Early thrusting and folding in the Snow Lake camp, Manitoba: tectonic implications and effects on volcanogenic massive sulfide deposits

Abstract

The Chisel sequence occurs within the Snow Lake arc assemblage (SLA) of the Paleoproterozoic Trans-Hudson Orogen. It is host to six economic volcanogenic massive sulfide (VMS) deposits that formed within a single time-stratigraphic ore interval. The SLA was previously interpreted to share the same deformation history as the ca. 1.86–1.84 Ga Burntwood Group turbidites of the Kisseynew Basin to the north; however, early isoclinal folds identified in the arc assemblage cannot be correlated with structures in the turbidites and therefore developed prior to Burntwood Group deposition. This indicates that the SLA underwent a significant deformation event prior to 1.86 Ga, likely due to intraoceanic accretion or pre-1.86 Ga accretion to the Amisk collage during closure of the ancestral Manikewan Ocean. Evidence for this early folding event is recorded in the Chisel sequence deposits where it has influenced the geometry of the deposits and the location of the ore interval. The deposits are isoclinally folded by these folds, and a major early thrust fault at the Lalor deposit, the Lalor-Chisel fault, displaced the hanging-wall rocks of the deposit and the ore interval. Further flattening, transposition, and elongation of the ore lenses occurred during post-1.84 Ga collision of the SLA with the partially hidden Archean Sask craton. Although the VMS deposits are polydeformed and sulfides and precious metals were mechanically and hydrothermally remobilized at the metre scale, their primary base metal zoning remains intact and provides a tool to reconstruct the geometry of the deposits.