Authors / CoAuthors
Roy, I.G. | Milligan, P.R. | Duan, J. | Crowe, M.C.A.
Abstract
Mount Isa mineralized province has brought renewed interest of exploration in the greenfield regions in finding base metal (Pb-Zn-Ag) deposits which is often being hosted on black Urquhart shale which is electrically conductive medium. The new study is focused on understanding the structural framework which is favourable for mineralization; for example, the pathways for mineralized fluid, zone(s) of highly conductive region suggesting either conductive sulphide mineralization or the presence of the favourable host for mineralization. This prompts magnetotelluric investigations with frequency range 10-4 103 Hz for broadband and 10 20x103 Hz for audio magnetotelluric investigations. Higher frequency allows investigating relatively shallow and highly resolvable structure; on the other hand, lower frequency allows investigating structure seated deep below the horizontal surface. The new zone of investigations which is an upside down L-shaped region at the southwest corner of the Mount Isa region is designated as Mount Isa extension. The broadband magnetotelluric data are acquired in a regular rectangular grid fashion in, so as to help in building a 3D electrical conductivity image of the subsurface. In the current study we focus on analysing and modelling magnetotelluric data at a rectangular zone which is at the western end of the L-shaped region. We designate this zone as West Bloc and conduct detailed data driven analysis and modelling of MT data. We conduct dimensionality analysis and direction (or strike) analysis using phase ellipse and Mohr circle methods. In addition, we also conduct imaging in a vertical plane using apparent resistivity and phase pseudosections for both the transverse electric (TE) and the transverse magnetic (TM) modes of measured fields. We conduct inverse modelling on apparent resistivity and phase data. We initially make 1D Occam inversion in every MT sites. Using 1D Occam inversion results in every MT sites along a profile we create 2D resistivity section by stitching those results by appropriate gridding methods. We then conduct 2D inversion using ModEM software and generate several sections along profiles. We compare inverted 2D sections with the stitched sections. We then conduct 3D inversion using ModEM software and build 3D image of resistivity variation. In our every inversion run either 1D or 2D or 3D our starting model remains a half space. We observe, although there is a general agreement in stitched 2D sections with the 2D sections generated by 2D and 3D inverse modelling that the modelling results differ substantially. We conclude such observation is consistent with the following fact: 1) the conductivity structure in the study area is not purely 1D, 2D or 3D rather a hybrid one and 2) the half space starting model for higher dimension inversion scheme is too coarse to overcome the issues of local minima of optimization. Therefore, most pragmatic approach is to build a prior model using a lower dimension inversion scheme before any higher order inversion run. We would implement the foretold strategy of inverse modelling in future exercise.
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nonGeographicDataset
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82563
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- Report
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2015-01-01T00:00:00
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