Northland airborne magnetic and radiometric survey : a geological interpretation

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Stagpoole, V.M.; Edbrooke, S.W.; Christie, A.B.; Davy, B.W.; Caratori Tontini, F.; Soengkono, S.; Cox, S.C.; Isaac, M.J. 2012 Northland airborne magnetic and radiometric survey : a geological interpretation. Lower Hutt, N.Z.: GNS Science. GNS Science report 2011/54 113 p. + CD

Abstract: A new perspective of the geology of Northland is provided by low-level aeromagnetic and radiometric data, acquired in the region, from February to August 2011. The geophysical data are analysed together with existing ground-based gravity survey data and reviewed against the published geology of the region. Complex processing and modelling of the geophysical data have yielded unprecedented images that are used to aid interpretation, and a selection of individual anomalies are modelled in detail. The new datasets provide a great deal of information that is used to interpret geological structures and boundaries and help interpretation in the third dimension (depth). Many of the geological features described in the review are not on published maps. This report is intended to provide an overview of the data and an interpretation of the regional geology. It is not intended for identification of exploration targets, although the potential for the application of the new data in mineral exploration is described along with some mineral exploration geochemical data. Some key findings include: Structure of the Dun Mountain-Maitai terrane ophiolite can be interpreted from offsets and fine detail observed in the Junction Magnetic Anomaly; The southern boundary of the Mt Camel terrane is interpreted from the magnetic data; The greywacke basement in eastern Northland is generally magnetically quiet and geological structures and boundaries within the greywacke are not clearly imaged; The extent of the Tangihua Complex massifs is clearly imaged and the limited depth extent of the massifs is confirmed from geophysical analysis; Numerous anomalies in the dataset are associated with volcanic domes and lava flows. The Kaikohe-Bay of Islands and Puhipuhi-Whangarei volcanic fields are clearly imaged, as well as dacite domes between Maungaturoto and Mangawhai Heads. Lava flows from Waipoua Volcano are prominent in the magnetic data; Several volcanic features exhibit negative magnetic anomalies suggesting they are reversely magnetized; The northern edge of the predominantly offshore Kaipara Volcano is located beneath North Kaipara Head. Magnetic depth solutions suggest that the bulk of the volcano lies below 700 m depth; Several anomalies that have no surface geological expression are observed in the magnetic data. Geophysical modelling suggest that three circular anomalies (west of Whangaruru Harbour, east of Kaitaia and west of Houhora Heads) may be associated with large intrusions and/or buried volcanoes of intermediate composition (e.g. quartz diorite and andesite respectively) that extend to within a few hundred metres of the ground surface; Contrasting radiometric character in the Caples terrane, Northland Allochthon and Pliocene and Quaternary dune sands may be used to differentiate geological units (auth)