Geology of the rotorua geothermal system

Abstract

The Rotorua geothermal system is located in the south part of Rotorua Caldera, which collapsed during and after the eruption of Mamaku Ignimbrite some 140 ka ago. Drillholes provide geological and hydrological information to 300 m depth. The principal production aquifers are Mamaku Ignimbrite and the younger Rotorua City domes (rhyolite lavas). Poorly permeable lacustrine siltstones interbedded with sands, gravels and tephra cap the aquifers and form an aquitard. The Mamaku Ignimbrite aquifer has been drilled in the east and south of the field where it contains fluid at or near boiling point. The ignimbrite drops from south to north across exposed and buried caldera collapse scarps. The Whakarewarewa hot springs feed from fractured ignimbrite close inside an embayment of the caldera's southern boundary where the sedimentary cover is thin and faulted. Fluid may ascend from depth into Mamaku Ignimbrite via Ngapuna and Rotoatamaheke Faults. Rotorua City domes comprise a buried N-S ridge rising at either end to form north and south domes; both contain mostly sub-boiling water up to 190°C which flows laterally through the outer 40 m of permeable rhyolite as indicated by temperature data. Water rises into rhyolite across a sharp boundary with Mamaku Ignimbrite along the east side of the domes. Kuirau Fault along the west flank of north dome channels near-boiling fluid into the aquifer from depth. Cold groundwater is drawn into a low pressure and low temperature anomaly from sediments to the west. The Fenton Park aquifer comprises sands and gravels in the shallow sedimentary sequence which contain hot water derived possibly from Whakarewarewa, the south dome or the Rotoatamaheke Fault.