Comparison of hydrothermal fluids and fields of the northern Okataina Volcanic Centre: Fifty years ago to present time

Hydrothermal features in the Okataina Volcanic Centre, North Island, New Zealand, are found predominantly along the northern and southern boundaries of the centre. Very little work has been done studying the northern fluids, although yearly samples are collected from a handful of locations for monitoring purposes. In this study, we examine fluids from the Tikitere and Rotoma-Tikorangi geothermal fields, which are located along the northern OVC boundary near Lakes Rotoiti, Rotoehu and Rotoma and compare them to published, historical data from the region and more recent monitoring data where available. Samples taken from the lakeshores of Lake Rotoiti and at Waitangi Soda Springs indicate bicarbonate-chloride rich fluids that show minimal changes since 1970. Alkali geothermometer temperatures estimating reservoir temperatures range from around 220 at Waitangi Soda Springs to 290 degrees C at Manupirua Spring and show great consistency over the years despite their equilibrium immaturity. In contrast, slightly further south and higher in elevation, the Hells Gate Thermal Park, a popular tourist location, has a wider range of hotter geothermometer temperatures and features acid-sulfate fluids. There have been noticeable changes here since 1970, with new pools formed, and an evident enrichment in stable water isotopes, indicating greater water rock interactions and greater evaporation have occurred over time. Overall, hydrothermal fluids from the northern Okataina Volcanic Centre show good stability and low eruption risk at present time.