Geophysical characteristics of the southern Coromandel Volcanic Zone, New Zealand, and associated epithermal deposits

Abstract

Geophysical data from the southern Coromandel Volcanic Zone provide information critical to the investigation of epithermal mineral deposits and their regional structural setting. Regional gravity data show steep linear gradients that coincide with major faults bounding the Hauraki Rift to the west and the Waihi caldera to the east. Regional magnetic data are dominated by a large bi-polar anomaly coincident with the Waihi caldera, which may result from a sub-caldera intrusion, and otherwise by high-amplitude shorter wavelength magnetic anomalies associated with the volcanic rocks. High-resolution aeromagnetic and radiometric data reveal distinct signatures associated with epithermal deposits. Extensive magnetically quiet areas clearly delineate the location and extent of hydrothermal alteration zones around the deposits, whereas more localised zones of high K/Th ratios in these magnetically quiet areas delineate areas of intense potassium metasomatism. Epithermal deposits exhibit gravity signatures with two contrasting modalities: i) small negative anomalies (e.g.<30 gu at Golden Cross and Scotia) and ii) small positive anomalies (e.g. 40-50 gu at Karangahake and Waihi-Favona). Near-surface, low-density clay-altered andesite and/or enhanced fracture volume can account for small negative anomalies. However, positive gravity signatures show that significant mass anomalies must occur at greater depths; these may be either dense intrusions and/or zones of concentrated sulphide mineralisation.