Economic Geology
Sudbury Basin Mineral Deposits: Nickel, Copper, and the Impact Origin
The Sudbury Basin in northern Ontario is the site of one of the most unusual ore-forming events in Earth's history. Rather than the typical magmatic, hydrothermal, or sedimentary processes that produce most mineral deposits, Sudbury's nickel-copper-platinum group element (PGE) ores were triggered by a bolide — an asteroid or comet — that struck the Archean Shield approximately 1.849 billion years ago. The impact melted a roughly 200 km wide area of crust, generating the Sudbury Igneous Complex (SIC), and set off ore-forming processes that no other known impact has replicated at the same scale.
The Impact Event
Evidence for the impact comes from several lines of geological investigation. Shatter cones — striated, conical fracture surfaces formed only by hypervelocity shock waves — are preserved in the rocks surrounding the basin. The SIC itself contains microscopic evidence including planar deformation features in quartz and high-pressure polymorphs of silica (coesite and stishovite) that cannot form under ordinary tectonic pressure.
Geophysical modelling suggests the original impact structure was at least 250 km in diameter before erosion and subsequent tectonic compression collapsed it into the current 60 × 30 km elliptical basin. The bolide diameter has been estimated at 10–15 km — comparable to the Chicxulub impactor responsible for the end-Cretaceous extinction, though the Sudbury event predates it by roughly 1.8 billion years and struck an anoxic world with no multicellular life.
Structure of the Sudbury Igneous Complex
The SIC is broadly layered from base to top:
- Sublayer: An irregular unit at the base and footwall embayments of the SIC. It is noritic to quartz-gabbroic in composition and hosts the primary nickel-copper-PGE ore. The sublayer is interpreted as a mixing zone between the impact melt sheet and sulphide liquid that sank to the base.
- Norite: The main intrusive rock of the SIC, dominated by orthopyroxene and plagioclase. Norite is economically significant because it forms the walls of many ore-bearing embayments.
- Quartz Gabbro: Above the norite, this unit transitions upward toward a more silica-rich composition.
- Granophyre: The uppermost unit, representing the silicic fraction of the melt that rose to the top. Some granophyre is partly derived from melted country rock incorporated into the melt sheet.
Overlying the SIC is the Onaping Formation — a thick sequence of fall-back breccia, suevite (impact melt-bearing breccia), and reworked material that settled back into the crater in the hours and years after impact.
The Ore System
The dominant ore minerals are pentlandite [(Fe,Ni)₉S₈], pyrrhotite [Fe₁₋ₓS], and chalcopyrite [CuFeS₂]. Pentlandite is the primary nickel mineral; chalcopyrite contributes most of the copper; and both pyrrhotite and pentlandite carry small amounts of cobalt. Platinum-group minerals — braggite, cooperite, and michenerite — occur in trace quantities but add significant value.
The ore formed when the impact melt sheet became saturated in sulphur. A sulphide liquid separated from the silicate melt and, being denser, sank toward the base. As it sank, it scavenged nickel, copper, and PGEs from the surrounding silicate melt through a process of liquid-liquid partition. The final sulphide accumulations collected in the footwall embayments — irregularly shaped depressions in the underlying country rock — which explains why ore shoots at Sudbury are spatially complex and require detailed three-dimensional modelling to mine efficiently.
Mining History and Current Operations
Commercial nickel extraction at Sudbury began in 1886, initially prompted by the accidental discovery of ore during construction of the Canadian Pacific Railway. By the early 20th century, Sudbury accounted for the majority of the world's nickel supply. The two dominant producers were Inco (now Vale) and Falconbridge (now Glencore), operating dozens of individual mines along the basin margin.
Total historical production from the Sudbury district through 2020 exceeds 9.5 million tonnes of nickel, 9.2 million tonnes of copper, and substantial cobalt and PGE output. Active mines as of 2026 include Vale's Creighton, Garson, and Coleman operations, and Glencore's Fraser, Strathcona, and Nickel Rim South mines.
Environmental remediation around Sudbury has been ongoing since the 1970s, following decades of roast-yard and smelter emissions that acidified local soils and waterbodies over a wide area. Regreening efforts have replanted more than 11 million trees and substantially reduced sulphur dioxide emissions from smelting operations.