The behaviour of antimony released from surface geothermal features in New Zealand

Abstract

Antimony-rich geothermal features in the Taupo Volcanic Zone (TVZ) of New Zealand's North Island drain directly into surface aquatic environments. The discharge from Champagne Pool, a mixed alkali-chloride/sulfate hot spring at Wai-O-Tapu Geothermal Field, contains up to 194μg/L Sb. The discharge from Frying Pan Lake, a chloride-sulfate hot spring at Waimangu Geothermal Field, contains up to 21.5μg/L Sb. At Champagne Pool, downstream concentrations of Sb show distinct diurnal variations, particularly in winter, when concentrations in the early morning were less than half those measured mid‐afternoon. Changes in sulfide–sulfate equilibria and direct stibnite oxidation may explain this phenomenon. In the discharge from Frying Pan Lake, Sb exhibits little diurnal variation.Most (>80%) of the dissolved Sb released from Champagne Pool is removed from solution at Alum Lake, an acid-sulfate hot pool containing elevated levels of dissolved sulfide. Therefore relatively little Sb is discharged into the freshwater drainage in the area. However, in the absence of a sulfide-rich feature at Waimangu, most of the Sb discharged from Frying Pan Lake remains dissolved, and is transported into the freshwater drainage system of Lake Rotomahana. The contrast in Sb behaviour between these two sites confirms the importance of dissolved sulfide and low (<3) pH conditions in the precipitation and removal of dissolved Sb downstream of geothermal features. Otherwise, largely conservative behaviour can be expected.