Hydrothermal Redox Dynamics in the Central Crater at White Island, New Zealand

Abstract:

The White Island stratovolcano of New Zealand has an active shallow S-rich hydrothermal system present in the Central Crater. In this hydrothermal system, subsurface Fe-rich mineralization is prevalent in the unconsolidated volcaniclastic sediments. Development of these laterally coherent Fe-rich layers may alter the fluid and gas flow and mineralization dynamics of the hydrothermal system. Here we assess the redox chemistry and pathways leading to the formation of the subsurface Fe-rich layer(s). Based on fluid chemistry, mineralogical analyses and geochemical modeling, the subsurface Fe-rich layers are mainly composed of jarosite and goethite and form at the interface between the atmospheric (O₂-rich) and hydrothermal (reduced) systems. Cyclic mixing of volcanically-related fluid/gas pulses and precipitation is conducive to both jarosite and goethite forming in the same layer. Macroscopically, the formation of these Fe-rich layers decreases the permeability and porosity in the crater fill directly affecting fluid and gas flux and heat released throughout sections of the crater. Microbial communities are present in the Fe-rich layers, but how and to what extent microbial activity is interacting within these layers requires further investigation. Overall, the subsurface Fe-rich layers present at White Island represent a confined and small scale version (a few centimeters) of a redox interface potentially applicable to larger hydrothermal and ore forming systems.