Life and death of a sinter archive: Evolution of siliceous hot-spring deposits (Holocene) on the dynamic Paeroa Fault at Te Kopia, Taupō Volcanic Zone, New Zealand

Abstract

Siliceous hot spring deposits, or sinter, host a variety of extremophile microorganisms in modern geothermal settings, and are often touted as an important candidate in the search for early life on Earth or elsewhere in the solar system. However, the preservation potential of microbial textures within silica may be compromised over geologic time scales, particularly in active tectonic and/or volcanic environments. The Te Kopia geothermal field, Taup¯o Volcanic Zone (TVZ), New Zealand, situated along the dramatic Paeroa Fault (with up to 500 m offset, and demarcating the eastern boundary of the central TVZ), provides a natural laboratory to investigate the preservation state of microbial textures/microfossils in young (<5140 years BP) siliceous sinter that was originally deposited from circum-neutral alkali-chloride geothermal fluids (~100 ◦C to ambient) discharging at the land surface. Since its formation, various processes – mechanical (landslides), biological (vegetation), chemical (acidic leaching) – have rapidly degraded the relict sinter. Combined field mapping and petrographic observations of sinter textures indicate a prolonged history of alkali-chloride fluid surface discharge, with multiple stages of fluid flow and changing flow directions over an expansive area, prior to a shift to acid-sulfate thermal fluid chemistry associated with a drop in the local water table, possibly related to fault activity. Mineralogy (X-ray diffraction) and in situ geochemistry (electron microprobe analysis, synchrotron X-ray fluorescence) illustrate the effects of acidic hydrothermal alteration, with accelerated recrystallization of opal-A to opal-CT and quartz, and the leaching and overprinting of geochemical tracers of sinter growth and microbial textures (e.g., Rb, Ga, Al, Fe). Results suggest that despite the proportionally minor amount of vent geyserite sinter compared to lowtemperature palisade sinter in the study area, the dense nature of spring vent-related geyserite enhances its preservation potential compared to the more porous, filamentous microbial textures of the distal sinter apronrelated palisade texture.