CGG flew the Helitem® time-domain electromagnetic (TDEM) as a test survey over the Lalor Volcanogenic Massive Sulphide (VMS) deposit in early 2012. Lalor is a deep VMS deposit within the Chisel Lake basin in the North-western region of Manitoba, Canada.
The deposit was flown with Helitem® transmitting at 2 MAm2 dipole moment, and 6ms pulse at 30Hz, and a 10ms pulse at 15Hz. A block of lines between Lalor and the Chisel Lake area were covered. Initial results were exciting, as a broad, strong conductor of low amplitude was immediately apparent in the field data, and in preliminary Differential Conductivity™ sections. The conductor was most apparent in middle to late-time data in the derived B-field results.
A broad conductive belt was mapped with two higher conductivity cores that correlated with the Lalor deposit and the “South Bullseye” conductor previously detected with ground TDEM. The best results were achieved at 30Hz with 6ms halfsine pulse (10.7ms off-time). The 15Hz (10ms pulse) data also showed the conductor, and although the improved induction into a strong conductor of the 10ms pulse enhanced the signal, the reduction in data stacking at 15Hz base frequency resulted in very little net gain of signal-to-noise over 30Hz data.
HELITEM mapped two higher conductivity cores that correlated with the Lalor deposit and the “South Bullseye” conductor previously detected with ground TDEM"
The survey data were inverse-modeled with two approaches: Multiple plate-like conductors were forward modeled and adjusted until a reasonable fit was achieved using LeroiAir; and the UBC 3D TDEM modeling inversion was run independently (by UBC). The plate modeling returned a reasonable match to known geology, although the fit of the model decay curve to the full off-time of the field data was not optimal. This can be explained in part by the non-platelike conductivity that is in the area of the deposit, including stock work and a reported “thick zone of hydrothermal alteration” (Hudbay).
The 3D finite element inversion performed at UBC also showed a thick zone of conductivity, with cores at the location and depth of the Lalor deposit and the South Bullseye conductor. The 3D inversion results (and the Differential Conductivity™) also show the dip of the deposit, and particularly match the known dip of the Chisel Horizon on which the two deposits lie.
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