The Waihi-Waitekauri region hosts numerous low-sulfidation epithermal
deposits that both outcrop and occur at depth. Zones of intense hydrothermal
alteration enclose the Au-Ag bearing quartz vein deposits. Hydrothermal
alteration includes the destruction of magnetite by hydrothermal fluids,
pervasive silicification adjacent to quartz lodes and potassium enrichment due
to adularia and illite deposition. High-resolution airborne magnetic and
radiometric, and new gravity data delineate distinct geophysical anomalies
associated with hydrothermal alteration and mineralisation. Five magnetically
quiet zones occur in the region varying in size from 3 to 34 km2 and in geometry
from circular to irregular zones. These zones represent the near-surface extent
of strong to intense hydrothermal alteration associated with fossil geothermal
systems. All epithermal deposits in the region occur in magnetically quiet
zones. Over 120 new gravity stations together with existing data across the
Waihi district have defined a positive Bouguer gravity anomaly coincident with
the magnetically quiet zone. Eastern and western gradients of 50 gu/km bound the
anomaly that contains two flat gravity highs west and east of the Waihi deposit.
Strong potassium signatures in airborne radiometric data correlate with
outcropping zones of alteration and are consistent with the presence of adularia
and illite minerals. Hydrothermally altered zones are represented by areas of
low Th/K ratios as a result of low thorium concentrations in andesite and
elevated potassium levels in altered zones.