Geophysical inversion contributions to mineral exploration: lessons from the Footprints project

Abstract

Magnetic and gravity inversions are used to create 2D or 3D models of the magnetic susceptibility and density, respectively, using potential field data. Unconstrained inversions generate an output based on mathematical constraints imposed by the inversion algorithm. Constrained inversions integrate lithological, structural, and petrophysical information in the inversion process to produce more geologically meaningful results. This study analyses the validity of this assertion in the context of the NSERC-CMIC Mineral Exploration Footprints project. Unconstrained and constrained geophysical inversions were computed for three mining sites: a gold site (Canadian Malartic, Québec), a copper site (Highland Valley, British Columbia), and a uranium site (Millennium – McArthur River, Saskatchewan). After initially computing unconstrained inversions, constrained inversions were developed using physical property measurements, which directly link geophysics to geology, and lithological boundaries extracted from an interpreted geological model. While each derived geological model is consistent with the geophysical data, each site exhibited some magnetic complexity that confounded the inversion. The gold site includes regions with a strong magnetic signature that masks the more weakly magnetic zone, thereby hiding the magnetic signature associated with the ore body. Initial unconstrained inversions for the copper site yielded solutions with invalid depth extent. A consistency between the constrained model and the geological model is reached with iterative changes to the depth extent of the model. At the uranium site, the observed magnetic signal is weak, but the inversion provided some insights that could be interpreted in terms of an already known complexly folded geological model.

The Henkel Petrophysical Plot: Mineralogy and Lithology From Physical Properties

“…The strong magnetic contrast between the arithmetic mean susceptibility of the outer Border and Guichon phases and the less magnetic country rock and the inner more felsic phases (1 order of magnitude) leads to the generation of a significant magnetic anomaly which has been modeled to define the 3‐D geometry of the outer shell of the intrusion (Vallée, Farquharson, et al, ). Density values also exhibit a similar trend with higher values (2.80 g/cm 3 ) being associated with the more mafic Border Phase to values around 2.66 g/cm 3 being associated with the most felsic Bethsaida Phase (Vallée, Morris, et al, ; Figure 12). Unfortunately, this minimal variation in density contrast together with limited gravity observations means that the pluton has a broad negative anomaly, but gravity surveys cannot resolve the geological contact associated with the mafic Border Phase.…”

“…Using density values reported by Ager et al () the best‐fit gravity profile model calculated by Roy and Clowes () included a contrast of −0.15 g/cc between the outer more mafic units and the inner more felsic phases. Using the same geologically and seismically constrained approach on a more recent airborne gravity data set gave more consistent coverage but with less detail (Vallée, Morris, et al, ). Using this low‐resolution airborne gravity data, they were able to define a sharp western margin of the batholith, but their inversion was unable to detect any internal structure of the pluton.…”

“…Mahmoodi and Smith (), for example, apply fuzzy k‐means clustering on borehole susceptibility, density and gamma logs to characterize lithological zones in the Sudbury intrusive complex. These data, especially density and magnetic susceptibility when correlated to lithologies and mineralogies, are essential inputs to constrain the geophysical inversion and geological interpretation of magnetic and gravity surveys (Vallée, Farquharson, et al, ; Vallée, Morris, et al, ). In this paper, we dissect systematic changes in density and magnetic susceptibility and how these relate to geological processes including igneous rock formation, weathering and sedimentary transport, metamorphism, alteration, and ore formation.…”

The Henkel Petrophysical Plot: Mineralogy and Lithology from Physical Properties

“…Using density values reported by Ager et al, (1973) the best-fit gravity profile model calculated by Roy and Clowes (2000) included a contrast of -0.15 g/cc between the outer more mafic units and the inner more felsic phases. Using the same geologically and seismically constrained approach on a more recent airborne gravity dataset gave more consistent coverage but with less detail (Vallée et al, 2019b). Using this low resolution airborne gravity data, they were able to define a sharp western margin of the batholith, but their inversion was unable to detect any internal structure of the pluton.…”

Laser-Induced Breakdown Spectroscopy—An Emerging Analytical Tool for Mineral Exploration

“…Such multi-parameter, mineral deposit footprints are now well documented at most ore system types and are widely used to vector toward ore zones within individual deposits and, more rarely, at continent to mineral district scales [17][18][19][20]. Geophysical imaging is proven particularly effective at mapping these deposit footprints because ore-forming processes tend to produce mineralogical changes that impact rock properties, e.g., density, magnetic susceptibility, conductivity [21]. However, some components of a mineral deposit footprint can be invisible to geophysical methods, either because the rock property changes between the host rock and the ore-forming mineral assemblage are relatively subtle and/or because the ore-forming process only resulted in trace element substitution into the main rock-forming minerals.…”