The Central Otago helicopter-borne magnetic and electromagnetic regional survey : a preliminary geological interpretation

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Martin, A.P.; Kellett, R.L.; Henderson, S.; Davies, B.; Coote, J.A. 2016 The Central Otago helicopter-borne magnetic and electromagnetic regional survey : a preliminary geological interpretation. Lower Hutt, N.Z.: GNS Science. GNS Science report 2016/26. iv, 59 p.

Abstract : Regional helicopter-borne magnetic and electromagnetic data was flown in 2007 over Central Otago in the South Island of New Zealand. This survey was conducted on behalf of gold exploration company Glass Earth New Zealand Limited and the Otago Regional Council. Here we present images re-gridded from the original data for magnetics (e.g. reduced to pole, first and second vertical derivatives, upwards continuation) and electromagnetics (at several frequencies) in conjunction with re-gridded, legacy gravity data (free air, Bouguer). These data are interpreted relative to the geology, which is dominated by a Paleozoic-Mesozoic meta-sedimentary belt (The Otago Schist) that is overlain by Paleogene to Quaternary, variably marine to non-marine sediments and dissected and overlain by Oligocene to Miocene igneous rocks. Similarities in structural style, lithology and geophysical response are noted in the northern half of the survey area, which approximately equates with the Rakaia Terrane. The southern half of the survey area roughly equates to the Caples Terrane, and the boundary between these terranes is approximately traced in the geophysical data. A map of geophysical linear anomalies is presented. The anomalies are interpreted to reflect primary lithological differences but polyphase ductile deformation has generated multiple generation foliation surfaces that may not be parallel to the primary lithological layering. The volume of Paleozoic-Mesozoic greenschist rocks newly identified in the Rakaia Terrane using the geophysics is comparable to the adjacent Wanaka Lithological Association, of which greenschist rocks are a significant feature. Greenschist rocks identified from geophysics act as an important marker horizon in the Otago Schist, giving a good indication of the scale and style of folding and degree of deformation in adjacent lithologies. This is particularly relevant in understanding the history of mineralised structures, such as the Hyde-Macraes Shear Zone, that hosts a world-class gold ± tungsten deposit. Volcanic facies can have different geophysical responses with phreatomagmatic units tending to correlate with lower magnetic intensities. Sedimentary basins are particularly well defined in the airborne electromagnetic data and are being exploited to target groundwater and alluvial gold resources. The 2007 magnetic and electromagnetic data is a valuable resource for better understanding the geology and mineral wealth of Central Otago. (auth)