Characterising The Hydrothermal Alteration Of The Broadlands-Ohaaki Geothermal System, New Zealand, Using Short-Wave Infrared Spectroscopy

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Journal Article: Characterising The Hydrothermal Alteration Of The Broadlands-Ohaaki Geothermal System, New Zealand, Using Short-Wave Infrared Spectroscopy

Abstract
Hydrothermal clay minerals present in the Broadlands-Ohaaki geothermal field were characterised by field portable short-wave infrared spectroscopy. Three major alteration zones, an upper smectite, a middle illite and a lower illite-chlorite, are spectrally separable. The zoning pattern is generally consistent with the thermal structure of the geothermal field, although occasionally zone boundaries cut present-day isotherms. The data indicate that temperature is the major control on clay zoning and permeability plays a subordinate role. Both beidellite and montmorillonite are common in the upper, low-temperature smectite zone. Kaolinite, mainly of low crystallinity, marks the margin of the field where cool acidic ground waters inflow. In the middle alteration zone, illite, dominantly K-rich, shows a narrow compositional variability. Some highly permeable zones are characterised by illite with low octahedral Al contents. Ammonium-bearing illite and buddingtonite are present locally in permeable horizons within the illite zone, where temperatures are above 200°C. Chlorite is most abundant in the lower alteration zone (temperature >250°C), although it also occurs unevenly in the upper and middle alteration zones. Chlorite varies from Mg- to Fe-rich varieties (but mostly with Mg# values <0.5), but no compositional trends with respect to depth are spectrally detectable.

Authors 
K. Yang, P. R. L. Browne, J. F. Huntington and J. L. Walshe








Published Journal 
Journal of Volcanology and Geothermal Research, 2001





DOI 
Not Provided
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Citation

K. Yang,P. R. L. Browne,J. F. Huntington,J. L. Walshe. 2001. Characterising The Hydrothermal Alteration Of The Broadlands-Ohaaki Geothermal System, New Zealand, Using Short-Wave Infrared Spectroscopy. Journal of Volcanology and Geothermal Research. (!) .