Geology, History and Stratigraphy of Hydrothermal Eruptions in the Rotorua Geothermal Field

Abstract

The study area is the Rotorua geothermal field (RGF), which is located in the southern end of the Rotorua caldera. Distribution of surface geothermal activity, hydrothermal eruption (HE) deposits and craters have been mapped. HE deposits formed in geological, historical and modern times were studied to record and describe their distribution, composition, petrography, lithology and stratigraphy.

Locations of all identified HE sites in the RGF have been plotted and shown on accompanying maps. At Ohinemutu and Sulphur Bay, HE breccias have been preserved only where deposits were subsequently silicified by hot spring outflows. At Whakarewarewa two HE breccias outcrop over about 0.5 km² and are up to 23 m thick. Study of stratigraphic relationships with tephras of known ages has determined one as Holocene (c. 20 ka) and one of Late Pleistocene age (c. 65 ka).

Four HE breccias were studied in detail. The mineral composition, physical description, field characteristics and spatial distribution of HE breccias are described and mapped. Clasts and matrix were examined by XRD analyses of whole (67) and clay fraction separates (16). Silica crystallinity indices (14) were determined by XRD to identify devitrification rates of opaline mineral species. The petrography of 67 breccia clasts and matrix samples were studied, and polished sections showed the only sulphide to be pyrite. Geothermal alteration of all prehistorical HE deposits was of advanced argillic to argillic and one modern day deposit was of sub propylitic epithermal alteration.

Study of a HE deposit formed in Kuirau Park on 26 January 2001 allowed properties and mineral composition to be examined and recorded before rapid oxidation and weathering destroyed its physical and mineral composition. Study of this and other twentieth century HE events confirms that these rapidly decompose to leave little or no recognisable deposit in the geological record.

Descriptions and all known historical records of HEs in the RGF have been collated to provide a comprehensive inventory of maps and appendices of these events. This information will greatly assist statutory authorities with mitigation of risk to lives and property from geothermal hazards as well as aid appropriate land use and emergency services planning.

Hydrothermal eruptions are ongoing in the RGF through time and produce craters 2 - 110m diameter with erupted deposit volumes of 1 - 500,000 m³ that have distributed over <5 m² to >0.5 km². These have occurred throughout at least Late Pleistocene to Holocene times and appear to have been triggered by such events as volcanic eruptions from the adjacent Okataina Volcanic Centre and related abrupt changes to the waterlevels of Lake Rotorua. Frequency and spatial distribution of HEs in historical times suggest human interference with the geothermal system is also a significant influence.

This study has brought together all records of known hydrothermal eruptions and other unusual geothermal disturbances in the Rotorua geothermal field. The information has plotted into GIS for use by statutory authorities and others. Two prehistorical HE deposits in Rotorua city have also been recognised and described for the first time, together with identification of their likely vent locations and possible eruption initiating events.