3D joint inversion of magnetotelluric and airborne tipper data: a case study from the Morrison porphyry Cu–Au–Mo deposit, British Columbia, Canada
2018
Z-axis tipper electromagnetic and broadband magnetotelluric data were used to determine
three-dimensional electrical resistivity models of theMorrison 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. Groundbased
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.
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