Geochemical and petrogenetic comparison between the VMS-endowed Blake River Group and the VMS-poor Stoughton-Roquemaure assemblage, Abitibi greenstone belt

Vite-Sánchez, Octavio (2025). Geochemical and petrogenetic comparison between the VMS-endowed Blake River Group and the VMS-poor Stoughton-Roquemaure assemblage, Abitibi greenstone belt Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de la Terre, 180 p.

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Résumé

The Abitibi greenstone belt is one of the most studied Archean volcanic provinces globally, known for its exceptional endowment in volcanogenic massive sulfide (VMS) deposits. However, the distribution of these deposits is highly uneven among its volcanic assemblages. This thesis investigates the petrological and geochemical factors controlling this differential endowment by comparing two end-member assemblages: the highly fertile Blake River Group (BRG), representing the Blake River Assemblage, and the poorly endowed Stoughton–Roquemaure (SR) assemblage. Over 4,000 whole-rock geochemical analyses of volcanic rocks were compiled, and principal component analysis guided the selection of key variables. Those variables were transformed into a classification diagram for volcanic rocks using immobile element ratios (Zr/Ti vs. Th/Yb), resulting in a unified framework of 12 geochemical groups. Petrogenetic models – including fractional crystallization (FC), assimilation-fractional crystallization (AFC), and magma mixing – were developed using the Magma Chamber Simulator. Despite the presence of similar geochemical groups in both assemblages, their internal distribution differs markedly. The BRG exhibits a bimodal nature with 62.4% mafic, 9.4% intermediate, and 28.2% felsic compositions, by number of samples (with no komatiitic rocks). The S-R assemblage is dominated by mafic rocks (64.7%), with intermediate (21.8%) and felsic (9.9%) compositions decreasing accordingly. Komatiites account for 1% of the samples and komatiitic basalts for 2.6%. Tholeiitic basalts and high-Th basalts are the dominant mafic groups in both assemblages, but tholeiitic basalts are more than twice as abundant in the S-R (37.1%). Conversely, medium-Th basalts are scarce in the S-R (8.1%), indicating two distinct clusters of mafic compositions, unlike the more continuous spectrum observed in the BRG. The S-R assemblage has a strong presence of calc-alkaline intermediate volcanic rocks, unlike the BRG. While felsic rocks are more abundant and geochemically diverse in the BRG, low-Yb felsic types dominate in both assemblages. Transitional felsic compositions are prominent in the BRG but nearly absent in the S-R, where tholeiitic felsic types are not recorded. Significant contrasts also emerge between VMS-endowed and VMS-barren areas. In the BRG, VMS-bearing formations are notably depleted in tholeiitic signatures and are instead characterized by transitional to calc-alkaline affinities, interpreted as the result of crustal interaction. The prolonged emplacement of the BRG (~9 m.y.) likely enabled the development of a mature transcrustal magmatic system, sustaining extensive felsic volcanism. Unlike the S-R, BRG magmatism was concentrated in a more localized area, potentially facilitated by deep transcrustal structures. However, the occurrence of tholeiitic magmas in the majority of the BRG formations indicates a long-lived mantle-derived thermal flux that replenished the magmatic system. In contrast, the S-R assemblage was emplaced over a shorter timespan (~3 m.y.) but covers a broader area across the Abitibi greenstone belt, although it does not each as far south. Despite being more voluminous overall, the S-R assemblage reveals a north–south geochemical zonation. Tholeiitic magmas dominate the central and southern sectors, while transitional to calc-alkaline compositions become more prominent toward the north – where the only known formal VMS deposit (Estrades) is located. This pattern aligns with the crustal architecture of the region: the central Abitibi rift zone, associated with the proto–Destor-Porcupine fault, facilitated decompression melting and plume-driven magmatism with minimal crustal interaction. In contrast, the northern Abitibi portion, likely underlain by thicker crust, exhibits stronger evidence for crustal interaction with the same mantle-derived magmas. This interaction likely occurred through assimilation–fractional crystallization (AFC) or magma mixing, both modeled using a high-MgO tholeiitic basalt and a TTG-like crustal end-member. These findings suggest that VMS fertility in the BRG reflects a combination of factors: prolonged magmatic input (not necessarily larger magma volumes), transcrustal structural controls, and sustained felsic volcanism. In contrast, although the S-R assemblage shows voluminous magmatism, its emplacement was likely more comparable to modern oceanic plateaus were abundant mafic magmatism is generated but limited interaction with the crust occurs, reducing the chances to generate complex magmatic systems in the crust. The strong presence of calcalkaline intermediate rocks, with compositions resembling the TTG end-member, along with the presence of two chemically distinct mafic rock populations, suggests that magma mixing was the dominant differentiation process in the S-R. This contrasts with the BRG, where geochemical trends are more consistent with progressive assimilation–fractional crystallization (AFC). This study provides a new framework to assess VMS potential in Archean terranes, integrating geochemistry from mafic to felsic compositions and petrology with crustal-scale processes.

Type de document:	Thèse Thèse
Directeur de mémoire/thèse:	Ross, Pierre-Simon
Co-directeurs de mémoire/thèse:	Mercier-Langevin, Patrick
Mots-clés libres:	basalt; crustal assimilation; archean greenstone belt; VMS fertility; magma mixing; petrogenetic modeling
Centre:	Centre Eau Terre Environnement
Date de dépôt:	19 juin 2026 15:47
Dernière modification:	19 juin 2026 15:47
URI:	https://espace.inrs.ca/id/eprint/17116

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