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: A note on steady flow into a submerged point sink

authors: Hocking, Graeme Charles

subjects: axisymmetric flow

abstract: The steady, axisymmetric flow induced by a point sink (or source) submerged in an unbounded inviscid fluid is computed. The resulting deformation of the free surface is obtained, and a limit of steady solutions is found that is quite different to those obtained in past work. More accurate solutions indicate that the old limiting flow rate was too high and, in fact, the breakdown of steady solutions at a lower flow rate is characterized by the appearance of spurious wavelets at the free surface.

text: A note on steady flow into a submerged point sink The steady, axisymmetric flow induced by a point sink (or source) submerged in an unbounded inviscid fluid is computed. The resulting deformation of the free surface is obtained, and a limit of steady solutions is found that is quite different to those obtained in past work. More accurate solutions indicate that the old limiting flow rate was too high and, in fact, the breakdown of steady solutions at a lower flow rate is characterized by the appearance of spurious wavelets at the free surface.

year: 2014

title: Māori & Psychology Research Unit Annual Report 2014

authors: Waitoki, Waikaremoana

abstract: Annual report of the Māori and Psychology Research Unit (MPRU) 2014. The unit was established in August of 1997. The unit is designed to provide a catalyst and support network for enhancing research concerning the psychological needs, aspirations, and priorities of Maori people. The MPRU is well situated to draw together skilled and experienced interdisciplinary research groups by networking and establishing working relationships with staff and students within the Faculty of Arts and Social Sciences, the University, and the wider community.

text: Māori & Psychology Research Unit Annual Report 2014 Annual report of the Māori and Psychology Research Unit (MPRU) 2014. The unit was established in August of 1997. The unit is designed to provide a catalyst and support network for enhancing research concerning the psychological needs, aspirations, and priorities of Maori people. The MPRU is well situated to draw together skilled and experienced interdisciplinary research groups by networking and establishing working relationships with staff and students within the Faculty of Arts and Social Sciences, the University, and the wider community.

year: 2014