Kent Balas – Aurora Minerals Group, Astana, Kazakhstan, kbalas@aurora.kz
Flemming Effers – SkyTEM™ surveys, Aarhus, Denmark, fe@skytem.com
In 2016 Aurora Minerals Group (AMG) introduced the SkyTEM™ airborne time-domain electromagnetic (“TEM”) acquisition system to Kazakhstan. The system acquires high quality TEM data. One of the challenges with TEM data, in general, is that the collected data cannot be translated into real geological features directly. To resolve real geology several transformations of the data need to be completed. To date, the state of the art code to complete this step has been the Arjun air code (Wilson et al., 2006). Jovan Silic and Intrepid geophysics have recently re-written this Inversion code to advance the original code, improving the quality of the subsurface conductivity model in areas of steep terrain and high conductivity contrast (Silic et al., 2015).
Introduction
Aurora Minerals Group has been contracted to conduct exploration in North Kazakhstan. The target area is composed of Silurian and Devonian plutonic rocks intruding a Proterozoic marine sequence. The area has been subjected to significant crustal shortening, major faults trend to the north-east and north-west. In 2016 a SkyTEM™ helicopter TEM survey was conducted, targeting linear magnetic features that are associated with mineralisation found during historical exploration work. The results from this survey were inverted using the 2.5D Inversion code described in (Silic et al., 2015). The targeted area was a linear magnetic anomaly, the first vertical derivative magnetic image is shown below in Figure 1. The elevation range within the survey area is ~380m RL – 280m RL.
Figure 1 - The magnetic anomaly targeted with the AEM survey, first vertical derivative magnetic image shown.
SkyTEM AEM survey
The SkyTEM™ survey was completed during October 2016 targeting a linear magnetic anomaly. The survey flight lines were oriented to be roughly perpendicular to the strike of the magnetic anomaly. The purpose of the investigation was to see if there was a resistive zone associated with the linear anomaly, indicating that there would be strong quartz stock-work veining that could host gold and base metal mineralisation. The total survey was for 614 linear km. Flight lines and the Digital elevation model are shown in Figure 2 and Figure 3.
Figure 2 - TMI first vertical derivative overlain with SkyTEM EM survey flight lines
Figure 3 - Digitial elevation model for the survey area
Figure 4 - Setting up the SkyTEM™ system prior to the survey
Figure 5 - Helicopter lifting off to conduct the survey
Electromagnetic data Inversion
The inversion of the SkyTEM™ data was completed using the Intrepid 2.5D code described in (Silic et al., 2015). The inversion results create a subsurface conductivity model with geometry that reflects real geological features. These data can then be integrated with other geological data to make inferences about possible locations for mineralisation.
Results of Inversion
The Inversion results in a subsurface conductivity/resistivity model that reflects real geological features. The data output comes in the form of an ASCII file with X,Y,Z coordinates and an associated conductivity and resistivity value. These data can then be imported into a 3D geological modelling package such as Micromine, Surpac or Datamine and modelled as a conductivity/resistivity “block”, see Figure 6. Once these data are imported, the Geologist is then able to add other geological data and correlate this with geophysical data.
Figure 6 - 3D finite element mesh of Log conductivity values
Resistive zone associated with mineralization
Figure 7, below, shows a surface Log conductivity image and a -300m RL conductivity image. The images clearly define a resistive zone. This resistive zone is strongly correlated with high grade assays from historical work. Figure 8 shows the surface conductivity model correlated with an historical prospect map with the results of a geochemical survey and several trenches. Au grades up to 4 g/T, Ag to 25 g/T and Cu up to 0.5% are seen associated with the resistive zone defined by the SkyTEM™ survey.
Figure 7 – Surface conductivity (A) and 300m RL depth slice through the conductivity model, units are Log mS/m
Figure 8 - Surface conductivity model overlain with historical work showing a geochemical anomaly and assay grades from trench sampling. Au grades upto 4 g/T, Ag grades up to 25 g/T and Cu grades up to 0.5% are seen
Conductive zone at depth
A second feature visible in the Intrepid inversion is a conductive zone at depth at approximately the 200m level, see Figure 9. This conductive zone is interpreted to be a paleochannel that runs parallel to the magnetic anomaly. This zone may be an excellent target for alluvial gold mineralisation. It is interpreted that a paleochannel has cut into the alteration zone associated with the Quartz stock-work veining from the adjacent resistive zone. The features defined in the image outline a large alluvial system.
Figure 9 - 200m RL slice through the Intrepid conductivity model
Conclusion
The SkyTEM™ survey and subsequent Intrepid data inversion have been successful in defining both conductive and resistive zones that are associated with mineralisation. The primary zone of interest defined by the survey and subsequent data inversion is the resistive zone that correlates with the magnetic anomaly.
The secondary features of interest are the possible paleochannel adjacent the magnetic anomaly, this zone probably sourced minerals from a clay rich alteration zone, concentrating gold in an alluvial system of ~8km long by 1 – 1.5km wide.
The workflow used during this survey and subsequent data processing is industry leading and will be used during 2017 to delineate further targets for mineral exploration.
References
Silic, J., Paterson, R., FitzGerald, D., Archer, T., 2015. Comparing 1D and 2.5 D AEM inversions in 3D geological mapping using a new adaptive inversion solver, in: 14th International Congress of the Brazilian Geophysical Society & EXPOGEF, Rio de Janeiro, Brazil, 3-6 August 2015. Brazilian Geophysical Society, pp. 184–189.
Wilson, G.A., Raiche, A.P., Sugeng, F., 2006. 2.5 D inversion of airborne electromagnetic data. Explor. Geophys. 37, 363–371.
