Case study: Applying SkyTEM and Dias technologies

Integration of airborne (SkyTEM) and ground (Dias) electrical survey methods to identification of a new gold-bearing deposits in Northern Kazakhstan

Authors:

V. Kaminskiy (Promiseland Exploration, Ltd)

E. Taikulakov (Promiseland Exploration, Ltd)

Y. Janseitov (Aurora Minerals Group)

A. Darayev (Aurora Minerals Group)

 

Introduction

The study area is located in the North Kazakhstan region about 250 km north-west from the famous Vasilkovskoe deposit. The terrain is a slightly rugged and prospective for gold ore deposits, including being in association with sulphide mineralization. For confidentiality reasons, the exact location of the area has not been disclosed.

In 2017, the SkyTEM airborne EM survey method was conducted over the study area (more than 1600 line kilometers within a scale of 1: 40,000). SkyTEM data were interpreted by Promiseland exploration, Ltd. As a result, SkyTEM data analysis revealed a number of prospective conductive anomalies, which may be associated with mineralization (sulphidization) zones. Some of these anomalies (large) were subject to modelling, in order to determine the parameters, and some areas, due to the large scale of the survey, were recommended for verification by ground geophysical methods at scale of 1: 5000 for the purpose of detailing.

The ground geophysical survey conducted at four sites (Figure 1) using a direct current /pole-dipole configuration of induced polarization method. Data were obtained by Dias Geophysics and further processed and inverted in 3D. As the results of three-dimensional inversions, polarized high-resistance objects were identified as possible drilling targets, which may be interpreted as different type of gold-bearing objects (not associated with massive sulfides).

Part of the drilling program was performed based on interpretation of SkyTEM data. In the southeast area, gold-bearing mineralization associated with sulphides was revealed at depth of over 100 m and remains open at depth. Drilling operations are still ongoing. The gold content was recovered to reach 10 g/t over 9 m interval.

Geological settings

Chlorite-sericite-clay and quartz-chlorite-sericite schists, metamorphosed polymictic sandstones and siltstones, tuff argillites, tuff aleurolites and tuff sandstones take part in the structure of the area under study.

In the vicinity of fault zones and dikes, quartzite and jasper quartzite are developed along siliceous schist, which belong to the Lower-Middle Ordovician age. The upper part of the formation is composed of polymictic and quartzitic sandstones with aleurolitic and argillaceous lenses. Intrusive formations within the area are developed quite widely and represented by dykes, dyke-like subvolcanic bodies. These bodies are composed of gabbro, diabases, trachyandesites, mica lamprophyres, plagiogranite-porphyries, granite-porphyries, breccia lavas and breccia lavas lava-breccia of the main composition. The geological map of the study area is shown in Figure 1.

Figure 1. Geological map of the study area. Green lines represent the flight path of SkyTEM survey. Blue area corresponds to the place where gold deposit is found. Colored grids (on the left) represent the areas where ground electrical surveying was carried out.

All units described above are covered by mainly continuous Cenozoic cover of loose deposits, represented by clays, loams, sandy loams, whereas sands and pebbles are not common. Listed rocks deposits form a sufficiently powerful (from 40 to 100 m) low-resistance layer.

Geophysical methods

SkyTEM system

A study area was covered by airborne geophysical survey in a scale of 1:40 000 using a SkyTEM 312M system. Overall flight path of two blocks were more than 1 600 linear kilometers (Figure 1). SkyTEM technology is a helicopterborne transient EM method (Figure 2 illustrates the scheme of a SkyTEM system). The effective loop area of a system is approximately 2500 m2, which provides a dipole moment of up to 500,000 NIA.

Figure 2. A scheme of a SkyTEM system.

Figure 2 shows a transmitter coil, which is fixed on the suspension system and towed behind helicopter emitting electro-magnetic (EM) pulses. A system is also equipped with GPS receivers, laser altimeters and a two-component (XZ) sensor for measuring the secondary EM signal. The magnetometer towed on the external suspension 9 m apart along the X axis.

