Rabone, S.D.C. 1991 Residual total force magnetic anomaly map, Coromandel Region, 1:100 000. Lower Hutt: DSIR Geology & Geophysics. New Zealand Geological Survey report M 183. 16 p.
Abstract
Data from a low-level constant terrain clearance aeromagnetic survey carried out in 1977-78 in the Coromandel region by Amoco Minerals as part of a regional mineral exploration programme has been redrawn on two 1:100 000 scale maps at 50 nT contour intervals. The resulting magnetic anomaly patterns basically show the distribution of the basement greywackes, intermediate-acid volcanics, and subvolcanic-plutonic intrusives that make up the geology of the region. Pervasive hydrothermal alteration causing demagnetisation of the volcanics is widespread and results in extensive areas of low-amplitude, long wavelength anomalies superimposed on the primary magnetic anomaly pattern. As alteration has been strongly controlled by structural features, the distribution of these areas of subdued magnetic anomalies may indicate significant elements of the regional structure. A striking feature of the resulting anomaly pattern is a series of narrow northeast trending belts of low amplitude, long wavelength magnetic anomaly. Four belts or "troughs", - Coromandel-Waikawau Bay, Tapu-Kuaotunu, Puriri-Tairua and Te Aroha-Opoutere, are recognisable. These troughs probably represent zones of deep-seated fracturing and possible rift movement that have localised hydrothermal systems initiated by porphyry intrusives. A circular feature in the magnetic anomaly pattern in the Kapowai area corresponds closely to a mapped caldera structure. A similar pattern is apparent at Waitekauri,and is characterised by extensive hydrothermal alteration. This feature hosts a number of significant epithermal vein deposits adjacent to its margins, including the Waihi vein system. While the circular anomaly pattern may be controlled by the intersection of major north-northeast and northwest structural elements, its similarity to the Kapowai feature suggests the possible existence of a caldera-type structure in the Waitekauri area. (auth)
