Textures and mineralogy of siliceous hot-spring deposits at Geyser Valley, Wairakei, Taupo Volcanic Zone, New Zealand

Abstract

Geyser (Thermal) Valley is located within the Wairakei geothermal eld, approximately 7 km northeast of Taupo. Prior to the 1960s, this area was a popular tourist attraction, with over 240 geothermal manifestations (Gregg & Laing, 1951), including 22 actively playing geysers. Extraction of geothermal uids for power production at Wairakei began in the 1950s, causing a rapid drawdown of reservoir levels { e ectively severing the supply of deep uids to Geyser Valley by 1968. A concurrent transformation of surface activity in the valley saw the cessation of alkali-chloride uid discharge from geysers and hot-springs, and the onset of steam dom- inated conditions. Currently (2014), surface geothermal manifestations are characterised by empty geyser and hot-spring vents, areas of steaming, heated and altered country rock, and, the precipitation of e orescent salts and native sulphur. Field mapping and a review of aerial photographs in this thesis suggest that the spatial distribution of active and inactive geother- mal features at Geyser Valley are in uenced by local NE{SW and NW{SE-striking structures associated with tectonic extension in the TVZ. The textures, mineralogy and geochemistry of siliceous sinter deposits surrounding former hot-spring and geyser features are the main focus of this thesis. Analyses of the primary (depositional) and secondary (alteration) textures of these sinters were achieved using hand sample observation along with optical and scanning elec- tron microscopy (ESEM). Geyseritic, wavy-laminated, Particulate Siliceous Sediments (PSS), palisade, bubble-mat, domal-stromatolitic and plant-rich textures were identi ed in representa- tive sinter samples. The comparison of these textures with modern and other fossil examples has allowed determination of depositional facies for the sinters, which range from hot (>90 ) subaqueous vent environs, to cool (<40 ), distal marsh-type areas. The mineralogy and geo- chemistry of the samples were determined using various methods (XRPD, oriented clay XRD, XRF and EDS/EDAX). Amorphous opal-A compositions dominate the Geyser Valley sinters, with only a few samples exhibiting an opal-A/CT mineralogy, indicating that silica phase mat- uration (diagenesis) is at an early stage in these deposits. Interpretation of secondary textures, mineralogy and geochemistry strongly suggest interaction of acidi ed steam (H2SO4) with most Geyser Valley sinter deposits, which exhibit distinct dissolution and re-precipitation textures, early silica phase maturation in some cases, and, the development of volumetrically minor alter- ation clay minerals (montmorillonite and kaolinite). Despite numerous scienti c studies focusing on the impacts of reservoir exploitation at Wairakei, the sinter deposits at Geyser Valley have been largely overlooked. This thesis contributes to the knowledge of reservoir utilisation and its associated e ects upon surface geothermal features. With a known date for the cessation of sinter formation at Geyser Valley, these deposits provide a potential gauge for evaluating natural rates of silica diagenesis and alteration, having been subjected to steam-dominated conditions for over 50 years.