Near-surface geophysical surveys at the Duport gold deposit, Ontario, Canada: Relating airborne responses to small-scale geologic features

Abstract

Near-surface geophysical measurements using magnetometer, magnetic susceptibility, terrain conductivity, and time-domain electromagnetic instruments were made at the shear-hosted Duport gold deposit on Cameron Island in Shoal Lake, western Ontario, Canada, to help relate airborne total magnetic intensity (TMI) and helicopter electromagnetic survey data to small-scale geologic features. The magnetic airborne response provides a weak indication of a narrow anomaly within the Duport deformation zone, and the airborne electromagnetic response provides an indication of enhanced conductivity in the northwest of Cameron Island. In contrast, surface magnetic responses are dominated by the narrow 10,000–15,000 nT magnetic anomaly of a talc-chlorite-dolomite schistose basalt unit, a feature barely visible in the airborne TMI data. This geologic unit hosts the veins containing gold mineralization, so the surface TMI data provide a valuable response for delineating the corresponding rocks. Modeling of the TMI data indicates that the unit has a susceptibility of up to 0.7 SI and a corresponding magnetite content of up to 20%. The TMI data also reveal along-strike variations in the magnetic anomaly providing information on the component of ultramafic rocks in the protolith of the unit. The surface geophysical data allow the enhanced conductivity in the northwest part of Cameron Island to be attributed to several narrow sulfidized zones containing up to 10% pyrrhotite. Additional conductive and positively magnetized zones are associated with concentrations of 5%–10% of magnetite. The near-surface geophysics at the Duport deposit provided a rapid and inexpensive method for defining the magnetic and electrical properties of the geologic units at the site and for defining the exact location, width, and internal structure of features observed in the airborne geophysical data. Availability of magnetic susceptibility measurements from a single drill core enhanced the accuracy of the interpretations and the ability to relate the near-surface geophysical responses to geologic features.