Mt. MilliganΒΆ
Mt. Milligan is a Cu-Au (copper-gold) porphyry deposit situated in north central British Columbia. Like many deposits of this type, the individual rock units and alteration products have physical properties that can be detected with geophysical surveys. In particular:
- magnetite content will alter the magnetic susceptibility;
- pyrite, chalcopyrite, and bornite affect chargeability; and
- significant mineralization and fluid-filled fractures will alter the electrical conductivity.
Geophysical surveys have the potential to provide quantitative information about the distribution of these physical properties. Magnetic, DC resistivity, IP, and airborne EM data have been collected at Mt. Milligan. In this case history the focus is upon magnetic data gathered on the ground over a 1.2 by 1.0 km area, which overlies a segment of the deposit known as the MBX East deposit.
A 3D magnetic susceptibility model is obtained directly by inverting surface total field anomaly data. The resulting physical property model is compared with a rock model constructed from geologic information from 600 drill holes and with a 3D model of gold concentration. The susceptibility model has features that correlate with various geologic boundaries and rock units. Most notably, the recovered susceptibility displays an anticorrelation with the greatest gold concentration. This anticorrelation between gold concentration and susceptibility provides an important constraint that helps define the distribution and geochemical control of the orebody.
DC resistivity and IP surveys were also inverted, and the resulting 3D model of chargeability displayed a positive correlation with the greatest gold concentration. The inversion of DC and IP data are discussed in a seperate section.
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Reference
UBC-GIF work on this deposit is published in Li and Oldenburg, 1996, “Inversion of geophysical data over a copper gold porphyry deposit: a case history for Mt. Milligan”, Geophysics, Vol. 62, No. 5, pg 1419-1431.