Siliceous hotspring (sinter) deposits along the Paeroa Fault (TVZ): Archives of geothermal fluid history

Abstract

Volcanic and tectonic activity in New Zealand is concentrated in the Taupo Volcanic Zone, a back-arc wedge in the central North Island. The Paeroa Range is a significant structural feature in the TVZ, which coincides with the Paeroa and Ngapouri Fault Zones, and was principally created by uplift along the Paeroa Fault scarp. Of the numerous geothermal systems associated with the range, Mangamingi/Pukemoremore, Te Kopia and Ngapouri were examined in depth. Preserved geothermal sinter deposits at these sites were studied using a range of geochemical and palaeontological techniques (XRPD, GADS, XRF, SEM, and AMS as well as thin section analysis) with the aim of defining their past geothermal fluid compositions and past palaeoenvironment in the context of regional tectonic controls. A comparison between the three sites found that they varied in age and fluid composition, and were predominantly produced by siliceous fluids. Sinter composition, age and microbial fossil composition were inferred to reflect paleoenvironmental conditions at the time of deposition. Subsequent alteration, including opal diagenesis and alteration by acidic overprinting, was further indicative of changes in the geothermal regime. Variability of the geothermal environments found along the Paeroa Range indicates that they are strongly influenced by structural controls, including uplift/faulting, silica cementation, and structural permeability. Further, mixed-acidic geothermal deposits may be a more appropriate analogue for extraterrestrial life on Mars, than previously considered near-neutral alkali chloride deposits due to their similarity to the sinter deposits found on Mars.