An impact of cosmic proportions
Roughly 1.85 billion years ago, a bolide estimated at 10-15 kilometers in diameter slammed into the continental crust of the Canadian Shield. The impact formed the Sudbury Structure, the second-largest confirmed impact crater on Earth. The initial crater was a transient cavity that quickly collapsed, eventually forming a complex, multi-ring basin originally estimated to be around 200 kilometers across. The immense energy release—many millions of times greater than the largest nuclear weapon—melted a vast volume of the surrounding rock, creating a superheated sheet of impact melt up to 2.5 kilometers thick.
This molten rock formed what is now known as the Sudbury Igneous Complex (SIC). The impact also created a network of deep, radiating and concentric fractures in the bedrock surrounding the crater. Before the main sheet of impact melt could cool and solidify, immense pressure injected portions of it deep into these newly formed cracks. These injections of magma, some extending over 30 kilometers from the crater's edge, are the "Offset Dikes." They are geologic hypodermic needles that shot metal-rich magma deep into the country rock.
From magma veins to mineral deposits
The magma that formed the Offset Dikes is a specific type of rock known to geologists as quartz diorite, though it can have a granodiorite composition. As this molten material flowed into the fractures, an important process called magmatic sulfide segregation occurred. The silicate-based melt and a sulfur-rich melt were immiscible, like oil and water. Heavier elements with an affinity for sulfur, nickel, copper, and Platinum Group Elements (PGEs)—were scavenged by the sulfide liquid. This dense, metal-rich liquid separated and settled within the dikes as they cooled.
The result is a series of linear ore bodies containing high concentrations of pyrrhotite, pentlandite, and chalcopyrite, the primary ore minerals for nickel and copper. Famous examples like the Copper Cliff and Worthington offset dikes host some of the most valuable mineral deposits on the planet. The Copper Cliff dike alone accounts for about 15% of all the ore ever mined in the Sudbury district. The total pre-mining resource of the Sudbury camp is estimated to have been over 1.6 billion metric tons of ore. Mining operations in the region continue to produce billions of dollars in revenue annually.