INVERSION OF GEOPHYSICAL-DATA OVER A COPPER-GOLD PORPHYRY DEPOSIT - ACASE-HISTORY FOR MT MILLIGAN

In this paper, we invert magnetic, DC resistivity, induced polarizatio n (IP), and airborne electromagnetic (EM) data from the Mt. Milligan c opper-gold porphyry deposit and jointly interpret the inversion result s with available geological and mineralization data. The inversions ar e carried out over an area that encloses an intrusive stock, known as the MBX Stock, and the resulting mineralization zone surrounding it. T he earth model is discretized into a large number of cells having cons tant physical properties, and distributions of magnetic susceptibility , conductivity, and chargeability are obtained by minimizing a model o bjective function subject to adequately fitting the corresponding data . A 3-D magnetic susceptibility model is obtained directly by invertin g 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 a lgorithm and composited into a 3-D conductivity model. The physical pr operty models are compared with a rock model constructed from geologic information from 600 drill holes and with a 3-D model of gold concent ration. The physical property models have features that correlate with various geologic boundaries and rock units. More notably, the recover ed chargeability and susceptibility seem to reflect the distribution o f mineralization: chargeability highs coincide with the greatest gold concentration, while the susceptibility displays an anticorrelation wi th it. Overall, our inversion results are consistent with the geology and mineralization models for the Cu-Au porphyry deposit, while the an ticorrelation between gold concentration and susceptibility provides a n important constraint that helps define the distribution and geochemi cal control of the orebody.