Formation of large-scale impact melt dikes: A case study of the Foy Offset Dike at the Sudbury impact structure, Canada

Abstract

Hypervelocity impacts frequently result in the formation of dikes in the crater floor and central uplift. Impact melt-bearing dikes in large terrestrial impact basins – such as the Offset Dikes at the Sudbury impact structure – occur at a large scale, often tens of meters wide and several kilometers in length. The Offset Dikes host significant Ni–Cu–PGE deposits, which include several well-known mines, such as Totten and Copper Cliff that have been mined for nearly 100 years. The Offset Dikes typically consist of a clast- and sulfide-rich core and clast-poor margins. Their formation has been a subject of debate for decades. The most widely proposed model is the early emplacement of clast-poor impact melt shortly after the impact event, followed by the later emplacement of clast- and sulfide-rich impact melt in the center of the dike. An alternative hypothesis is that a single pulse of clast-rich impact melt flowed into the fractured target rocks and flow differentiation resulted in a clast-rich core and a clast-poor margin. In this study, we examine field and petrographic relationships of the Foy Offset Dike to better understand its emplacement. Our results show that the characteristics of the Foy Offset Dike – namely the gradational nature of the contact between the clast-rich and clast-poor phases, the alignment of clasts sub-parallel to this contact, the geochemical similarities and the presence of sulfides within both phases of the dike – are more consistent with the single injection and flow differentiation hypothesis.