Exploration targeting using 3D geophysical inversion: A new approach
An integrated geological and magnetic data inversion modelling case study
By Taronish Pithawala, Geosoft Inc. | December 10, 2013
The beauty of Geosoft VOXI is that as you learn about a property, you can add those measurable rock properties to the model and constrain it even further. It’s a very quick and efficient way to assess large tracts of ground given the right exploration target.
As mineral exploration continues to focus on deeper, more complex bodies, the need for a more complete subsurface image has never been greater. In order to mitigate the rising costs of drilling, targets must be elucidated by combining as many pieces of the exploration puzzle as possible. Geophysical methods have long been used by the mineral exploration industry to help define drilling targets. But conventional interpretation using 2D geophysical section and plan maps is becoming less reliable in deep and complex exploration plays.
Explorers can visualize the subsurface in a more instructive way by using 3D inversion of geophysical data. However, inversion is often considered a specialist tool used towards the end of an interpretation process, if at all. Moreover, it’s often difficult to reconcile geophysical earth models with known geology – making the subsurface picture more ambiguous instead of clearer.
With the introduction of VOXI Earth Modelling, Geosoft offers a new approach to inversion of magnetic and gravity data that reduces the time and effort required to generate 3D models.
The Justin Gold Project
The Justin property covers 18,000 acres in a known gold producing district in the southeast Yukon (Figure 1). The property’s most prospective regions are alteration zones that formed when a granodiorite stock intruded into calcareous limestone to form hornfels and skarn. Mineralisation consists of skarn style replacement mineralisation overprinted by quartz calcite stockwork veining. Both the skarn style and vein style mineralisation contain anomalous gold values.
In 2010 property owner Aben Resources followed up an airborne magnetic anomaly in an area of extensive scree cover and found some indications of mineralization on surface (Figure 2). Subsequent drilling intersected the POW zone, a greenfield discovery of intrusion related gold mineralization that remains open in all directions (Figure 3). The discovery hole returned 60 m grading 1.19 g/t gold, including 21m grading 2.47 g/t gold.
Because magnetics played such a key role in pinpointing the mineralization at POW, Aben’s geological consultant Terralogic Exploration considered VOXI a logical tool for finding new targets on Justin that had similar magnetic signatures but lacked outcrop.
After drilling several holes into POW, Terralogic had enough data to build a 3-D wireframe model of the zone using Geosoft’s wireframing tools (Figure 4). At the same time, Geosoft used the aeromagnetic data to run an unconstrained magnetic inversion over the zone as a first pass at a 3D earth model. But when the two models were compared, the magnetic anomaly appeared to stretch beyond the geological boundaries determined by the drilling (Figure 5).
After consulting Terralogic’s geologist, Geosoft ran a second magnetic inversion using VOXI’s iterative reweighting inversion (IRI). IRI (Figure 6) provides a more focused result that should be more geologically reasonable than the unconstrained model, but ultimately relies on the judgment of the geologist for validation. As a final check on the IRI, Geosoft built a more complex model using the drillhole geology (Figure 7). The results were similar to those of the IRI.
The reconciliation of the geology and geophysics provided by the IRI process gave Terralogic’s geologist the confidence that VOXI’s magnetic inversion would pinpoint other drill targets on Justin that were similar to POW. The whole process, from building the original wireframe to checking the IRI inversion against known drillhole data to coming up with new targets for drilling, took a matter of days.
VOXI proved to be a valuable tool on the Justin project, where the principal host of the mineralization responds well to magnetics but other geological information is limited. As a result of IRI, Aben now has several drill ready targets outside the known area of mineralization that they would not have had the confidence to drill otherwise (Figure 8). Being able to delineate and assess targets quickly and efficiently in areas where there is little to no outcrop saved the company considerable time, money and uncertainty.
The new targets outlined by VOXI at Justin demonstrate the value of collaboration between geologists and geophysicists on a software exploration platform designed to encourage iterative interpretation using all the pieces of the exploration puzzle.
The downturn in the mining industry has hit the junior sector particularly hard and Aben is no exception. Once the market returns to life, Aben plans to raise the funds to continue working on Justin. Drilling the targets delineated by VOXI will be one of the company’s top priorities.
Also read the Earth Explorer article, Geophysical inversion modelling delivers new targets in the Yukon.
Figure 1: Aben Justin Gold Property Regional Topographic Map.
Figure 2: Regional Scale Airborne Magnetic Map; POW Zone outlined in red.
Figure 3: Inset map of POW Zone showing DDH traces in deep purple.
Figure 4: 3D wireframe model built using DDH section maps; mineralisation hosted in intrusive contact halo within the hornfels and skarn units.
Figure 5: Unconstrained susceptibility inversion result with positive magnetic anomaly in blue; isosurface value = 0.012 SI. Geophysical model is overly smooth, crossing several geological boundaries. Additionally, susceptibility contrast is very low (0.012 SI) over a large volume.
Figure 6: Iterative Reweighting Inversion (IRI) focused susceptibility model in blue; isosurface value = 0.04 SI. Geophysical model correlates to known geology, with larger susceptibility contrasts (>0.04 SI) contained within the hornfels and skarn units.
Figure 7: Susceptibility model constrained with DDH data in green; isosurface value = 0.04 SI. Drillhole constrained geophysical model correlates highly with known geology but is also very similar to the IRI model (without drillhole constraints); blue isosurface = 0.04SI.
Figure 8: Regional plan view of zone outlined in black in Figure 2. IRI focused susceptibility model in purple; isosurface value = 0.01 SI. IRI method confirms known targets in drilled zones and finds potential DDH targets for follow up in untapped areas.