3D joint inversion of magnetotelluric and airborne tipper data: a case study from the Morrison porphyry Cu–Au–Mo deposit, British Columbia, Canada

Abstract

Z‐axis tipper electromagnetic and broadband magnetotelluric data were used to determine three‐dimensional electrical resistivity models of the Morrison porphyry Cu–Au–Mo deposit in British Columbia. Z‐axis tipper electromagnetic data are collected with a helicopter, thus allowing rapid surveys with uniform spatial sampling. Ground‐based magnetotelluric surveys can achieve a greater exploration depth than Z‐axis tipper electromagnetic surveys, but data collection is slower and can be limited by difficult terrain. The airborne Z‐axis tipper electromagnetic tipper data and the ground magnetotelluric tipper data show good agreement at the Morrison deposit despite differences in the data collection method, spatial sampling, and collection date. Resistivity models derived from individual inversions of the Z‐axis tipper electromagnetic tipper data and magnetotelluric impedance data contain some similar features, but the Z‐axis tipper electromagnetic model appears to lack resolution below a depth of 1 km, and the magnetotelluric model suffers from non‐uniform and relatively sparse spatial sampling. The joint Z‐axis tipper electromagnetic inversion solves these issues by combining the dense spatial sampling of the airborne Z‐axis tipper electromagnetic technique and the deeper penetration of the lower frequency magnetotelluric data. The resulting joint resistivity model correlates well with the known geology and distribution of alteration at the Morrison deposit. Higher resistivity is associated with the potassic alteration zone and volcanic country rocks, whereas areas of lower resistivity agree with known faults and sedimentary units. The pyrite halo and ≥0.3% Cu zone have the moderate resistivity that is expected of disseminated sulphides. The joint Z‐axis tipper electromagnetic inversion provides an improved resistivity model by enhancing the lateral and depth resolution of resistivity features compared with the individual Z‐axis tipper electromagnetic and magnetotelluric inversions. This case study shows that a joint Z‐axis tipper electromagnetic–magnetotelluric approach effectively images the interpreted mineralised zone at the Morrison deposit and could be beneficial in exploration for disseminated sulphides at other porphyry deposits.

Deep Mineral Exploration of the Jinchuan Cu–Ni Sulfide Deposit Based on Aeromagnetic, Gravity, and CSAMT Methods

“…The success of this method is verified by the inversion of magnetic data and the direct current resistivity data of Cristalino iron oxide copper gold deposit in northern Brazil. Lee et al [18] obtained the resistivity model consistent with the regional geology through the 3D joint inversion of magnetotelluric and Z-axis tipper electromagnetic data, which not only shows the mineralization belt interpreted for the Morrison porphyry Cu-Au-Mo deposit but is also conducive to the exploration of the disseminated sulfide of other porphyry deposits.…”

Three-Dimensional Audio-Magnetotelluric Imaging including Surface Topography of the Cimabanshuo Porphyry Copper Deposit, Tibet

“…The resistivity values beneath the 0.3% Cu belt predominantly range from 100 to 1000 Ωm, which was suggested to contain potassic intrusive rocks with low-content or/and disconnected metal sulfides. The average grade of Cu ore in Cimabanshuo is ~0.2629-0.5726% [23], which is essentially the same as that of Morrison (~0.3%, [15]). According to the idealized model in Figure 1, the most likely explanation for the high-resistivity body nearest to the Cu ore body is the potassic intrusive rocks.…”

“…This caused electrical (DC) and electromagnetic (EM) soundings to become frequently used ways to image the underground structure of porphyry Cu deposits [14]. For instance, 3D joint inversion of magnetotelluric sounding (MT) and Z-axis tipper electromagnetic data were used to image intrusive rocks and alteration zones beneath the Morrison porphyry Cu-Au-Mo deposit in British Columbia, Canada [15]. [12] and [15].…”

“…For instance, 3D joint inversion of magnetotelluric sounding (MT) and Z-axis tipper electromagnetic data were used to image intrusive rocks and alteration zones beneath the Morrison porphyry Cu-Au-Mo deposit in British Columbia, Canada [15]. [12] and [15]. Among these EM methods, audio-frequency magnetotelluric soundings (AMT) are commonly used to study the structure of metallic deposits [16].…”

“…According to the forward modelling studies of Dong et al [19], such clear topographic relief in Cimabanshuo cannot be ignored, otherwise, the reliable underground electrical structure cannot be obtained by inversion [20]. Furthermore, as mentioned above, the porphyry metallogenic system is a complex three-dimensional (3D) system; therefore, a [12,15]. Among these EM methods, audio-frequency magnetotelluric soundings (AMT) are commonly used to study the structure of metallic deposits [16].…”