Acidic, Iron-Rich Terrestrial Hot Springs of the Central North Island, New Zealand

Abstract

Interest in the possible environments that could host past or present life onMars has drawn attention to iron-rich acid-sulphate terrestrial hot spring systems and their deposits on Earth. These systems have received relatively little attention when compared with the more common and voluminous alkali chloride springs and their associated sinter/travertine deposits. To expand knowledge of terrestrial iron-rich acid-sulphate systems, this thesis examines the waters and deposits associated with three iron rich, low pH spring systems at localities within the geothermal systems at White Island, Waiotapu (Orange Springs) and Tikitere within the Taupo Volcanic Zone, New Zealand. One occurrence of purely steam-heated deposits is also examined from the Te Kopia system. The geological settings are contrasting, between the andesite at White Island and rhyolite for the other localities. Chemical data from water and solids (precipitates and their substrates) are presented, along with textural and mineralogical information. The waters show some systematic influence of the host lithology (e.g. elevated Ca and Mg, and lesser degrees of light rareearth element enrichment at White Island), and these differences are transmitted through to the bulk chemistries of the deposits. However, the three main localities studied also show extensive diversity on small spatial scales (down to centimetres) in the chemistry of both discharging fluids and solid precipitates, as well as mineral modes and textural morphologies, yielding a wide range of iron-rich and sulphurous deposits at each site. The formation of poorly crystalline iron-rich precipitates from acidic aqueous environments appears to result in similar deposit morphologies across many depositional settings, and the style of deposit precipitation shows potential for the preservation of biosignatures in these environments. This research adds three new sites to the chemical, textural and mineralogical database of acid-sulphate hydrothermal environments, and sets the groundwork for developing an approach to understanding and quantifying this spatially highly variable and complex group of hot spring deposits.