Geophysical exploration of the Puhipuhi epithermal area, Northland, New Zealand

The Puhipuhi epithermal area, which occurs in a region of graywacke basemen t partially covered by basalt and lake-bed deposits, is characterized by bo th large-scale and small-scale geophysical anomalies. Known occurrences of locally intense alteration or silicification are typically associated with strong gravity, resistivity or LP anomalies. Gravity data define a complex negative residual anomaly (up to -50 gu) which has been used to identify an d delineate a large area (about 20 km(2)) of low-density presumably clay-al tered, graywacke basement rocks. This zone, modeled as extending to a few k ilometers depth, encompasses, but is more extensive than, the known areas o f alteration and has a close spatial association with the basalt cover rock s. Short-wavelength gravity minima and maxima, which indicate that the most intense alteration of the basement rocks occurs below the basalt, correlat e, in part, with the inferred location of hydrothermal upflow zones. The co ntrol on the location of these zones and their relationship to the location of the basalts is not well known; however, if the basalts acted as a cap r ock to the geothermal system, then these areas merit further exploration. H igh (greater than or equal to 100 ohm-m) and low (less than or equal to 10 ohm-m) resistivity and high (greater than or equal to 30 mS) IP anomalies o ccur in association with known silicification, clay alteration and sulfide mineralisation, respectively. In addition, magnetic data help constrain the relative timing of hydrothermal alteration and basaltic volcanism and indi cate that mineralisation was broadly synchronous with volcanism. (C) 1999 E lsevier Science B.V. All rights reserved.