The textures and mineralogy of the Black Terraces, Waipahihi, TVZ

Abstract

The Waipahihi Valley, is 4 km south of Taupo, New Zealand, where the “Black Terraces” a geothermal derived siliceous sinter was situated. They were a popular tourist site in the early 20th Century. The region has been subject to changes as a result of development and as such the landscape has been altered including the lands geothermal features, especially since the introduction of geothermal and hydroelectrical power generation in the region in the 1950’s. Since then the terraces have become overgrown and are no longer an obvious feature of the landscape. However the site remains actively geothermal and provides water from to the hot pool complex downstream, presently (2014) the source stream remains thermal. The present temperature is consistent with records with a pH that is consistent with an alkali chloride geothermal system. The terraces were mapped and sampled revealing a number of textures predominantly from lower temperature, distal, marsh and channel settings with some stromatlitic textures observed. Analysis of hand samples has shown that there has been no modification of the primary sinter morphology and that primary porosity, where there is any, has been lost due to infilling with detrital soil and vegetative ingrowth. Dipteral larval fossils were identified as part of the sinter structure, providing an upper thermal limit of ~51°C where present. Larval fossils were found within some stromaolites indicating perhaps different temperature waters forming similar structures. Chemical and mineral analysis using XRF and XRD reveal a compositionally uniform deposit which has undergone no significant alteration of primary features apart from mechanical breakdown at the sinter surface-soil boundary. These analyses indicted that the silica phase remains Opal-A. Which was supported by SEM images revealing Opal-A nano and microspheres encrusting microfossils including diatoms, and microbial filaments of various thicknesses from palisade filaments >10μm across to.prostrate filaments as low as ~1μm. Many of which were observed in thin section. Which indicate temperatures ranges encompassing those expected from vent to apron to marsh .The source of the black silica is likely microbial when preserved in laminae and due to its dispersal may be indicative of Calothrix mats, however black pigment is also seen with in marsh facies where bark has been silicified. The relatively high water table revealed by the stream and a former vent which is now a small thermal pool. Indicate that a drop of only a few meters in the water table is significant enough to cease sinter from forming and to start the process of burial removing a habitat for significant thermal flora and fauna.