Exploration and Discovery with the Geotech VTEM Airborne Electromagnetic System

R. B. Barlow¹, Andrei Bagrainski², Bob Lo²

1. Geosource Geoscience Consulting, Toronto 2. Geotech Limited, Canada

ABSTRACT Ore deposits are non renewable and as more discoveries are made, new deposits gradually become harder to find. One approach to help sustain the discovery rate and therefore the world demand for metals, is to support new exploration technology development in the hope that some developments will prove successful and overcome the law of diminishing exploration returns. In 2006, the VTEM system flew approximately 250,000 line kilometers world wide. The low noise characteristics, exploration depth and spatial resolution make the system adaptable to the search for economic deposits such as VMS, Ni-Cu-PGE, Sedex type, and Uranium. This off-time system, measuring in the absence of the primary field, is proving to have an extremely wide conductivity aperture made possible by a lengthy series of measuring times and a large effective dipole moment. This aperture has been further extended by the addition of a series of B-field channels. A recent discovery of a Ni-Cu-PGE deposit and an example of a poorly conducting sphalerite occurrence are given to illustrate the system versatility and effectiveness. Introduction The VTEM system, designed by Geotech Limited, has flown more than 500,000 line kilometers since 2004. While pleasantly surprised by the growth statistics, and grateful to its loyal clients, the company is putting a parallel effort in the advancement of VTEM technology. The development of the B-field VTEM system addition has necessitated a modification of the original receiver specifications to enable a close approximation of the B-field measurements as well as dB/dt. The off-time VTEM transient measurements have a unique property in that they measure the secondary field only, in the absence of the primary field. Impulse systems, like VTEM put a large amount of energy into the early time portion of the decay. These early time measurements are equivalent to higher frequencies which have historic importance for mapping targets with weaker conductivities. As well, the long measuring time records late time channels, or low frequency equivalents useful for mapping conductors associated with Ni-Cu-PGE mineralization where the combination of conductivity and width often require significant late time measurements to allow target analysis.

By example, this paper will examine VTEM results over a new Ni-Cu-PGE discovery by Golden Chalice Resources Ltd. in Langmuir Twp., east south-east of Timmins and a test area near Wawa that has known sphalerite occurrences that do not responded well to other EM systems tested. VTEM Specifications The VTEM system operates at 30 Hz in countries powered by 60 Hz power and 25 Hz in countries with 50 Hz power. A standard duty cycle of 42 to 45 percent is used to create a wide pulse, thus extending the range of targets were run-on from the turn-on portion of the pulse is negligible.

Figure 1: An example of a Hi-power pulse with shorter pulse width to enable a higher current draw. Above panel shows the impulse response measured by the receiver and the lower panel represents the current wave form. The VTEM (Versatile Time-Domain ElectroMagnetic) system uses a central loop configured transmitter with a 26.1 metre diameter Tx loop and a dipole moment of approximately 425,000 nIA. The Tx-Rx loop altitude is normally 30 to 35 m and the effective dipole moment at the conductor compares more than favorably with fixed wing systems which must fly much higher. Golden Chalice Discovery – Langmuir Twp A Ni-Cu-PGE discovery was made in Langmuir Twp earlier this year by Golden Chalice Resources Ltd. using the results of a VTEM airborne survey and interpretation. The survey was flown with a line spacing of 75 metres at a nominal EM bird height of 35 metres and 65 metres for the magnetometer bird. Langmuir Township geology consists mostly of a komatiite volcanic suite of ultramafic and mafic flows and intrusives that rap around the southern portion of the Shaw Dome. The area is covered by overburden and has less than 5 percent outcrop in most parts of the Township.

The Montreal River fault cuts the area in a north north-westerly direction and has likely generated accompanying structural displacements as interpreted by conductor trends in various parts of the survey area.

Discovery Trend

Figure 2: Showing a portion of the flight path with anomalies over a second vertical derivative image. Montreal River fault is in black with diabase dikes in orange and geological contacts as thin black lines. The line spacing is 75 metres. The discovery trend, shown near the northwest corner of the survey, has a strike length of approximately 450 metres. Some minor displacement within this trend is evidenced by the slight shift northward of the east end of the trend.

