New and existing gravity and aeromagnetic data are used to determine the
structure of the Waihi region and to investigate any relationship between
structure, hydrothermal alteration and epithermal mineralisation. The 24 mGal,
10 km diameter, negative Bouguer gravity anomaly which occurs south of Waihi has
been modelled as a I.5 km deep asymmetrical basin. This basin, the Waihi
Depression, is interpreted to be infilled with low-density volcanic rocks of the
Uretara Formation and Whitianga Group, overlying older andesites of the Waipupu
Formation. To the north and west, the margins of the depression are defined by
the Waihi and Mangakino Faults, which have been modelled with dips of 45 - 60
to the south and west respectively. The southern and eastern boundaries of the
Waihi Depression are defined by smooth, shallow dipping (15 - 20) boundaries.
The Waihi Depression is interpreted as a fault-angle depression, resulting from
southeast extension in the early Pliocene. The Waihi Fault is modelled as an
approximately E-W feature, located to the south of Waihi. Small 2 - 4 mGal
positive Bouguer gravity anomalies occur north and west of the margins of the
depression, and correlate with known areas of hydrothermal alteration and areas
of low magnetic gradient.