Continual Improvement: Geophysical examples from the Eagle Ni-Cu Mine

As explorers are forced to look deeper and in more challenging geophysical environments, is becoming more important for geoscientists to fine-tune their survey methods in order to gain maximum value from limited data sets.

Two examples of this continual improvement will be discussed from the Eagle Ni-Cu Mine in Michigan, USA. The high grade Eagle Ni-Cu mine is hosted by an ultramafic Proterozoic magmatic feeder that intruded into the metasediments of the Proterozoic Baraga Basin. There is limited outcrop in the area due to a covering of glacial till and a thick portion of the Baraga Basin is very conductive and magnetic due to pyrrhotitic sediments. The till thickness varies from 0 to over 200 m and complicates the interpretation of gravity survey data. The very conductive metasediments limits the depth of penetration of conventional surface based ground EM surveys and complicates the interpretation and limits the radius of detection of borehole TEM surveys.

The ability to interpret borehole TEM surveys was improved significantly by lowering the base frequency of the transmitter in order to allow the host response to decay enough that the very conductive, constrained massive sulfide response could be better observed. Complex borehole TEM responses with multiple off-hole type responses became more conventional in nature and the off-hole response of the Eagle mineralization could be detected.

Till thickness had previously been determined by drill hole intercepts and the resonant frequency of passive seismic noise. To extend the coverage of these values, frequency domain data from a Resolve survey was modeled using 1D inversion software over the entire exploration block. Once the till thickness was interpreted, the gravitational response of the till was computed and subtracted from the airborne and ground gravity surveys. This improved the interpretation of the gravity data.

Presented by:  Dick West, Technical Director, Exploration Technology & Group Resources, Lundin Mining Corporation.
Recorded on March 3, 2015, as part of the Geosoft Explorer Speaker Series presented at the PDAC.

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