Old Paradigms, Fresh Ideas: Innovative Perspectives on Gold Research and Exploration in Québec

The Larder Lake gold district in Ontario and the Val-d’Or gold district in Québec present strong potential for the exploration of orogenic gold deposits, as well as magmatic-hydrothermal deposits (e.g. Upper Beaver). Understanding how these deposits were formed requires thorough knowledge of the geodynamic evolution of these two regions. These regions are characterized by an abundance of post-volcanic intrusions emplaced between ~2695 Ma and ~2660 Ma, providing an excellent opportunity to use their geochemical signature to reconstruct the geodynamic evolution of these sectors.

Our results, combining total rock geochemistry data, U-Pb data from multiple intrusions and the study of zircon trace elements, reveal distinct dynamics for these two regions. The Larder Lake area is characterized by large volumes of intrusions displaying sanukitoid-type signatures, spatially and temporally associated with small volumes of tonalite–trondhjemite–granodiorite (TTG)-type magmas in the form of dykes. The majority of these intrusions (sanukitoid and TTG) are oxidized (ΔFMQ > 1.5). Conversely, the Val-d’Or area is characterized by magmatism dominated by TTG-type intrusions and a limited number of magmas with a sanukitoid signature. These intrusions are generally more reduced than those in the Larder Lake area, with ΔFMQ values below 1.5. In both districts, variations in magma composition and degree of oxidation do not appear to be correlated with their age of emplacement. This observation contrasts with the evolution generally proposed across the Abitibi Belt.

Now primarily viewed as an environmental contaminant, mercury was used for exploration in Abitibi as early as the 1960s. Studies in the Noranda camp during the 1960s and 1970s identified Hg halos above concealed volcanogenic massive sulfide deposits. At Norbec, their footprint exceeded that of alteration. The strongest anomalies were associated with mineralization rich in Zn, Ag or tellurides. This approach contributed to the discovery of Delbridge and helped interpret targets at Millenbach and Corbett. At Sigma, step heating distinguished forms of Hg associated with gold-bearing veins, porphyries and host rocks.

Despite these findings, Hg has largely fallen out of routine exploration practice. Its volatility, variable lithological backgrounds, analytical limitations and metamorphism have complicated anomaly interpretation. Yet these complications may hold the very information being sought. Advances in direct combustion, in situ mineral geochemistry and isotope analysis now allow investigations beyond bulk concentrations to examine Hg hosts, timing and remobilization.

Recent studies identify an enriched Hg-Te signature in some Archean sanukitoid-associated gold systems, distinct from conventional orogenic systems. Observations from various Abitibi deposits nevertheless show that Hg is not a universal gold indicator: its distribution depends on the system, mineral assemblage and geological history.

This presentation revisits Hg as a longstanding exploration pathfinder and a potential tool for distinguishing metallogenic systems: a forgotten element deserving a fresh look.

Mineral systems are valuable geological markers of the evolution of geodynamic environments. In this presentation, the mineral system concept is applied to gold mineralizations, with a focus on the Keraf shear zone within the Arabian-Nubian Shield, the world-largest track of Neoproterozoic juvenile crust and one of the main Pan-African gold provinces.

To investigate this system, an 80-km-long regional geological transect was conducted across the central part of the Keraf deformation corridor to constrain the tectono-magmatic evolution of the rocks hosting the gold mineralization. Geochemical data and supra-chondritic Hf and Nd isotopic signatures reveal two juvenile magmatic flare-ups along the Keraf island arc-back-arc complex between ca. 840–810 Ma and ca. 760–655 Ma. Early arc crust was reworked during the second peak of arc magmatic activity. The ~185 Myr-long lifespan of the Keraf island arc and the predominant juvenile nature of magmatism may have promoted crustal-scale gold fertilization. Besides, a first minor magmatic-hydrothermal gold event was related to early Keraf arc magmatic accretion. The second mineralizing event is represented by a system of extensional veins hosted by these intrusive bodies, highlighting a strong rheological control on orogenic gold mineralization. The mineralized veins formed through a fault-valve mechanism as a response to episodic and localized reactivation of crustal strike-slip shearing up to ca. 550 Ma.

These new field- and laboratory-based data allow us to reinterpret this suture zone, related to the assembly of Gondwana, as a Neoproterozoic intra-oceanic arc system.

The West Menarik property is located in the La Grande Archean subprovince, in Eeyou Istchee James Bay. The volcano-sedimentary sequences are cut by intermediate to felsic intrusions and overlain by an alluvial-fluvial sedimentary basin. An integrated mineral system approach was used to reassess the gold potential of the area along three complementary axes:

- Structural architecture: A reinterpretation of the structural framework, combining field data, geophysics and high-resolution imagery, highlights a hierarchical fault network within a transpressive relay. A quantitative validation confirms the predictive capability of this architectural interpretation on the distribution of gold mineralization and identifies favourable preferential alignment.

- Magmatic and geochemical signatures: The multivariate classification of intrusions using immobile elements distinguishes several intrusive facies with contrasting geochemical signatures. A specific facies, enriched in chalcophile and volatile elements, stands out due to an anomalous gold background and a diffuse metal footprint in the host rock.

- Metallogenic fields: Mapping of hydrothermal footprints reveals the coexistence of two distinct fields on the property: a magmatic-hydrothermal-dominated field spatially associated with fertile intrusions and gold veins of orogenic type. This duality, characteristic of prolific Archean belts, opens new exploration perspectives for the project.

The Archean Au-Cu-Ag Troilus deposit, which hosts 14.5 Moz of gold equivalent (combined measured, indicated, and inferred resources), is located within the Frotet-Evans Greenstone Belt (Eeyou Istchee James Bay, Québec, Canada). The deposit is divided into four main mineralized zones (Z87, J, Southwest, and X22), two of which (Z87 and J) were mined by open pit between 1996 and 2010, producing more than 2 Moz of gold and 70 kt of copper. Historically classified either as a deformed and metamorphosed porphyry-style deposit or as an orogenic gold deposit, Troilus extends over more than 7 km in length and 1.5 km in width and exhibits a wide range of geological characteristics that make its interpretation complex. The integration of historical datasets with numerous modern exploration tools, including detailed logging of oriented drill core, core magnetic susceptibility measurements, multi-element geochemical analyses using four-acid digestion, large-scale soil and till geochemical surveys, geophysical surveys, and 3D geological modelling, combined with academic research and field mapping campaigns, has significantly improved, and continues to improve, the understanding of the deposit and has led to the development of a new metallogenic model for Troilus. A polyphased intrusion-related/VMS Au-Cu-Ag model with trace Zn-Mo-Pb-Co is now in favour. Troilus-style mineralization and alteration extends for more than 20 km along key stratigraphic trends, implying mineralizing processes were large-scale and more likely protracted. This new model has implications for exploration, which continues both at the deposit and regional scales.