The facies architecture of a paleoproterozoic basaltic andesite synvolcanic intrusion, Flin Flon, Manitoba, Canada

Abstract

The Paleoproterozic 1920 unit occurs at the base of the Hidden formation, overlying greater than 90 million tons of volcanogenic massive sulfide deposits at Flin Flon, Manitoba, Canada. The 1920 unit is a laterally discontinuous, Fe-Ti-P-rich basaltic andesite (Fe2O3 12 wt.%, TiO2 1.1 wt.% and P2O5 0.30 wt.%), consisting of massive, pillowed and in situ brecciated basaltic andesite. The 1920 unit is characterised by facies, textures and structures similar to those associated with subaqueous basaltic lavas. However, the presence of blocky and fluidal peperite along the upper contact, and lens-shaped inclusions of tuff within and along its basal contact, as well as the discordancy of the upper contact suggest that the predominantly massive, basaltic andesite was emplaced as a high-level, synvolcanic sill into wet, unconsolidated volcaniclastic deposits. The presence of a breccia containing clasts of the 1920 unit within the package of volcaniclastic rocks overlying the 1920 unit, and a pillowed facies within the 1920 unit, suggest that, locally, the 1920 unit breached the seafloor. An abundance of synvolcanic dikes within the footwall to the 1920 unit, including the presence of a basaltic andesite dike that merges into the 1920 unit, indicates the possible location of the feeder for the 1920 unit in the northwestern portion of the area.