Magmatism and Tectonics: Intersecting Perspectives on Archaean Metallogeny

The transition from Archaean to Paleoproterozoic marks a pivotal period in Earth’s history, reflected by significant changes in the lithosphere. With relatively few rocks from the Siderian (2500-2300 Ma) and Rhyacian (2300-2050 Ma) periods preserved in the geological record, this transition remains poorly understood. The late Siderian to Rhyacian terrains of the southern West African Craton (sWAC) offer a valuable window into this enigmatic interval of Precambrian history. Over the past century, numerous studies have investigated this Paleoproterozoic record to decipher its tectonic, lithological, geochemical, geochronological, structural, metamorphic, and metallogenic characteristics. In the context of the West African eXploration Initiative (WAXI) project, Perret et al. (2025) integrate all available published datasets to provide new insights into the late Siderian-Rhyacian tectonic evolution of the sWAC. This presentation will showcase this comprehensive data compilation, emphasizing tectonic and magmatic features of the sWAC, and will compare them with Archean metallogenic provinces to discuss secular changes recorded in Precambrian convergent-margin metallogenic cycles.

Archean cratons comprise greenstone belts overlying vast granitic gneiss complexes coring crustal-scale domes. The mafic-ultramafic rocks of the greenstone belts imply partial melting of a mantle with a higher average temperature than the present-day. The ubiquitous presence of migmatites that crystallized at a depth of about fifteen kilometers reflects a high geothermal gradient and a crust significantly affected by partial melting. Magmatic rocks from the iconic Pilbara and Barberton cratons have Nd model ages from 4.0 to 3.0 Ga and U-Pb zircon ages from 3.7 Ga to 2.7 Ga. These data nourish a discussion on the growth-differentiation modes of the crust and associated mineral systems attributed to a succession of mantle plumes or gently dipping subduction zones.

An alternative model is that, following the formation of a primitive mafic crust, the differentiation of the Archean crust is controlled by its partial melting and convection for more than a billion years. In this context, gold deposits on the one hand associated with the formation of the mafic crust remobilized in shear zones controlling the circulation of hydrothermal fluids, and uranium deposits on the other hand, located in late-Archean potassic or peraluminous granites, provide specific markers of the growth and differentiation of the crust by magmatism and hydrothermalism.

Mineralization types are often associated with specific tectonic environments. A review of geological and geochemical data will show that modern-style ridges and arcs did not exist in the Archaean, and a periodically-destabilized stagnant-lid crust system is favoured. The presentation will examine how exploration strategies can be adapted to non-uniformitarian interpretations of the geology.

Periods of mantle overturn lasting up to 100 My produce enhanced magmatism, crustal mobility and tectonism. Mantle upwelling zones deliver large volumes of basalt and komatiite, inducing crustal reworking and possible breakup of continents. During rifting, primitive melts often ascend along crustal/lithospheric discontinuities, localizing komatiites (Ni potential), and layered intrusions (Cr-PGE). Greenstone volcanics following the rupture develop mafic-felsic cycles (2-10 My). During the felsic stage, high-level magma chambers develop, creating the heat engine that drives the shallow hydrothermal cells that leach and concentrate metals to form VMS deposits. Lateral mantle flow during overturns presses against continental lithospheric keels, setting them into motion. The weak and buoyant Archean oceanic crust does not subduct, but imbricates or subcretes beneath the leading edge of the drifting Archaean continent, forming a Kenoran-style orogen. Imbricating convergent margins would be characterized by downthrusting of abundant altered basaltic crust, representing a plausible source for auriferous metamorphic fluids. Late-stage faults on the edges of indentors would allow fluids to drain upwards abruptly and deposit gold. Underthrusting of crustally-derived sediments could transfer LILE and metals to the mantle, creating the fertile metasomatized source needed to form late-stage fluid-rich sanukitoids, some of which develop Mo-Cu-Au porphyry-type mineralization.

This study focuses on TTG (tonalite-trondhjemite-granodiorite) type plutonism in Abitibi, its relationship with crustal evolution and metallogenic environments. The trace element signature (La/Yb, Sr/Y, Eu/Eu*, Nb, Ta) of 32 dated intrusions classified according to their FI-FII-FIII fertility (Lesher et al., 1985) connects mineralization systems to melting depth and crustal architecture. At the same time, the crystallization depths of these rocks were estimated on the basis of their normative mineralogy. The chronological evolution of these intrusions combined with their melting and crystallization depths reveal crustal thickening as a result of tectonism. Furthermore, the distribution of TTG types shows an early compartmentalization of the Abitibi crust into three distinct domains.

In the North-East domain, the predominance of FI-type intrusions indicates early crustal thickening and an arc context favourable to 1) porphyry deposits (e.g, Chibougamau) within shallow synvolcanic intrusions; 2) intrusion-related gold mineralization (e.g., Windfall) related to deeper LILE-enriched syntectonic intrusions. The southern Abitibi region is dominated by FIII and FIIb-type intrusions, reflecting thin crust and high heat flows associated with major VMS camps (e.g., Rouyn-Noranda). The North-West domain displays FII-FI evolution, reflecting greater melting depths caused by crustal thickening related to tectonism. This domain is punctuated by troughs or thinned thermal corridors favourable to minor VMS camps (e.g., Normétal). This study demonstrates the use of compositional variations in TTG as an indicator of crustal processes and metallogenic potential.

Granitoids of the TTG suite are the most voluminous felsic rocks in Archean cratons. As such, the generation, mobilization, and crystallization of these magmas played a major role in the evolution of Archean crust. TTG suites represent large-scale mass and heat transport in the crust, the extent and consequences of which can only be fully understood by reconstructing the magmatic history of these rocks. In this presentation, I will discuss various factors and processes that influenced the formation and evolution of TTG magmas, from source to sink, using the Kapuskasing Uplift in the southern Superior Province as an example. This region is a natural laboratory for studying TTG petrogenesis, as it represents a crustal cross section with potential source rocks in the lower crust and granitoids of TTG affinity at different crustal levels. The chemical compositions of these TTGs reflect a complex combination of source influences and magmatic processes, and therefore cannot be easily linked to one variable in their formation. Further work is required to fully understand the magmatic history of these rocks and therefore their overall significance to the evolution of the crust, including potential connections to volcanism and ore deposit formation.