Geodynamic setting, crustal architecture, and VMS metallogeny of ca. 2720 Ma greenstone belt assemblages of the northern Wawa subprovince, Superior Province

Abstract

The greenstone belts along the northern margin of the Wawa subprovince of the Superior Province (Vermilion, Shebandowan, Winston Lake, Manitouwadge) formed at ca. 2720 Ma and have been interpreted to be representative of a rifted-arc to back-arc tectonic setting. Despite a common inferred tectonic setting and broad similarities, these greenstone belts have a significantly different metallogeny as evidenced by different endowments in volcanogenic massive sulphide (VMS), magmatic sulphide, and orogenic gold deposits. In this paper, we examine differences in geodynamic setting and crustal architecture as they pertain to the metallogeny of each greenstone belt by characterizing the regional-scale trace-element and isotopic (Nd and Pb) geochemistry of each belt. The trace-element geochemistry of the Vermilion greenstone belt (VGB) shows evidence for a transition from arc-like to back-arc mafic rocks in the Soudan belt to plume-driven rifted arcs in the ultramafic-bearing Newton belt. The Shebandowan greenstone belt (SGB) has a significant proportion of calc-alkalic, arc-like basalts, intermediate lithofacies, and high-Mg andesites, which are characteristic of low-angle, “hot” subduction. Extensional settings within the SGB are plume-driven and associated with komatiitic ultramafic and mid-ocean ridge basalt (MORB)-like basalts. The Winston Lake greenstone belt (WGB) is characterized by a transition from calc-alkalic, arc-like basalts to back-arc basalts upward in the strata and is capped by alkalic ocean-island basalt (OIB)-like basalts. This association is consistent with plume-driven rifting of a mature arc setting. Each of the VGB, SGB, and WGB show some isotopic evidence for the interaction with a juvenile or slightly older differentiated crust. The Manitouwadge greenstone belt (MGB) is characterized by isotopically juvenile, bimodal, tholeiitic to transitional volcanic lithofacies in a back-arc setting. The MGB is the most isotopically juvenile belt and is also the most productive in terms of VMS mineralization. The Zn-rich VMS mineralization within the WGB suggests a relatively lower-temperature hydrothermal system, possibly within a relatively shallow-water environment. The Zn-dominated and locally Au-enriched VMS mineralization, as well as mafic lithofacies and alteration assemblages, are characteristic of relatively shallower-water deposition in the VGB and SGB, and indicate that the ideal VMS-forming tectonic condition may have been compromised by a shallower-water depositional setting. However, the thickened arc crust and compressional tectonics of the SGB suprasubduction zone during hot subduction may have provided a crustal setting more favourable for the magmatic Ni–Cu sulphide and relative gold endowment of this belt.