magma to penetrate the crust and NE-SW oriented faults permit hot fluids to ascend and convect as individual hydrothermal systems.  The Jurassic greywacke basement hosts the heat source critical to geothermal systems, the Waiora Formation is the permeable reservoir hosting fluids and the Huka Falls Formation is the cap rock to the system. 304 drill cutting samples and six drill core samples were collected from three wells across the Tauhara field (TH9, TH10 and TH12) at 20 m intervals, and a variety of petrographic, mineralogical and geochemical techniques were applied to the samples.  New information on fluid flow pathways (feed zones), alteration types, alteration zones, hydrothermal conditions and mineral inferred temperatures were identified and interpreted from the data collected.

Using both alteration mineralogy data (from powder x-ray diffraction, short wave infrared or SWIR, and petrography) and geochemical data (from x-ray fluorescence and portable x-ray fluorescence), three main zones of alteration were identified across all wells:  A shallow zone of argillic to intermediate argillic alteration followed by a transition zone with increasing depth as propylitic alteration begins to dominate and an intense propylitic alteration zone at the deepest levels.

Mineral inferred temperatures are generally cooler than measured well temperatures in all wells except in TH10.  Here, mineral inferred temperatures agree with hotter measured well temperatures (> 280 °C) and strong potassic alteration indicates that TH10 is closest to the major upflow zone, making south Tauhara a prospective area for geothermal production.  In TH12, mineral inferred temperatures are inconsistent with measured well temperatures implying a recent increase of fluid temperature.  However petrographic evidence of an illite-calcite overprint suggests a cooler fluid at the bottom of well TH12 which is supported by petrography in this study showing pervasive illite alteration.  TH12 is perhaps more marginal to the upflow zone and the change in temperature regime may be related to blind faults at depth bringing the cooler fluid in.  An alteration halo based on geochemical data was identified in the vicinity of the dike in TH9 at ~1400 mRF by high alteration index values determined by pXRF data, which is useful because illite and chlorite detected by SWIR did not show any appreciable mineralogical alteration halo.

The pXRF technique on cuttings correlated well with lab XRF on powdered cutting samples crushed using a ring mill, showing excellent correlation (rs >0.8) for elements As, Ba, Ca, K, Nb, Rb, Sr, Y, and Zr but poorer correlation (rs <0.7) for others.  Samples analysed by XRF were classified as rhyolite/dacite or andesite/andesite-basaltic.  One sample was reclassified as andesite/andesite-basaltic rather than ignimbrite and this finding was corroborated by downhole pXRF geochemical data which show it belongs in the andesitic lithic crystal breccia unit. These results mean that downhole pXRF geochemical trends of immobile elements (Zr, Y, Nb) can be used to refine lithological boundaries, which cannot be identified from visual logging alone.  In addition, downhole immobile element trends are able to separate lithology packets and distinguish subunits or internal stratigraphic variations.  Conversely, variations in mobile element concentrations (Ba, K, Ca, As, Rb and Pb) indicated feed zones or permeable zones of fluid movement by depletions or elevations in element concentrations.  The (AlOH / H2O) ratio derived from SWIR data was plotted against depth downhole and changes in the ratio appear to coincide with some of the locations of feed zones (fluid pathways) in TH9 and TH12.

This study is the first to apply pXRF to geothermal drill cuttings and has proven that the data collected can be used to aid visual logging, and quantify alteration intensity.  Application of rapid geochemistry on existing and future wells will generate detailed geology models and enable correlation between wells and across fields.  This newly developed approach can be applied worldwide to any geothermal fields, provided that appropriate quality assurance techniques are used to ensure consistency of data.

year: 2015

title: Are all teenagers emotionally illiterate? Charting a neuroscientific storying of normative deficit

authors: Payne, Monica A.

subjects: adolescents

abstract: Defining ‘emotional illiteracy’ is a task located within the broader context of expert (and subsequently public) assumptions regarding the normally expectable competencies of the age group concerned. In the late 1990s a series of neuroscientific studies reporting adolescents' limited ability to recognize emotional states from facial expressions seemed to present radically new developmental benchmarks. Although these studies were subsequently subjected to considerable methodological and interpretive criticism, some incautious assertions regarding teenagers' general inability to respond appropriately, especially in emotionally charged situations, continued to flourish. This paper charts the creation and maintenance of these ideas over the past decade to illustrate how, when primary sources are not carefully checked, powerful messages for which there is dubious empirical evidence can become incorporated into expert advice. It also suggests the importance of linking neuroscientific claims to other strands of contemporary Western efforts to define and contrast normative and disordered emotional behaviour in adolescence.

text: Are all teenagers emotionally illiterate? Charting a neuroscientific storying of normative deficit Defining ‘emotional illiteracy’ is a task located within the broader context of expert (and subsequently public) assumptions regarding the normally expectable competencies of the age group concerned. In the late 1990s a series of neuroscientific studies reporting adolescents' limited ability to recognize emotional states from facial expressions seemed to present radically new developmental benchmarks. Although these studies were subsequently subjected to considerable methodological and interpretive criticism, some incautious assertions regarding teenagers' general inability to respond appropriately, especially in emotionally charged situations, continued to flourish. This paper charts the creation and maintenance of these ideas over the past decade to illustrate how, when primary sources are not carefully checked, powerful messages for which there is dubious empirical evidence can become incorporated into expert advice. It also suggests the importance of linking neuroscientific claims to other strands of contemporary Western efforts to define and contrast normative and disordered emotional behaviour in adolescence.

year: 2012

title: A Maori language and literacy institute: looking to cyberspace

authors: Roberts, Ngaere

subjects: te reo Māori

abstract: In this paper, it is argued that many of the problems associated with the lack of coordinated research and development activities relating to the revitalisation of Māori language could be resolved by establishing a Māori Language and Literacy Institute. The possibility of establishing such an Institute in cyberspace is an attractive one in that it reduces overheads, allows for an immediate link between researchers, educational providers and iwi, and provides opportunities for wide dissemination of findings and for ongoing discussion.

text: A Maori language and literacy institute: looking to cyberspace In this paper, it is argued that many of the problems associated with the lack of coordinated research and development activities relating to the revitalisation of Māori language could be resolved by establishing a Māori Language and Literacy Institute. The possibility of establishing such an Institute in cyberspace is an attractive one in that it reduces overheads, allows for an immediate link between researchers, educational providers and iwi, and provides opportunities for wide dissemination of findings and for ongoing discussion.

year: 2000-10