The
Karangahake adularia-sericite epithermal gold-silver deposit, located at the
southern end of the Waitekauri Corridor, Hauraki Goldfield, occurs as quartz
veins with associated alteration haloes. Integrated geophysical studies were
undertaken to determine the expression, location, and extent of these alteration
zones. Aeromagnetic data show an extensive, magnetic quiet' zone over
Karangahake and the southern Rahu Ridge that represents a zone of magnetite
destruction resulting from hydrothermal alteration. Analytic signal data are
used to define the boundary of this zone; the resulting 6 km2 area is
elongate north-south with an irregular boundary. A detailed gravity survey over
the area reveals a positive residual gravity anomaly, up to 30 gu in amplitude,
over the Karangahake deposit. Density measurements on near-surface rocks give
low values indicating that the source of this positive gravity anomaly must be
deep; possible causative bodies include an igneous intrusion or dense sulfide
mineralisation. To the west and east of the host Waipupu Formation, strong
gravity gradients of 100 gu/km and 65 gu/km reflect the faulted boundary of the
Hauraki Rift, and the edge of the Waihi Basin respectively. High resolution
airborne radiometric data define a 2.2 km2 area of potassium
enrichment. This area is elongate north-south and has an irregular boundary. It
coincides approximately with the centre of the magnetic quiet zone and the peak
of the gravity high, and correlates with known areas of intense mineralisation.
Anomalously high potassium counts and elevated K/Th ratios correlate with areas
of potassic alteration that have undergone K-metasomatism. Underexplored
portions of this zone may indicate areas of gold-silver prospectivity. Combined
geophysical data therefore reveal the extent of hydrothermal alteration, and the
central zone of Kmetasomatism that is most likely to contain
orebodies.