Figure 3: Stacked profiles of Total Field Magnetics (blue) and second vertical derivative (red) in the first panel, 15 early and mid time EM channels (130 to 960 microseconds) in the middle panel and 10 mid to late time channels (1130 to 6340 microseconds) in the lower panel.

Line Conductance VTEM Tau BH Tau Depth Dip/Direction 3510 2.8 S/m x 25 m 60 m 82.5 (N) 3515 2.5 S/m x 60 m 90 m 70(N) 3520 3.0 S/m x 60 m 4.24 ms 4.6 ms 75 m 85(S) 3525 3.0 S/m x 65 m 90 m 72.5(S) 3530 3.5 S/m x 40 m 95 m 90

Table 1: Modeling Results for the discovery zone. A prism model was used with Maxwell software. Line 3520 is approximately the centre of the trend. Hand calculated Tau’s are shown for VTEM and BH results.

Figure 4: Geology (after Houle et al, 2005) of Shaw, Carman, Eldorado and Langmuir Townships. The black outline shows the Golden Chalice ground. The red arrow indicates the discovery zone.

Figure 5: Resistivity – depth image of line 3520 showing a wide prismatic conductor at the discovery hole. The inset on the right shows the diffusion pattern of the electric field at each time gate. The late time portion of line 3520 in figure 3 suggests a near vertical conductor with some complication in dip. The dispersion has slowed in the area of the conductor which appears to continue downward. The image is clipped in the areas not covered by resistivity-depth computation (figure 5) to eliminate the possibility of griding artifacts. The early time channels have stopped at the top of the conductor while later time channels continue to greater depths before eventually terminating with the last channel displayed. A section along line 3515, shown in figure 6, portrays a near identical image and shows a possible vertical section of the komatiite assemblage to the south with two anticline structures and a syncline in the middle. This feature is imaged in part on line 3520.

Figure 6: A similar image to line 3520, line 3515 shows a clear section of the likely komatiite assemblage to the south of the target.

Bore Hole EM Results The discovery drill hole was surveyed by Quantec Geosciences Ltd. for Golden Chalice using a Geonics Protem three component borehole EM with a 200 x 200 metre loop at surface. The chemical analysis of the drill core revealed 1.14% nickel over 72.5 metres including 13.1 metres of 1.74% nickel and 17.5 metres of 2.23% nickel, along with the associated copper, cobalt and platinum group elements. Nickel mineralization is associated with disseminated, fracture filled and blebs of sulphides throughout the 72.5 metre core length. Platinum and Palladium values over one 17.5 metre section were 0.20 g/t and 0.50 g/t respectively.

Figure 7: On the left, the z-component from the discovery hole showing the position of high grade sections in red and, on the right, the x- component. The bore hole data agrees well with initial VTEM interpretations above, however, modeling does not conclude with a good fit to the data when using one or even two prisms. The two high grade zones may be disconnected electrically from each other as evidenced by the field reducing to zero between the two high grade zones. Another account for the field reducing to zero would be that the probe is inside (symmetrically) the massive zone. More drilling will be required to resolve the three dimensional geometry of the deposit.

Kambalda Type Model Green, A. H. et al, 1981 have compared other similar Ni-Cu-PGE occurrences in Langmuir Township to the equivalents of Western Australian deposits classified as the “Kambalda type” model. This model has the ore melt pooling in topographic traps on the floor of the “magnesium-rich” komatiite flow rocks. The ore section is usually stratified with more massive sections grading into more disseminated concentrations near the top of the ore zone. Retrograde metamorphism, if present, can displace or reconstitute the original stratiform assemblage. Further drilling and analysis is required to place the discovery in its proper context, but early observations suggest the trends which populate the arcing lithological boundary, as depicted by magnetic rocks to the south and relatively non magnetic rocks to the north, should be classified as high priority targets. Donaldson, et al, 1986, classified komatiite flow rocks in terms of MgO content with a series starting with spinifex textured komatiites (MgO 20 to 32 %) through cumulate komatiites (MgO 40 to 90%) to the end member dunite (MgO >90%). The geology map shown in figure 4, modified after Houle et al, 2005, has a close resemblance to the regional setting at Kambalda in Western Australia with ore zones near the contacts of komatiite flows. The VTEM survey has identified a stratigraphic horizon of conductors that includes the discovery zone at the contact of magnetic and non magnetic rocks.

