Understanding the South Range Sudbury impact structure: a study of shear zones, impact breccias and PGE occurrences in the Vermilion and Crean Hill mines area, Sudbury, Canada

Abstract

The Sudbury impact structure formed when a bolide collided with the southern Superior craton margin ca. 1.85 Ga ago. During the impact, the target rocks were brecciated and melted, which resulted in the formation of a thick impact melt sheet, the Sudbury Igneous Complex (SIC), underlain by brecciated basement rocks. Nickel-copper-platinum group elements (Ni-Cu-PGE) deposits formed at the base of the SIC as contact deposits, in association with offset dikes that flowed from the melt sheet into the basement rocks (offset deposits), and as sulphide veins and pods of disseminated PGE mineralization that were emplaced into the brecciated basement rocks (footwall deposits). The deposits and their host rocks were subsequently modified by orogenic events that reworked the South Range of the impact structure. The thesis discusses the formation of syn-impact breccias below the SIC-basement contact, the structural and metamorphic modification of the impact structure during subsequent orogenic events, and the effect of syn- and post-impact processes on the formation and remobilization of (Ni-)Cu-PGE mineralization at the contact-type Crean Hill deposit and offset-type Vermilion deposit. The breccias formed by in situ shock melting of the footwall rocks during the impact, and were modified by contact metamorphism during cooling of the SIC and by syn-tectonic deformation and metamorphism after the impact. The latter resulted in the formation of regional folds and south-side-up-dextral shear zones during northwest-directed shortening of the impact structure. (Ni-)Cu-PGE mineralization at the Crean Hill and Vermilion deposits was emplaced as sulphide melts immediately after the impact and was remobilized by metamorphic semi-metal melt and hydrothermal fluids into the breccias and shear zones during and after regional deformation. These results illustrate how both syn- and post-impact processes contributed to the formation and modification of Ni-Cu-PGE deposits in Sudbury.