The Matahana Basin Resistivity Anomaly: Implications for Geothermal Exploration in the Taupo Volcanic Zone

Abstract

In 1986, a 593m hole was drilled into a low-resistivity anomaly, identified by Schlumberger-arraymapping, at Matahana Basin in the north-westem part of the Taupo Volcanic Zone. Unlike comparable exploration holes in other low-resistivity anomalies in the this hole was cold. We show that the low-resistivity material can be identified as ignimbrites older than The low resistivity is caused by a process of devitrification within the ignimbrites, leading to the formation of conductive minerals (clays and zeolites) within the rock matrix. At near ambient temperatures this process causesmore than an order-of-magnitudereduction in resistivity after ca. 1Ma. Conductive rocks produced by this process are expected to exist at depth throughout most of Misinterpretation of the resulting low-resistivity anomalies as indicatinghydrothermal conditionsmust be avoided.