Geophysical anomalies at Puhipuhi reflect both diverse
lithologies and different degrees of hydrothermal alteration. Most significant
is a strong negative gravity anomaly (up to 50 g.u.), encompassing but extending
beyond areas of known alteration, which is modelled as a deep zone (up to 3 km)
of reduced density in the host greywacke basement. This zone, interpreted as
being due to alteration (at least in part), closely correlates with the extent
of overlying basalt flows. The gravity data indicate that the most intense
alteration occurs below the basalts and correlates with the location of
previously inferred hydrothermal upflow zones. These observations suggest that
the basalt may have acted as a cap rock to a mineralising geothermal system.
Aeromagnetic data identify highly variable but typically low magnetisations in
the basalts which suggests that alteration may have been contemporaneous with
volcanism. These geophysical data at Puhipuhi confirm the locations of centres
of known alteration and suggest potential exploration targets below the basalt
cap where there is no surface manifestation of alteration. Further implications
of these results are discussed in Locke et al. (1999).