Snapshot of hot-spring sinter at Geyser Valley, Wairakei, New Zealand, following anthropogenic drawdown of the geothermal reservoir

•Textures of Geyser Valley siliceous sinter – prevalence of high-temperature sedimentary facies.•Mineralogy of sinter – minor alteration despite decades of exposure to acidic steam condensates (H2SO4) owing to reservoir drawdown.•Structural control on geothermal features at Geyser Valley – cross-cutting faults and extension into Wairakei system, Taupo Volcanic Zone.•Occurrence of efflorescence, silica residue, particulate siliceous sediment (PSS) and desmid algae. From the 1950s, extraction of thermal fluids to generate electricity at Wairakei, Taupo Volcanic Zone, New Zealand, heralded a rapid transformation of surface manifestations at nearby Geyser Valley, 1km NW of the power station. Active geysers and hot-springs fed by alkali-chloride waters ceased by 1968, replaced by acidic steam-dominated conditions. Field relationships indicate geothermal features are fault-controlled. Siliceous hot-spring (sinter) deposits represent spring-vent to distal marsh settings. The dominant mineralogy is opal-A, with some minor clay alteration. Some textures show silica dissolution and re-precipitation. Thus, only minor alteration and diagenesis has occurred, with vegetation overgrowing the extinct sinter.