Inversion of geophysical data over a copper gold porphyry deposit: A case history for Mt. Milligan

Abstract

In this paper, we invert magnetic, DC resistivity, induced polarization (IP), and airborne electromagnetic (EM) data from the Mt. Milligan copper‐gold porphyry deposit and jointly interpret the inversion results with available geological and mineralization data. The inversions are carried out over an area that encloses an intrusive stock, known as the MBX Stock, and the resulting mineralization zone surrounding it. The earth model is discretized into a large number of cells having constant physical properties, and distributions of magnetic susceptibility, conductivity, and chargeability are obtained by minimizing a model objective function subject to adequately fitting the corresponding data. A 3-D magnetic susceptibility model is obtained directly by inverting surface total field anomaly data. 3-D conductivity and chargeability models are formed by compositing 2-D sections recovered by inverting DC/IP pole‐dipole data. The airborne EM data are inverted with a 1-D algorithm and composited into a 3-D conductivity model. The physical property models are compared with a rock model constructed from geologic information from 600 drill holes and with a 3-D model of gold concentration. The physical property models have features that correlate with various geologic boundaries and rock units. More notably, the recovered chargeability and susceptibility seem to reflect the distribution of mineralization: chargeability highs coincide with the greatest gold concentration, while the susceptibility displays an anticorrelation with it. Overall, our inversion results are consistent with the geology and mineralization models for the Cu-Au porphyry deposit, while the anticorrelation between gold concentration and susceptibility provides an important constraint that helps define the distribution and geochemical control of the orebody.