ABSTRACT

A 5 cm diameter nodule recently recovered from the northern slopes of Mount Ruapehu, Taupo Volcanic Zone, New Zealand consists of bladed natroalunite, quartz and rutile. The natroalunite crystals are zoned, having K-rich cores and Na-rich rims, and are enriched in Ba and Sr over Rb. The strontium isotopic composition (0.70535) is close to the average for Ruapehu lavas and the δ³⁴S (‰) CDT value of +16.1 is similar to dissolved sulphate in Ruapehu crater lake. The nodule probably originated as a precipitate from hydrothermal fluids at low pH, and was subsequently baked by hot lava.