Inversion and quantitative analysis of airborne geophysical survey data from the Taupo Volcanic Zone

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Stagpoole, V.M.; Caratori Tontini, F.; Soengkono, S.; Rattenbury, M.S.; Henderson, S. 2015 Inversion and quantitative analysis of airborne geophysical survey data from the Taupo Volcanic Zone. Lower Hutt, N.Z.: GNS Science GNS Science report 2013/51 iv, 35 p. + 1 enclosure + 1 CD

Abstract: Aeromagnetic and aerogravity data, acquired by Glass Earth Ltd over a large part of the Taupo Volcanic Zone (TVZ) in 2005, were studied using two- and three-dimensional inversion techniques. Spectral estimation from the long wavelength magnetic anomalies indicates that the Curie isotherm is about 6 to 7 km deep under the central TVZ, in agreement with pervious magnetic modelling and the depth of well-located earthquakes in the region. Inversion of aeromagnetic and aerogravity data, using the modified Fourier approach, yielded maps that show the distribution of magnetisation and density. Three dimensional inversion of a subregion of the magnetic survey produces results in good agreement with the regional magnetisation map. The derived magnetisation map exhibits increasing magnetisation from west to east that ranges from -1.5 A/m to 3 A/m, similar to laboratory measurements of volcanic rocks from the TVZ. Distinct zones of reduced magnetisation correlate with known geothermal fields and low resistivity anomalies. Density distribution derived from the aerogravity survey correlates poorly with geology, although at longer wavelength scales, density anomalies correlate with the overall geological structure of the region. There is a statistical corelation of both magnetisation and density with gold occurrences and mineral exploration samples that have geochemical analyses of silver >1.0 ppm and gold >100 ppb. These mineral occurrences and samples appear to be correlated with higher density regions, with regions that have a horizontal magnetisation gradient in the range from 0.3 to 0.9 x 10-3 A/m2. This suggests that many gold occurrences and geochemical anomalies are recorded in higher density zones at the edges of magnetisation anomalies, and potentially relate to mineralisation along faults and structural lineaments that form geological boundaries and zones of up-flow of mineral-rich geothermal fields. The inversion of potential field data may therefore provide a way for focussing future exploration efforts. (auth)