SkyTEM system is operated in dual-moment mode, using a “low” moment (Figure 3a) and a “high” moment (Figure 3b). Measurements were made in time domain (in off-time mode) on 14 channels (from 20 ms to 617 ms) for the "low" moment and 26 channels (from 0.13 ms to 11.6 ms) for the "high" moment respectively. A SkyTEM system with proper configuration can provide penetration of the interpretable signal to 300 m.

Figure 3. Pulse configuration of SkyTEM system: (a) “Low” moment; (b) “high” moment.

Dias system

Based on a SkyTEM data interpretation, several sites were assigned for verification by ground electrical survey method. Induced polarization (IP) method, powered on direct current, was applied. Three-dimensional multi-electrode data was collected in a pol-dipole configuration using the Common Voltage Reference technology (CVR). Ground electrical survey (4 sites) was conducted by Dias Geophysics (Canada) (Figure 4).

Figure 4. Location of study areas by ground electrical survey using the Dias technology.

The specific feature of the CVR technology is that the receiving electrodes are arranged in a regular grid that fills the entire area. Usually, up to 300 receiving electrodes are used simultaneously. Potential electrodes are set up together with the receiving ones, and single potential electrode is carried out to the conventional “infinity” (at a distance of about 3 km apart from the study area). Data collection with the mode of direct current/IP is then performed using all standard geometries of pole-dipole configuration, but without relocations of potential electrodes. This provides uniform three-dimensional coverage of subsurface. Overall (N2-N)/2 measurements are collected at each site, where N is the number of receiving electrodes. Three-dimensional models of electrical conductivity and chargeability distributions were obtained. Depth of these models reaches several hundred of meters.

Interpretation and integration

“Low” moment of SkyTEM data was not considered at the interpretation stage. Since this configuration is mainly used for groundwater and engineering studies. It does not carry significant information about bedrock conductors.

About thirty conductivity anomalies were identified in the “high” moment data, some of which were drilled in the summer of 2018. Borehole illustrated in Figure 5 corresponds to sulfide mineralization with high (up to 10 g/ton) gold content per 9 m interval.

Figure 5. A SkyTEM anomaly originated from sulfide mineralization with high gold content.

Anomalies that were recommended for ground-based verification and covered with Dias electrical survey did not confirm that conductive anomalies originate from the deep seated structurers (bedrock source). However, several large anomalies of chargeability were identified (Figure 6, Figure 7 and Figure 8). Anomalies are shown on horizontal slices at absolute elevation levels of 200, 100, and 0 m, which correspond to approximate depths of 150 to 350 m (with average elevations of 325 to 350 m).

Figure 6. Horizontal slices of recovered chargeability from 3D inversion model of Dias data, at absolute elevation 200 m (approximate depth of 150 m).

Figure 7. Horizontal slices of recovered chargeability from 3D inversion model of Dias data, at absolute elevation 100 m (approximate depth of 250 m).

Figure 8. Horizontal slices of recovered chargeability from 3D inversion model of Dias data, at absolute elevation 0 m (approximate depth of 350 m).

Consolidated view of three-dimensional chargeability models, obtained from Dias data by three-dimensional inversion (Figure 9).

Figure 9. Three-dimensional distribution of chargeability based on the results of three-dimensional inversion of Dias data.

Conclusion

We have demonstrated an efficient and smart method of geological prospecting for gold-bearing deposits. Applied airborne electrical exploration method demonstrated good opportunities to reveals sulphidization zones. Ground electrical survey allows identify anomalous objects associated with massive or disseminated mineralization or zones of silicification. Currently, drilling program in the area is still ongoing.

 

Published in magazine “Zoloto i technologii” September№3(41) 2018.

Case study was presented at MINEX Russia 2018 by Geophysical Consultant V. Kaminskiy.