Preliminary geological interpretation of airborne geophysical datasets, West Coast, New Zealand

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Rattenbury, M.S. 2015 Preliminary geological interpretation of airborne geophysical datasets, West Coast, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2014/71 56 p.

Abstract: Airborne magnetic and radiometric data for the South Island’s West Coast have recently been acquired for the New Zealand Government to assist the resource industry to more effectively target their exploration for minerals, coal and petroleum. The government survey was flown at 200 metre line spacing and a target ground clearance of 50 metres. This high resolution survey is usefully augmented by publicly available industry-acquired airborne geophysical data in North Westland and South Westland. The more obvious magnetic and radiometric anomalies have been described from these datasets and put into their geological context. The aeromagnetic data show a wide range of anomalies that generally correlate well with known geological mapping units of differing magnetic susceptibility. Strongly positive total magnetic intensity anomalies are typically associated with igneous rocks; particularly Late Paleozoic-Mesozoic ultramafic rocks (Pounamu Ultramafics and Dun Mountain Ultramafic Group), Mesozoic mafic rocks (Alpine Schist metavolcanics, Kirwans Dolerite, Arnott Basalt, and the Hohonu, Buller and other dike swarms) and some of the Early Cretaceous granitic intrusions (Rahu, Separation Point and Darran suites). Sedimentary rocks are generally weakly magnetic; the stand-out exception being the Callaghans Greensand Member and other Miocene unconformity-related units. The largely metasedimentary Alpine Schist is more magnetic at high metamorphic grade. Unconsolidated Quaternary sediments associated with magnetic anomalies include titanomagnetite coastal sands and ultramafic-rich till deposits. Numerous large amplitude anomalies in areas of extensive Quaternary cover are interpreted to be caused by buried granitic intrusions, dike swarms or basaltic flows. The weakly magnetic Greenland Group metasedimentary rocks and midPaleozoic Karamea Suite intrusions are difficult to distinguish with aeromagnetic data. The radiometric data are of limited use in forested areas and correlate best with mapped geology in rocky alpine areas, modern river channels and coastal dunes. The K, Th and U channels record emissions from near surface materials and in many places these have been transported and intermixed from multiple sources. (auth)