Using magnetic susceptibility measurements to define mineral exploration targets

Using magnetic susceptibility measurements to define mineral exploration targets

Mineral explorationists rely on quality data processing to help minimize exploration risk and effectively define new targets. When teams collect multiple sets of surface and borehole geological and geophysical data, Mira Geoscience uses the Common Earth Model framework to integrate all data sets for superior interpretation and better decision-making.

In the case of porphyry copper-gold deposits, the identification of a high temperature hydrothermal centre often associated with magnetite is frequently an attractive exploration target. Traditionally, magnetic inversion models derived from surface or airborne data are used to identify magnetic bodies at depth. However, regional magnetic trends and/or the presence of high magnetic susceptibility geological units may obscure the magnetic signature of such discrete bodies. By combining ground or airborne magnetic survey data with measured drillcore susceptibility data, the resolution and geometrical accuracy of the magnetic inversion is greatly increased.

Case study: Triumph Gold Corp’s Freegold Mountain Project, Yukon, Canada

The Freegold Mountain Project is a great example of producing useful alteration modelling through geologically-driven constrained magnetic inversion of ground and airborne magnetic surveys guided by drillhole magnetic susceptibility measurements. This helped Triumph Gold identify valuable new exploration targets based on a porphyry copper-gold deposit model.

Utilizing drillhole magnetic susceptibility measurements to build a starting susceptibility model for the magnetic inversion, and then fixing the model susceptibility where intercepted by drilling, effectively uses all available sources of subsurface magnetic information. This approach allowed Mira Geoscience to improve the representation of drilled magnetic zones within the inversion model to highlight residual susceptible material in underexplored areas. GOCAD Mining Suite was used to interpolate the downhole susceptibility data via simple kriging within the vicinity of logged boreholes in each rock type of the geologic model.

VP Geophysics Suite (VPmg) was used to test the conformity of the provided 3D geologic model with the different magnetic surveys through forward modelling. Some adjustments were performed on the starting model to minimize the residual values due to discrepancies between the geological model and the observed data. The drillhole magnetic susceptibilities were interpolated into the model using simple kriging and homogeneous inversions allowed optimization of magnetic susceptibility values for each modelled lithology in regions beyond the influence of drillhole data. Finally, a drillhole constrained heterogenous magnetic susceptibility inversion was performed. This inversion resolves anomalies not explained by the geological and physical property inputs while preserving magnetic susceptibility information in model cells intercepted by the drillholes.

In the end, the drillhole-constrained magnetic inversion model successfully identified pipe-shaped magnetic anomalies interpreted to represent high heat centres marked by potassic alteration and the presence of magnetic material (see figure above). Triumph Gold’s 2020 field season confirmed that the anomaly observed in the Keirsten Zone was in fact associated with a mineralized hydrothermal system. A similar magnetic anomaly was confirmed in 2020 drilling, near the Revenue Deposit. Magnetic anomalies are also observed under and in the vicinity of the gold and copper rich WAu and Blue Sky Breccias, which Triumph plans to explore in the coming exploration season.

Notes on software

GOCAD Mining Suite (GMS) is the only 3D interpretation environment that allows the creation of plausible geologic models that honour all geoscientific data, whether the data are sparse or dense. It performs especially well in the context of a large amount of borehole information to constrain 3D geological and geophysical data modelling. GMS, along with VPmg, enables Mira Geoscience to interactively interpret and model geologic data using borehole physical property measurements with potential field data. This unique setup, in concert with four different inversion styles, gives our consultants and clients full control to test geological hypotheses through forward modelling, extend geologic models, or discover new prospective areas.