The famous nickel mine at Sudbury in Ontario, Canada was created by a huge asteroid that hit the spot over 1.8 billions ago. It left an impact crater known as the Sudbury Basin. Such asteroid impacts provided most of the heavy metals accessible to mines because most of the metals in the earth settled into the core of the planet: Rare Earth – Brian Koberlein –
… fortunately Earth was also bombarded by meteors in its early history, and this made mining practical in two ways. The first is that asteroids and meteors themselves contain vast quantities of heavy metals. By some estimates, a single mile-wide asteroid could contain twenty trillion dollars worth of precious metals. Since most of the asteroid bombardments occurred after Earth’s crust formed, these metals were deposited near Earth’s surface, making them easier to obtain. There is some evidence that most of accessible heavy metals are extraterrestrial in origin due to this process.
The second is due to large impacts such as Sudbury. With large impacts, part of the Earth’s crust are melted. As a result, deposited material then settles in layers as it re-cools. Heavier elements settle at the bottom of the crater, while lighter ones settle near the top. As a result, heavy metals are concentrated at the bottom layer of the crater, producing rich veins of ore. Impacts can also create other useful byproducts, such as impact diamonds and pockets of oil. Chicxulub crater (caused by the famous dinosaur extinction asteroid) near the Gulf of Mexico is a region with plentiful oil deposits, for example.
Here’s a nicely made video describing the formation of the Sudbury basin: