Jean M. Legault

ZTEM tipper AFMAG and 2D inversion results over an unconformity uranium target in northern Saskatchewan

Summary An airborne AFMAG demonstration test survey was conducted using the ZTEM tipper electromagnetic prospecting system over an unconformity uranium occurrence in northern Saskatchewan, Canada. These tests were conducted in early 2008 and were flown to test the system’s capability to penetrate and define the basement geology below a thick sequence of Athabasca sandstones, as well as defining resistivity structure relating to possible fault-controlled alteration zones. The AFMAG method utilizes naturally occurring audio telluric fields from worldwide sferic activity as its primary field source and is capable of large penetration depths – in the order of hundreds of metres to km. The AFMAG field tests corroborate previously airborne transient electromagnetic, magnetic and ground follow-up EM surveys. A 2D inversion algorithm has been modified to account for the air layer required for the ZTEM tipper data. The results agree with the known geology to below >500m.

ZTEM airborne tipper AFMAG test survey over a magmatic copper‐nickel target at Axis Lake in northern Saskatchewan

An airborne AFMAG demonstration test survey was conducted using the ZTEMTM tipper electromagnetic prospecting system over a known magmatic copper-nickel occurrence in northern Saskatchewan, Canada. These tests were conducted in early 2008 and were flown to test the system’s capability to detect the conductive but relatively weakly mineralized sulphide deposits and to define potential extensions at depth. The AFMAG method utilizes naturally occurring audio telluric fields from worldwide sferic activity as its primary field source and is capable of large penetration depths – in the order of hundreds of metres to km. The AFMAG field tests corroborate previously airborne transient electromagnetic, magnetic and ground follow-up EM surveys. 2D forward modelling results agree with the >300m vertical depth extent as defined in subsequent diamond drill testing.

Ground IP-Resistivity, And Airborne Spectrem And Helicopter ZTEM Survey Results Over Pebble Copper-moly-gold Porphyry Deposit, Alaska

Survey results from ground IP-Resistivity, Spectrem airborne electromagnetics and magnetics, and ZTEM tipper AFMAG helicopter electromagnetics are compared over the Pebble porphyry copper deposit, in southwest Alaska. The IP data display higher amplitudes associated with the increased sulphides over the Pebble alteration halo that vector towards the deposit. Lower resistivities are associated with the chargeable alteration halo and slightly higher resistivities occur in the less chargeable core at Pebble West. The deeper Pebble East was not detected in IP due to insufficient penetration. The airborne EM surveys, Spectrem and ZTEM, appear to map the resistivity contrast with more resolution than the ground IP resistivity survey, due to higher sampling and better depth of investigation. Spectrem conductors coincide with the rich pyrite mineralization, sulphide veins and alteration, but the depth of investigation is at the detection limit of Pebble East. The ZTEM tipper AFMAG survey appears more sensitive to weak resisitivity contrast especially at depth. Both Pebble West and East are well detected in the ZTEM results.

Airborne Inductive Induced Polarization Chargeability Mapping of VTEM Data

Airborne inductive induced polarization (AIIP) effect has been widely recognized in airborne time domain EM system data. AIIP chargeability mapping opens new and exciting areas in mineral exploration for airborne time domain EM systems in the search for sulphides and clay minerals. An AIIP chargeability mapping tool based on CSIRO/AMIRA Airbeo is created for VTEM data, with examples from Mt Milligan, British Columbia, Canada and Tullah, Tasmania. Using the Cole-Cole frequency dependent resistivity, the tool examines the VTEM decay data spectrally and selects the decay associated with the lowest RMS error from a set of decays generated by varying chargeability m and time constant T within specific ranges, giving a constant frequency factor c, while the background resistivity is inverted. The parameter mo used to generate the decay is the AIIP apparent chargeability.

3D inversion of SPECTREM and ZTEM airborne electromagnetic data from the Pebble Cu–Au–Mo porphyry deposit, Alaska

Geological, geochemical, and geophysical surveys have been conducted in the area of the Pebble Cu–Au–Mo porphyry deposit in south-west Alaska since 1985. This case study compares three-dimensional (3D) inversion results from Anglo American’s proprietary SPECTREM 2000 fixed-wing time-domain airborne electromagnetic (AEM) and Geotech’s ZTEM airborne audio-frequency magnetics (AFMAG) systems flown over the Pebble deposit. Within the commonality of their physics, 3D inversions of both SPECTREM and ZTEM recover conductivity models consistent with each other and the known geology. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West. The high grade CuEqn 0.6% ore shell is not consistently following the high conductive trend, suggesting that the SPECTREM and ZTEM responses correspond in part to the sulphide distribution, but not directly with the ore mineralization. As in any exploration project, interpretation of both surveys has yielded an improved understanding of the geology, alteration and mineralization of the Pebble system and this will serve well for on-going exploration activities. There are distinct practical advantages to the use of both SPECTREM and ZTEM, so we draw no recommendation for either system. We can conclude however, that 3D inversion of both AEM and ZTEM surveys is now a practical consideration and that it has added value to exploration at Pebble. This case study compares 3D inversion results from SPECTREM 2000 fixed-wing time-domain airborne electromagnetic (AEM) and ZTEM airborne audio-frequency magnetics (AFMAG) systems flown over the Pebble Cu–Au–Mo porphyry deposit in south-western Alaska. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West.