Poorly Conducting Sphalerite Targets The following is an example of early time measurements over a resistive area containing known sphalerite occurrences. It is interesting to note that high sphalerite content of the famous Kidd Creek ore body rendered its conductivity very low and an associated graphite conductor was the prime cause of the large airborne response. Other high content sphalerite deposits (ie SEDEX Zn-Pb-Ag deposits; 4 to 16 % Zn) are often poorly conducting and therefore represent a challenge for AEM systems.

(a) (b)

qu time channel images w 0, 110, and 130 microseconds – a, b, c, & d respectively). Oval shows a seri microseconds. Arrow shows a target gaining in strength over 60 microseco The arrow in figure 8 points to a known sdrilling. The anomalies on both sides of the arrow target are behaving in a similar manner and are therefore of priority interest. A high dipole moment is required to

(c) (d)

Figure 8: Showing a se ence of early ith anomalies plotted (70, 9

es of targets fading over 60nds.

phalerite target which is confirmed by

map these targets as well as a distribution of early time gates which are ompensated for an tter turn-off characteristics.

gure 9: T l at 1130 icroseconds. All res es have vanished for the above conductor trends.

he relationship between poorly conducting targets and sphalerite (Zn) ore grade aterial in VMS deposits is not well understood. Sphalerite is usually present ith pyrite and/or pyrrhot and often in combination with copper and lead ulphides.

rea Conductance S Range Mean

Tau millisecond Range Mean

c y transmi

A

B F

C

D

E

Fi he three small areas, Zn-conductor in the middle, have reached a current saturation levem

pons

Tmw ite s A

A 0.5-1.0 0.8 ---- <0.01 B 0.2-0.5 0.4 ---- <0.01 C 1.5-4.0 2.8 0.01-1.6 1.1 D 4.2-5.9 4.3 4.5-4.9 4.7 E 0.4-0.9 0.5 0.01-0.6 0.2 F 0.3-1.8 1.1 0.01-1.5 0.1 Table 2: Conductance and Tau statistics of conductor trends outlined in figure 9. The statistics shown in table 2 demonstrate the mapping ability of the early time hannels combined when combined with a large dipole moment. It is hoped we an obtain the assay results from core when it becomes available.

cc

This area was flown by another airborne sys er of magnitude less

a dipole m conductors were detected in this survey.

mmary

a selecti uir Township su on the geological ilarity to Type” model in Western Australia. The VTEM survey

rovided a base of low noise data, specifically in the late time channels. Strong es (time constant

illiseconds) require clean late time data to compute meaningful analysis.

ns of the

rget.

over a sistive area containing a number of known sphalerite occurrences. In order to

et a respectable response from these targets with transient EM systems, a le moment is necessary to generate currents within these targets. As

ell, sampling the relevant portion of the decay spectrum in order to “light up”

used

onaldson, M.J., Lesher, C.M., Groves, D.I. and Gresham, J.J., 1986,

tralian ccurrences, Economic Geology, v76, no. 6, p1503-1523.

oole and Guilmette, 2005, Geology of Carman and Langmuir Townships, OGS

tem with an ordof oment and no Su Are on of the Langm rvey was basedsim the “Kambaldapconductances (conductivity S/m X thickness m) and Tau valumConductivity- depth imaging was found to be helpful when combined with 3D modeling. Bore Hole EM is very helpful for guiding the drill program. A large loopground system will hopefully supply information on the deeper regiota The later example, illustrates the use of early-time channels acquiredregstrong dipowpoorly conducting features of economic importance, is required. Acknowledgements The authors would like to thank Golden Chalice Resources Limited for datain this paper. References DComparison of Archean dunites and Komatiites associated with nickel mineralization in Western Australia: Implications for dunite genesis, Mineralium Deposita, vol. 21, no. 4, p296-305. Green, A.H., and Naldrett, A.J., 1981, The Langmuir volcanic peridotite – associated nickel deposits; Canadian equivalents of Western Auso HPreliminary Map P3268.