10 March, 2010 (11:15 GMT), 5 Merrion Square, Dublin 2.

Speaker: Ian Ferguson, University of Manitoba, Winnipeg, Manitoba, Canada.
Title: Electrical and magnetic properties of the Duport gold deposit, northwest Ontario, Canada.

Abstract:

Ground geophysical measurements have been made using magnetometer, magnetic susceptibility, EM31 and TEM instruments at the shear-hosted Duport gold deposit in Shoal Lake, western Ontario, Canada in order to help relate airborne EM and magnetic survey data to geological features. Ground magnetic survey results show a broad anomaly correlating with an anomaly in the airborne magnetic data as well as several narrow discontinuous anomalies that are not as evident in the airborne data. The magnetic data have been modelled using steeply dipping plates possessing both induced and remanent magnetization. The broad anomaly is interpreted to be due to magnetite-rich gabbroic rocks with a Koenigsberger ratio of less than 1 whereas the narrow discontinuous anomalies are interpreted to be due to pyrrhotite-rich altered basaltic rocks with a Koenigsberger ratio exceeding 1. In-phase EM31 results delineate enhanced near-surface magnetic susceptibility in the pyrrhotitic rocks and
apparent susceptibility values derived from the response can be compared with the magnetic models. Quadrature EM31 and TEM results indicate that the pyrrhotite-rich rocks have significantly enhanced electrical conductivity. The combined electrical and magnetic results are consistent with pyrrhotite contents of 5-8% in the pyrrhotitic rocks and magnetite contents of 1-2% in the magnetite-rich rocks. Examination of the correlation between gold assay values and magnetic susceptibility measurements made on one drill-core indicates that highest gold content occurs at the margin of the pyrrhotite and magnetite-rich rocks.