Lithological influences on hydrothermal eruption potential; a case study from the Rotokawa geothermal field, Taupo Volcanic Zone, New Zealand

Abstract

Hydrothermal eruptions are violent explosive events driven by the expansion of super-heated water and steam produced from a suddenly ruptured, pressurized aquifer. They eject water, steam, and rock as ballistics, or in debris jets and wet surges. The explosive events are common to many geothermal fields in the Central North Island. The Rotokawa geothermal system, has experienced at least eight very large hydrothermal eruptions. This study examines the properties of the main lithologies sampled from the youngest hydrothermal eruption deposit of the Rotokawa field and investigates the fragmentation potential of the rock units via explosive decompression experiments. Four lithotypes were defined in this study that control the general petrophysical and mechanical properties of the samples: (1) glassy, pumice-rich tuff derived from variably welded ignimbrites; (2) mineral sandstone or micro-breccia comprising mainly quartz and feldspar crystals cemented by silica or calcite; (3) silicified polymictic breccias comprising clasts of hydrothermal breccias from earlier eruptions; and (4) fine-grained fluvial volcanoclastic mudstones originated from a paleo-lake environment. Samples are all variably overprinted by argillic to prophylitic alteration Three major alteration styles are identified: (A) Silicicification, which imparts very high decompression fragmentation thresholds by decreasing porosity/permeability; (B) Calcite cementation, which decreases the fragmentation threshold due to weak or brittle structures; and (C) Clay alteration, which replaces primary minerals and reduces porosity/permeability. With increasing degrees of alteration, the fragmentation threshold decreases, due to the weakness of the hydrous minerals involved. Dissolution of primary minerals weakens the rock strength of the sample and decreases the pressure needed to fragment. Overall, the effect of mineral replacement of primary minerals by hydrothermal minerals depends on the strength properties of the replacing minerals and can either positively or negatively impact the fragmentation threshold. The lithology and mineral assemblages of clast samples from the youngest Rotokawa hydrothermal breccia deposit, suggest an eruption source depth range of 300 - 350 m ( 240 °C ), hosted within the Wairoa Formation. This may indicate a source zone of interest to monitor or may produce future hydrothermal eruptions within the Rotokawa geothermal field.