Helicopter EM (ZTEM–VTEM) survey results over the Nuqrah copper–lead–zinc–gold SEDEX massive sulphide deposit in the Western Arabian Shield, Kingdom of Saudi Arabia

Magnetic and electromagnetic (EM) results from two helicopter EM surveys, a time-domain (VTEM) and AFMAG (ZTEM), are compared over the Nuqrah sedimentary exhalative (SEDEX) massive sulphide deposits in the Western Arabian Shield of the Kingdom of Saudi Arabia. The magnetic and EM data from both surveys map the major controlling structures that host the Nuqrah North and South deposits. Neither Nuqrah deposits stand out as distinctive aeromagnetic anomalies, but both EM surveys define the massive sulphide mineralised vent and bedded portions of the SEDEX orebodies. ZTEM is interpreted to be more capable in defining the larger, lower conductance and less mineralised distal portions of the SEDEX system. The modelled ZTEM also defines a down-dip extension of the Nuqrah South zone below a depth of 750 m. Magnetic and electromagnetic (EM) results from both time-domain (VTEM and AFMAG (ZTEM) helicopter EM surveys are compared over the Nuqrah sedimentary exhalative (SEDEX) massive sulphide deposit in Saudi Arabia. The magnetic and EM data map major controlling structures but only the EM surveys are able to define the Nuqrah deposits.

ZTEM and VTEM airborne EM survey results over PGM-Cu-Ni targets at East Bull Lake anorthositic complex, Massey, Ontario

Summary Heliborne VTEM and ZTEM surveys were flown over East Bull Lake Intrusive Complex near Massey ON. The VTEM results show good correlation with known occurrences and identify new areas for follow-up. The ZTEM survey results correlate well with known geology and provide indications of deep structure that agree with previous scalar and tensor AMT survey results. Comparisons between these data sets and the East Bull Lake geology and structure are showcased using 1D-2D-3D inversion modeling.

ZTEM and VTEM airborne EM and magnetic results over the Lalor copper‐gold volcanogenic massive sulfide deposit region, near Snow Lake, Manitoba

AbstractThe 14.4-Mt Lalor copper-zinc-gold deposit situated in the Flin Flon Greenstone Belt of north-central Manitoba is large (>900×700  m) but deeply buried (>550−1200  m) and was discovered using deep penetrating ground fixed-loop time-domain electromagnetic (EM). Results from three different airborne EM surveys over a five-year span permit comparisons of the system responses over the Lalor volcanogenic massive sulfide deposit region, assisted using 1D-2D-3D inversions. In late March, 2007, soon after its drilling discovery, Hudson Bay Exploration contracted Geotech to fly helicopter versatile time-domain electromagnetic (VTEM) surveys over the region west of Snow Lake, Manitoba. The deposit was not detected due to its large depth of burial (>550  m) and the lack of magnetic response. In 2009, Geotech carried out a helicopter z‐axis tipper electromagnetic (ZTEM) passive EM test survey over the Lalor deposit, which was successful and led to a larger survey over the area. In 2012, a second VTEM survey t...

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

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. In 2008, a ZTEM natural field EM and magnetic survey was flown over the world-class, >250 Mt SEDEX deposits at Howard’s Pass in the Selwyn Basin, Yukon. The AEM data provide insights into regional structures and intrusions. They also reveal a correlation between enhanced conductivity along strike and the clustering of deposits at Howard’s Pass.

Airborne Geophysics, Remote Sensing, UAV (Drone)-based Surveys and Mining Geophysics

Knowledge of occurrence and extent of quick clay is vital regional for hazard zonation and in detail for infrastructure projects. Quick clay poses a serious geohazard in Scandinavia and Canada (amongst others) as it practically liquefies at failure and thus leads to serious, retrogressive slides. To this end, geotechnical drillings and samples are analyzed, to indicate sensitive clay in an area. As quick clay has a higher resistivity than marine clays, geophysical methods looking at resistivity distribution provide a valuable tool in addition to geotechnical assessment. So far, this has mostly focused on electrical resistivity tomography (ERT) for detecting these subtle changes. Supporting a recent road development project close to Oslo AEM was suggested in order to link drill sites and fill the data gaps between them. Quick clay is not easily identified in the AEM data, but some possible occurrences agree well with the results from drillings. Especially where the sediment layer is thick, variations in electrical resistivity within this layer can be resolved. These subtle changes can be linked to quick-clay extend by comparison with borehole data and ground-based geophysical methods (ERT and IP). We discuss results from the combination of these methods and outline the possibilities and limitations of quick-clay mapping using AEM.