A textural and mineralogical study of the footwall rocks to the Sudbury Igneous Complex (North and East ranges)

Abstract

In the North and East range footwall rocks of the 1.85 Ga Sudbury Igneous Complex (SIC), quartz and plagioclase (An20-30), plastically deformed prior to the emplacement of the SIC, were thermally annealed, resulting in their replacement by aggregates of strain-free grains that coarsen towards the SIC footwall contact due to radiant heat. In the Victor footwall deposit, SIC East Range, granoblastic polygonal quartz was overprinted by dynamically recrystallized quartz, with microstructures indicative of bulging (BLG) recrystallization. Incipient melting at quartz-plagioclase interfaces in Victor footwall rocks is recorded as interstitial plagioclase seams in quartz with quartz-quartz-plagioclase dihedral angles that peak at ~40 ⁰, characteristic of wetting angles. Subsequent fluid-mediated modification of least-altered plagioclase (An20-30) generated porous albite (An0-10) through a coupled dissolutionprecipitation (CDP) mechanism, which was likely contemporaneous with the crystallization of secondary epidote-group minerals. In the Victor footwall, textures and mineral chemistry of plagioclase, epidote- and chlorite-group minerals support the notion that a relatively oxidized, acidic, Fe-bearing fluid with a temperature of ~310 – 385 ⁰C favoured quartz-plagioclase interfaces as grain-scale fluid pathways, preferentially dissolving the anorthite component of plagioclase and creating void space prior to formation of disseminated chalcopyrite proximal to massive Cu-rich sulfide veins in the footwall setting.