The Sudbury Basin

An impact crater that turned out to be an ore body

The Sudbury Basin, in the city of Greater Sudbury on the Canadian Shield, is the eroded remnant of one of the oldest and largest impact craters known on Earth. Something 10 to 15 kilometres across struck here 1.849 billion years ago. It was long taken for an asteroid; a 2014 analysis of siderophile elements, and of how much rock the impact melted, pointed to a comet instead. Debris was thrown more than 800 kilometres, and ejecta have turned up as far away as Minnesota. Estimates of the original crater run from 130 kilometres across to at least 200, and later deformation squeezed what was left into an oval about 62 kilometres long and 30 wide. Locally it is known as The Valley.

The collision punched through the crust and the basin filled with melted rock. As the melt cooled, a sulphide liquid separated out of it like oil from water, gathered up copper, nickel and the platinum metals, sank to the crater floor and solidified. The deposits lie mostly around the outer rim, where the mines are, and the ores carry nickel, copper, gold, silver, platinum, palladium, rhodium, iridium and ruthenium.

In 1856 a provincial land surveyor named Salter found his compass needle swinging between four and fourteen degrees near the site of the later Creighton Mine. He and Alexander Murray of the Geological Survey reported it and were ignored. The Canadian Pacific Railway changed that: in August 1883 a railway blacksmith, Thomas Flanagan, noticed rust-coloured rock in a freshly blasted cut northwest of present-day Sudbury, and the Murray Mine followed. Early ore went to a refinery in New Jersey, where the nickel at first came out as a pinkish grey material of no apparent value. The basin is now one of the world's largest suppliers of nickel and copper ores.

The impact origin was hard to prove after so much weathering, and the region had been volcanically active at about the same time. Reports in the late 1960s described shatter cones and shock-deformed quartz, and geologists reached a consensus by about 1970. NASA used the site to teach Apollo astronauts to recognise impact rocks such as breccias, and David Scott and James Irwin of Apollo 15, John Young and Charlie Duke of Apollo 16, and Gene Cernan and Jack Schmitt of Apollo 17 put that training to use on the Moon.

The smelting that built the city also polluted it with sulphur dioxide and metals. In recent decades emissions have been captured and work has gone into restoring the basin's land and water.