Abstract

Orebodies in the Missanabie-Renabie gold district (Archean Wawa subprovince, Ontario, Canada) are controversial in their genetic affiliation and have been considered both intrusion related and orogenic. The orebodies are hybrids composed of pre-orogenic, syn-orogenic, and late syn- to post-orogenic veins that spatially overlap within 2720.8 ± 1.4 Ma biotite tonalite of the Missinaibi Lake batholith. Gold-bearing, pre-orogenic laminated quartz veins with quartz-sericite-pyrite alteration halos (the Au1 hydrothermal event) account for the majority of gold endowment and are overprinted by (1) syn-orogenic, gold-barren quartz ± epidote ± chlorite ± K-feldspar ± hematite veins and breccias associated with hematite-bearing alteration zones (the Retrograde hydrothermal event) and (2) late syn- to post-orogenic, gold-bearing pyrite veins (the Au2 hydrothermal event).

These hydrothermal events are characterized by integrating field-based constraints with petrographic and scanning electron microscope-energy-dispersive spectroscopy imaging/analysis, lithogeochemistry and mass balance, and fluid inclusion studies including microthermometry and evaporate mound analysis. The results illustrate that the Au1 event relates to the unmixing of pyrite-stable, H2O-CO2 (XCO2 0.13–0.17; 3.7–8.0 wt % NaCl equiv), Au-Ag-Bi-Cu- Mo-Pb-Te-W-Zn-S-large ion lithophile element–bearing fluid. The Retrograde event reflects a pyrite-stable, Na-S-(± Ag, Bi, Te, W)–bearing H2O fluid (~5–10 wt % NaCl equiv) that evolved to a hematite-stable, more saline (up to ~23 wt % NaCl + CaCl2 equiv), Ca-K–enriched fluid in response to alteration-associated fluid-rock interaction. A lack of identifiable alteration and fluid inclusions associated with Au2 veins precludes constraints on the fluid chemistry of the Au2 event. Relationships among prograde metamorphic minerals, Au1 phases, and Retrograde phases indicate a pre-prograde metamorphic timing for the Au1 event and a post-peak metamorphic timing for the Retrograde and Au2 events. These contrasting fluid chemistries, alteration characteristics, and metamorphic timing relationships imply that the different hydrothermal events have divergent genetic histories. The chronology of the hydrothermal events with respect to deformation and metamorphism suggests that the intrusion-hosted, pre-orogenic, gold-bearing veins that formed across the Wawa gold camp during the Au1 event are genetically related to potassium-poor granitoid intrusions. These pre-orogenic veins were overprinted by syn-orogenic veins during the Retrograde and Au2 events as the intrusion-related/orogenic hybrid orebodies that characterize the Wawa gold camp formed.

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