Relative timing and controls on advanced argillic and conventional alteration of the Neoarchean Onaman volcanogenic massive sulfide deposit, Ontario, Canada

Volcanogenic massive sulfide (VMS) deposits with advanced argillic alteration display characteristics typical of conventional VMS and high-sulfidation epithermal deposits. Unlike conventional VMS deposits, these “hybrid” VMS are interpreted to have formed through a magmatic fluid contribution to an evolved, seawater-dominated, hydrothermal system. The close spatial association of advanced argillic alteration assemblages with more typical chlorite–sericite assemblages in the same VMS deposit has implications for the controls on magmatic versus seawater convective systems that have not been previously addressed in the literature. The Neoarchean Onaman assemblage (ca. 2780–2769 Ma) in northwestern Ontario hosts a conventional base metal VMS system with chlorite–sericite alteration that is overprinted by argillic and advanced argillic alteration, now manifested as a metamorphosed zone of kyanite–chloritoid–calcite–Fe–chlorite, that is associated with a barren, pyritic VMS deposit. The change from typical to advanced argillic alteration occurred during, or immediately following, the deposition of a tonalite clast-bearing heterolithic breccia, which recorded the uplift and subaerial exposure of the larger volcanic edifice. We postulate that the reactivation of synvolcanic structures during uplift, concomitant localized subsidence, and magmatism enhanced cross-stratal permeability and allowed for a direct magmatic volatile input into, or overprint on, an evolved seawater-dominated hydrothermal system. Our results suggest that geodynamic regimes exert a strong local, and possibly regional, control on the style, timing, and fluid characteristics of VMS hydrothermal systems. The association of advanced argillic alteration with barren pyritic massive sulfide also suggests that magmatic volatile input alone is not sufficient to enrich VMS systems in gold. This interpretation is consistent with observations from other magmatic–hydrothermal systems such as porphyry and epithermal deposits.