Helicopter AFMAG (ZTEM) EM and magnetic results over sedimentary exhalative (SEDEX) lead-zinc deposits at Howard’s Pass in Selwyn Basin, Yukon

Abstract

In 2008, a regional scale 24,675 line-km survey covering a 25,000 km2 area (1 km line spacing) was flown in the Selwyn Basin. The survey footprint straddles east-central Yukon and overlaps into the western North-west Territories. In March 2013, Yukon Geological Survey purchased the survey data and, in November 2013, released the data publicly. The Selwyn Basin area is prospective for sedimentary exhalative (SEDEX)-style Pb–Zn–Ag mineralisation and the z-axis tipper electromagnetic (ZTEM) survey data provide insights into regional structures and plutons in the region. The survey overflew the Howard’s Pass SEDEX deposits at the south-eastern edge of the Selwyn Basin survey area that hosts a ~250 million tonne resource with ~4.5% Zn and ~1.5% Pb. Airborne geophysics has not been extensively used in SEDEX exploration of the Selwyn Basin and the ZTEM survey is one of the few publicly available airborne audio-frequency magnetic (AFMAG) EM-magnetic datasets that offer the opportunity to study the deposit response at Howard’s Pass in close detail.Rock physical properties indicate that the lowest resistivities are associated with the Road River Group that contains the Pb–Zn mineralised horizon at Howard’s Pass, but also include graphitic shales in the same formation. Major NW–SE to ESE and minor NNW–SSE linear conductive trends correlate with known regional geologic, structural and inferred mineral trends that were previously not visible in magnetic results. At the deposit scale, a thin NW–SE trending conductive lineament extends along the > 37-km-long ‘Zinc Corridor’ horizon at Howard’s Pass, but must include both the Pb–Zn sulphide mineralisation deposit horizon as well as the surrounding graphitic black shales. 2D and 3D ZTEM inversions reveal zones of enhanced conductivity along strike and at depth that appear to correlate with the clustering of Pb–Zn deposits, which had not been previously noticed.