BHP has been developing its own Canadian potash mine — the Jansen project — in Potash Corp's backyard, and has already invested about $3.8 billion on it.
It was found at Petra's Cullinan mine in South Africa, which has yielded several massive gems, including the largest rough gem diamond ever discovered.
Kinross, which operates two mines in Russia and also owns the problem-plagued Tasiast operation in Mauritania, seeks to strengthen its portfolio in North America.
More environmentalists were murdered in Latin America in 2015 than anywhere else in the world, with anti-mining causes claiming the most lives, a report by Global Witness shows.