Rare Earth, Major, and Trace Element Geochemistry of Surface and Geothermal Waters from the Taupo Volcanic Zone, North Island New Zealand

Abstract

Geothermal fluids, surface water, and sediments were collected from the Taupo Volcanic Zone (TVZ) on the North Island of New Zealand. This region is characterized by hydrothermal activity with groundwaters feeding Lake Taupo, Waikato River, and the Wairakei and Wai-O-tapu hydrothermal fields of Rotorua. pH values ranged from near neutral (6.5–7.2) to alkaline (7.3–8.5). Surface water temperatures were between 15–16°C, while geothermal fluid temperatures ranged between 22 and 90°C. The major element chemistry of these waters describes the sample populations as geothermal and rhyolitic-sourced with some samples representing a mixture of these end-members. Using the rare earth element (REE) composition of the geothermal and surface waters, the competitive influences of mixing and water-rock interactions on fractionation were explored. Comparison of the REE composition of geothermal fluids, surface water, and sediments/ precipitates, indicate that temperature has a strong effect on REE fractionation in the sampled fluids. Eu- and Ce-anomalies (Eu/Eu*, Ce/Ce*) are visible in several samples and are related both to the inheritance of Eu and Ce abundances from geologic host materials and fractionation of these elements during water-rock interactions. These two anomalies are inversely related in the waters and sediments with sediments showing a positive relationship between Eu/Eu* and Ce/Ce* and waters showing an equally significant negative correlation between these two variables. The unique relationships between the Eu- and Ce- anomalies are indicative of the chemistry of the fluids and the nature of their interaction with the aquifer material such that precipitation of material during cooling led to the development of negative Eu-anomalies under reducing conditions. The REE, trace element and major element data demonstrate that competitive processes, including water-rock interaction at depth with aquifer wall-rock, differential leaching of REE from mineral phases and precipitation of secondary minerals from geothermal fluids result in highly evolved fluids discharging into the surface waters of the TVZ.