Calcite priming of a hydrothermal eruption at South Orakonui, Ngā Tamariki Geothermal Field, New Zealand

Abstract

On the 19th of April 2005, a small hydrothermal eruption took place at the Ngā Tamariki Geothermal Field, New Zealand. The 2005 eruption at South Orakonui cut through an existing hydrothermal eruption crater created in 1948 and ejected mud and rocks (breccia) up to 110 m from the vent. The breccia deposit thins exponentially from 2 to 3 m thick on the crater rim to <0.3 m approximately 70 m to the east with an estimated volume of ∼3.7 to ∼4.9 × 10³ m³ (dependent on crater rim thickness). Polymictic ejecta include rock fragments of pumice, tuff, cemented sand/pebbles, and mudstone that are supported in an unconsolidated mud-dominated matrix. Minor clasts of coarse sand to pebble conglomerate are preserved due to cementation by calcite or alternatively in fewer samples by opal/quartz. Included in the breccia are rare platy calcite vein fragments (up to 19 cm in length); the bladed crystal morphology indicating formation from a boiling fluid. Based on the measurement of fluid inclusions in platy calcite crystals the median homogenisation temperatures of 132° to 144 °C indicate that these vein fragments were sourced from 20 to 33 m below the water table. A model for the 2005 hydrothermal eruption is proposed in which pressurisation was due to the sealing of pores and fractures by calcite deposited from a near boiling-to-boiling fluid. Lesser deposition of opal/quartz by cooling contributed to the sealing. The 2005 eruption was preceded by gradual changes in surface thermal features over 12 years seen by the declining discharge of two springs, plus the short-lived appearance then demise of another. These changes are interpreted to reflect gradual and imperfect subsurface sealing by calcite and minor opal/quartz. The eruption trigger is not known and could be due to simple over pressurisation, a slight decline in the groundwater level (<1 m), or seismic activity. Priming of an eruption by calcite sealing has not been reported for other hydrothermal eruptions in New Zealand and globally is rare. Calcite sealing may have been significant in other hydrothermal eruptions but not recognised since calcite is easily destroyed by continuing thermal activity (steam, acid condensates) and/or by weathering as is currently taking place at South Orakonui.