Links between volcanogenic massive sulfide endowment and volcanic rock geochemistry: comparing two assemblages from the Archean Abitibi greenstone belt, Canada

Abstract

Volcanogenic massive sulfide (VMS) deposits exhibit stratigraphic and structural controls at the district scale. However, the factors contributing to the varying fertility of volcanic centers, assemblages, or entire greenstone belts remain unclear. Notably, in the Archean Abitibi greenstone belt (Canada), the Stoughton-Roquemaure (S-R) assemblage accounts for <1% of VMS tonnage of the belt, while the much less voluminous Blake River (BR) assemblage hosts almost half of the VMS tonnage. To compare the two assemblages and assess potential petrogenetic controls on VMS fertility, a compilation and filtering of whole-rock geochemistry (n = 4541 samples) has been completed. To explore the dataset, multivariate methods, such as Principal Component Analysis, were utilized. A Th/Yb versus Zr/Ti diagram was developed, on which twelve geochemical clusters were identified. The clusters range from mafic to felsic and from tholeiitic to calc-alkaline. Additionally, the study includes an examination of two ultramafic compositions—komatiitic basalts and komatiites. The Magma Chamber Simulator (MCS) was employed to model fractional crystallization (FC), assimilation-fractional crystallization (AFC), and magma mixing as possible petrogenetic processes for the formation of S-R and BR volcanic rocks. Several initial magma compositions ranging from ultramafic to mafic were evaluated, including different pressures, oxygen fugacities and water contents. Despite the presence of twelve geochemical groups, only two, tholeiitic basalts and high-Th basalts, accounts for almost half of the dataset (46%). Tholeiitic basalts, charachterized by low Th/Yb ratios (<0.2) and flat REE patterns are typically associated with extensional geodynamic contexts or large igneous provinces in the Phanerozoic. Contrastingly, high-Th basalts with high Th/Yb ratios (>0.5), LREE enrichments and transitional to calc-alkaline magmatic affinities are typically associated with continental/island arc environments in the Phanerozoic. In the Abitibi belt, tholeiitic basalts and high-Th basalts are however often intimately intercalated, at odds with modern tectonic environments. Our modeling suggests that magmas with moderate to high Th/Yb ratios and transitional to calc-alkaline signatures may result from a tholeiitic magma assimilating a TTG-like wall rock in the middle crust, explaining the association between the two types of rocks. Upon closer examination of predominantly volcanic geological formations hosting VMS, mafic to intermediate compositions mostly features moderate to high Th/Yb geochemical groups. However, not all successions with these characteristics are fertile. Felsic compositions occurring in the BR and S-R assemblages mostly feature (>90%) volcanic rocks with moderate to high Th/Yb ratios (>0.2-10). However, groups closely related to VMS activity feature moderate Th/Yb ratios (0.2-1) and are more abundant in the well-endowed BR. The petrogenetic models presented improve our understanding of Archean greenstone belt petrogenesis, and ongoing work will help further comprehend controls on VMS fertility.